Corona City Council
June 3, 2026

MEETING NOTES

TEMESCAL BASIN GROUNDWATER

SUSTAINABILITY PLAN

CITY OF CORONA
DWR PLANNING GRANT NO. 4600012652
Date: December 9, 2020 Time: 11:00 to 12:00
Location: Teams Web Conference Project No.: 46414
Subject: Temescal GSP Coordination with Chino GSA - Meeting 1 Attendees: Tom Moody, Katie Hockett, Kristian Alfelor, Melissa Estrada-Maravilla - , Corona DWP, Edgar Tellez Foster – Chino Watermaster, Chad Taylor, Gus Yates, and Maureen Reilly – Todd Groundwater
Introductions
The City of Corona is preparing a GSP for the Temescal Basin and wants to coordinate with Chino Watermaster as a neighboring basin.
• Our coordination will largely focus on the water budget and model for Temescal • We know Chino has a model that includes some of the Temescal Basin • We want to start the coordination process and plan for requesting data and information from the Watermaster
Chino has an Optimum Basin Management Plan first prepared in 2000 and updated in 2020. • This is the equivalent of a GSP for Chino
• Uses material physical injury in place of sustainability indicators • Available on watermaster website
Chino Valley model includes parts of the Temescal Basin and other neighboring basins as well.
• Needs to talk to consultant (Wildermuth, now West Yost) about how Temescal in represented in their model
Currently no plans to increase capture area or volume in their desalter system above the 40,000 AFY. The desalter wells are intended to function more or less the way ag wells functioned historically in terms of groundwater budget and flow patterns.

How does the Watermaster balance their desalter water quality goals with wetlands and other volumetric considerations?
• They have a monitoring and reporting program that includes groundwater elevation and vegetation monitoring in the Prado Basin Habitat Sustainability reporting, most recent in April 2020
• They also have a State of the Basin report that periodically reports on overall basin conditions
o The last State of the Basin report was through 2018
o They are working on the current report through 2020 now It seems like there is some variability in flow dynamics around the boundary between the two basins, how does the Chino model handle the dynamic flow in this area? • This is a question for Wildermuth / West Yost
Does the Watermaster have any agreements with OCWD?
• No, but they are bound by the Santa Ana River Watershed agreement that requires a certain outflow past Prado Dam.
• They do coordinate with OCWD in some data collection, but no operations commitments
How can we request data efficiently?
• Submit requests to request email from website and copy Edgar • They have some data sharing limitations for private well data • Special model runs can be completed, but there would be a passthrough cost • Can we request output data from existing/past model runs? o Yes, make request through the same channels
Does there desalter capture system capture all the water coming from the north, and do they rely on water coming from the Temescal Basin?
• Their desalter wells create a trough that captures water from both directions • The intent is to keep high TDS water from the former dairy operations from flowing to and entering the Santa Ana River, simulating historical agricultural pumping Chino Watermaster is interested in engaging with the preparation of the Temescal GSP.

MEETING NOTES

TEMESCAL BASIN GROUNDWATER

SUSTAINABILITY PLAN

CITY OF CORONA
DWR PLANNING GRANT NO. 4600012652
Date: November 30, 2020 Time:
2:00 to 3:00
PM
Location: Teams Web Conference Project No.: 46414
Subject: Temescal GSP Coordination with OCWD - Meeting 1 Attendees: Tom Moody, Katie Hockett, Kristian Alfelor, Melissa Estrada-Maravilla - , Corona DWP Chad Taylor, Gus Yates, and Maureen Reilly – Todd Groundwater, Adam Hutchinson – OCWD
The City of Corona is preparing a GSP for the Temescal Basin and wants to coordinate with OCWD in their capacity as the GSA for the neighboring Coastal Plain Basin as a neighboring basin with an approved alternative plan.
• Our coordination will largely focus on the water budget and model for Temescal • We’ve looked at the Coastal Plain water budget as presented in the alternative plan and want to coordinate our water budget in this area
• We want to start the coordination process and plan for requesting data and information from OCWD
• We also want to make sure we know if OCWD has questions or concerns about the Temescal
Did we receive the comments on the Plan Area chapter of the Temescal GSP? • Yes
• Adam wanted to make sure that the Temescal GSP note and incorporate management in the upstream Chino and Riverside-Arlington basins • Pumping for water quality management in these basins captures a large volume of the outflow that would otherwise come into Temescal and then on to Prado and eventually out to the Coastal Plain of OC basin

Geoscience is working on the Santa Ana River Integrated Model o OCWD has been working closely with Geoscience on this model o The model needs improvement, and OCWD is working with Geoscience on a version focused on the Prado area
• There is a recently published HCP prepared by Valley District that is watershed-wide and applies to the Basin and Prado
o It is under review now
o It is an attempt to combine all the HCPs into one framework Are you refining water budget, specifically inflow across basin boundary? • No, that term is relatively static because there is virtually no change between Prado dam and the OCWD rubber dam
• They assume the 1,580 AFY is pretty much all surface water flow What are OCWD’s main concerns relating to the Temescal GSP? • From a water quality perspective, they are focused on PFOS o They sampled in Temescal and see some PFOS in and around Temescal Creek and other waterways
o This included first flush sampling of many streams at the beginning of a significant storm
o Also sampled one of the new monitoring wells in Prado o Temescal Wash showed the highest PFOS surface water concentrations • OCWD is very concerned about maintaining habitat quantity and quality in the Prado wetlands. OCWD owns about 2,000 acres of the wetlands. o Least Bell’s vireo is a key management species.
o OCWD recently installed ten new shallow piezometers to measure water table depth in Prado, supplementing information from drive point piezometers installed previously.
o Adam is interested in having a multi-depth monitoring well near Prado Dam. o Water requirements for sustaining Prado habitat might have a secondary effect of increasing flow past Prado Dam.
• Adam wants the Temescal GSP to include a good and complete description of the entire upper Santa Ana River watershed and groundwater basins.

MEETING NOTES

TEMESCAL SUBBASIN

GROUNDWATER SUSTAINABILITY
PLAN

CITY OF CORONA
DWR PLANNING GRANT NO. 4600012652
Date: July 13, 2020 Time: 9 to 9:30 AM
Location: City of Corona Dept. of Water and Power Project No.: 46414 Subject: Initial Meeting with Eileen Navarro, Community Orgainizer Attendees: Melissa Estrada-Maravilla & Kristian Alfelor – Corona DWP, Jack Hughes– Kearns & West, Chad Taylor– Todd Groundwater, and Eileen Navarro

1. Brief introduction to SGMA and the GSP process
Chad gives a brief introduction to SGMA legislation

2. Introductions
Eileen recently finished college w/ degree in political science and community outreach.

She’s recently been working with Vice Mayor Casillas and learning about the Home Gardens community

3. Background information regarding the Home Gardens community Home Gardens is an unincorporated community with no elected officials or independent governance.

There aren’t any formal community groups, but there is active involvement on specific issues and social media engagement.

A large percentage of the community is Latinx with Spanish as their primary language. Lack of Spanish-language outreach has often been an impediment to engagement for the community. There has also been limited engagement due to fear of legal repercussions related to immigration status.

However, the community has come together in the past to lobby for specific items like sidewalks and Policing.

DWR Planning Grant No. 46000126552
There is a school, Home Gardens Academy, which serves as a local meeting place and information hub and there is also a library where meetings and gatherings are sometimes held
People are currently most engaged in COVID, schools reopening, an increase in car breakins, policing, and the potential for incorporation.

Active discussion about incorporating into either Corona or Riverside is occurring now.

The Incorporation discussion was started because of policing issues, and Eileen has been involved in information gathering and discussions on this topic.

Melissa asks if there have been any discussions regarding water service from Corona? Yes, there has been discussion of water service; approximately half of the community pays to Corona/Home Gardens WD while others pay directly to the City of Riverside. The divide is at McKinley Street.

Eileen is not aware of other water problems or questions from people in Home Gardens.

Melissa asks if Eileen is interested in being involved in the GSP process? And if so, would she like to be on the TAC?
Eileen indicated that she is interested in being updated during preparation of the GSP as an interested party, but she doesn’t think she’ll have time to be on the TAC because she is planning to start law school soon.

T:\PROJECTS\CORONA GSP 46414\OUTREACH AND MEETINGS\COMMUNICATIONS AND OUTREACH\MEETING NOTES - INTRODUCTORY MEETING WITH COUNCIL MEMBERS CASILLAS AND SCOTT 08-11-20.DOCX PAGE 1 OF2

MEETING AGENDA

TEMESCAL SUBBASIN

GROUNDWATER SUSTAINABILITY
PLAN

CITY OF CORONA
DWR PLANNING GRANT NO. 4600012652
Date: August 11, 2020 Time: 11AM to 12PM
Location: Teams Web Conference Project No.: 46414
Subject: Temescal GSP TAC Outreach Planning with Corona Council Members Attendees: Vice Mayor Jacque Casillas and Council Member Jason Scott – Corona, Katie Hockett, Kristian Alfelor, Melissa Estrada-Maravilla – Corona DWP, Jack Hughes & Joan Isaacson Kearns & West, and Chad Taylor Todd Groundwater

INTRODUCTIONS

PROJECT OVERVIEW
Chad gives a brief summary of SGMA and the GSP process and an introduction to the outreach requirements and plans

PURPOSE OF THE MEETING
Summary of the outreach and engagement we are planning for and to learn what we should know and consider for outreach in the communities Vice Mayor Casillas and Councilmember Scott work with/represent. Particularly on the areas designated as disadvantaged communities by the State of California and called out for inclusion in outreach and engagement by DWR in GSP preparation.

GSP COMMUNICATIONS AND OUTREACH REQUIREMENT SUMMARY
Jack notes that SGMA calls for consideration of all interests of all beneficial uses and users of groundwater. K&W has developed an Outreach and Involvement Plan that includes multiple avenues for public engagement including public workshops, pre workshop outreach, TACs,

DWR Planning Grant No. 46000126552
City Council and Board of Directors Meeting Presentations, plus website, fact sheet, translation, etc.

Focused Outreach (Jack/Joan)
Jack notes that there are areas in the Basin designated as Disadvantaged Communities, which are census block groups with less than 80% of the State's median household income or severely disadvantaged communities where (less than 60% of the State's median household income).

Vice Mayor Casillas notes that we should try to meet people where they are. There are multiple Catholic churches (St Edwards and Corpus Christi) that should be included.

Is there a way to incentive people to attend within the grant? • Hard to move grant funds toward incentives, and reductions in water bills run up on Prop 218 restrictions
• However, we could potentially coordinate with food distribution by providing fact sheets for distribution.

Council member Scott recommends using simple language in communications, reaching out to YMCA and mobile home parks, two work force housing developments 6th street near armory and near City Hall, and American Legion (Joe Domingus).

Vice Mayor Casillas mentions the Corona Norco Parent Teacher Center Spanish language radio add-buys
Are community members open to shorter content on alternative platforms? • Yes, especially younger people.
• Should consider packaging items developed for or during public meetings for alternative platform distribution later.

Discussion of character of districts and/or nearby communities (All) • Do people have any concerns pertaining to water, water quality, and the environment, and if so, what are they?
o They have not heard of concerns specific to water, except for questions about water bills from municipal providers.

Community Leader Meetings
Draft Notes
Tuesday, June 29, 2021
1:00 p.m. – 2:00 p.m.

Location: Zoom Virtual Meeting
Attendees
• Marven Norman, Center for Community Action and Environmental Justice (CCAEJ) City of Corona Department of Water and Power Staff
• Katie Hockett
• Kristian Alfelor
• Melissa Estrada-Maravilla
Consultant Team
• Chad Taylor, Todd Groundwater
• Maureen Reilly, Todd Groundwater
• Christian Mendez, Kearns & West
• Jack Hughes, Kearns & West
Notes
As a local leader, does water come up in a conversations with others in the community? • CCAEJ’s big focus is on air quality and other impacts to communities from warehouses and industrial uses. This has not been an issue near the City of Corona as far as CCAEJ knows.

Water is not a major focus for CCAEJ, but they have been asked to be involved in a lawsuit pertaining to impacts to groundwater from warehouse runoff.

What questions do you have about water today and tomorrow? • What happens if we start having more droughts?
• What are the plans if there need to be reduction in water use? Thursday, July 1, 2021
1:00 p.m. – 2:00 p.m.

Location: Zoom Virtual Meeting
Attendees
• Alma Marquez, Center for Community Action and Environmental Justice (CCAEJ) • Fauzia Rizvi, Western Municipal Water District
• Elizabeth Toups, IE Works
• Scott Goodell, IE Works
• Diana Meza, City of Corona Planning and Housing

City of Corona Department of Water and Power Staff
• Katie Hockett
• Kristian Alfelor
• Melissa Estrada-Maravilla
Consultant Team
• Chad Taylor, Todd Groundwater
• Maureen Reilly, Todd Groundwater
• Christian Mendez, Kearns & West
• Jack Hughes, Kearns & West
Notes
As a Local Leader, does water come up in a conversations with others in the community? • CCAEJ is interested in runoff pollution and its impact to groundwater. They would be interested in exploring this in communities outside the City of Corona. They are looking at initiating conversations on education and policy focusing on the impacts of runoff pollution to groundwater.
• Water is not often spoken about in the City of Corona Planning and Housing Commission. There needs to be more education on water conservation and groundwater. • The only time people speak about water is when there is something wrong. • There should be an awareness campaign about water issues like costs, drought, and conservation.

Are you aware of any private wells in your community?
• There is a community in Norco on Bluff Street that has a private well (this well appears to be outside the Temescal Basin boundaries).

What other groups/individuals should we invite to the Public Workshop on July 8? • Outreach to farmers markets, swap meets, and churches • CCAEJ can support by posting via social media
• Leela Project
• Local group that works with youth: could focus on job mobility • Outreach material should include: What's in it for me? Why should I care? • Include higher education and K-12
Other questions and comments from participants:
• What about future job availability?
• Alma Marquez expressed interest in the desalter tour

Temescal Basin GSP

APPENDIX I
Draft GSP Comments and Responses

December 14, 2021
Temescal GSA
755 Public Safety Way
Corona, CA 92878
Submitted via email: [email protected]
Re: Public Comment Letter for Temescal Basin Draft GSP
Dear Melissa Estrada-Maravilla,
On behalf of the above-listed organizations, we appreciate the opportunity to comment on the Draft Groundwater Sustainability Plan (GSP) for the Temescal Basin being prepared under the Sustainable Groundwater Management Act (SGMA). Our organizations are deeply engaged in and committed to the successful implementation of SGMA because we understand that groundwater is critical for the resilience of California’s water portfolio, particularly in light of changing climate. Under the requirements of SGMA, Groundwater Sustainability Agencies (GSAs) must consider the interests of all beneficial uses and users of groundwater, such as domestic well owners, environmental users, surface water users, federal government, California Native American tribes and disadvantaged communities (Water Code 10723.2).

As stakeholder representatives for beneficial users of groundwater, our GSP review focuses on how well disadvantaged communities, drinking water users, tribes, climate change, and the environment were addressed in the GSP. While we appreciate that some basins have consulted us directly via focus groups, workshops, and working groups, we are providing public comment letters to all GSAs as a means to engage in the development of 2022 GSPs across the state. Recognizing that GSPs are complicated and resource intensive to develop, the intention of this letter is to provide constructive stakeholder feedback that can improve the GSP prior to submission to the State.

Based on our review, we have significant concerns regarding the treatment of key beneficial users in the Draft GSP and consider the GSP to be insufficient under SGMA. We highlight the following findings:

1. Beneficial uses and users are not sufficiently considered in GSP development. a. Human Right to Water considerations are not sufficiently incorporated. b. Public trust resources are not sufficiently considered. c. Impacts of Minimum Thresholds, Measurable Objectives and Undesirable Results on beneficial uses and users are not sufficiently analyzed.

2. Climate change is not sufficiently considered.

Temescal Basin Draft GSP Page 1 of 13

3. Data gaps are not sufficiently identified and the GSP needs additional plans to eliminate them.

4. Projects and Management Actions do not sufficiently consider potential impacts or benefits to beneficial uses and users.

Our specific comments related to the deficiencies of the Temescal Basin Draft GSP along with recommendations on how to reconcile them, are provided in detail in Attachment A.

Please refer to the enclosed list of attachments for additional technical recommendations:

Attachment A GSP Specific Comments
Attachment B SGMA Tools to address DAC, drinking water, and environmental beneficial uses and users
Attachment C Freshwater species located in the basin
Attachment D The Nature Conservancy’s “Identifying GDEs under SGMA: Best Practices for using the NC Dataset”
Attachment E Maps of representative monitoring sites in relation to key beneficial users Thank you for fully considering our comments as you finalize your GSP.

Best Regards,
Ngodoo Atume
Water Policy Analyst
Clean Water Action/Clean Water Fund
Samantha Arthur
Working Lands Program Director
Audubon California
E.J. Remson
Senior Project Director, California Water Program
The Nature Conservancy
J. Pablo Ortiz-Partida, Ph.D.

Western States Climate and Water Scientist
Union of Concerned Scientists
Danielle V. Dolan
Water Program Director
Local Government Commission
Melissa M. Rohde
Groundwater Scientist
The Nature Conservancy
Temescal Basin Draft GSP Page 2 of 13

Attachment A
Specific Comments on the Temescal Basin Draft Groundwater Sustainability Plan

1. Consideration of Beneficial Uses and Users in GSP development Consideration of beneficial uses and users in GSP development is contingent upon adequate identification and engagement of the appropriate stakeholders. The (A) identification, (B) engagement, and (C) consideration of disadvantaged communities, drinking water users, tribes, groundwater 1 dependent ecosystems, streams, wetlands, and freshwater species are essential for ensuring the GSP integrates existing state policies on the Human Right to Water and the Public Trust Doctrine.

A. Identification of Key Beneficial Uses and Users
Disadvantaged Communities and Drinking Water Users
The identification of Disadvantaged Communities (DACs) and drinking water users is incomplete. The GSP provides information on DACs, including identification by name and location on a map (Figure 2-13). However, the GSP fails to clearly state the population of each DAC.

The GSP provides a density map of domestic wells in the basin (Figure 2-5). However, the plan fails to provide depth of these wells (such as minimum well depth, average well depth, or depth range) within the basin. This information is necessary to understand the distribution of shallow and vulnerable drinking water wells within the basin.

These missing elements are required for the GSAs to fully understand the specific interests and water demands of these beneficial users, and to support the consideration of beneficial users in the development of sustainable management criteria and selection of projects and management actions.

RECOMMENDATIONS
● Provide the population of each identified DAC.
● Include a map showing domestic well locations and average well depth across the basin.

Interconnected Surface Waters
The identification of Interconnected Surface Waters (ISWs) is insufficient, due to lack of supporting information provided for the ISW analysis. The GSP describes the use of aerial photos to analyze stream reaches and presents analysis of stream gage and groundwater elevation data.

The ISW section concludes with the following statement (p. 4-16): “In spite of these accuracy limitations, contours of depth to water measured in wells—in combination with depth to water data 1 Our letter provides a review of the identification and consideration of federally recognized tribes (Data source:

SGMA Data viewer) within the GSP from non-tribal members and NGOs. Based on the likely incomplete information available to our organizations for this review, we recommend that the GSA utilize the California Department of Water Resources’ “Engagement with Tribal Governments” Guidance Document (https://water.ca.gov/Programs/Groundwater-Management/SGMA-Groundwater-Management/Best-Management-Pra ctices-and-Guidance-Documents) to comprehensively address these important beneficial users in their GSP.

Temescal Basin Draft GSP Page 3 of 13

for the downstream end of the Bedford-Coldwater Subbasin (also shown in Figure 4-20) —indicates that there are only two areas in or near the Basin where depth to water is likely shallow enough to be within the root zone of vegetation or possibly discharge into stream channels or wetlands (Figure 4-20). One of the areas is the 2-mile bedrock reach of Temescal Wash between the Bedford-Coldwater Subbasin and Basin, and the other is the Prado Wetlands, where contouring suggests groundwater discharges into the wetlands. Depth to water in spring of 2017 was less than 20 feet downstream of about North Lincoln Avenue.” The spring 2017 depth to water data are the only data discussed when referring to depth to water. However, using seasonal groundwater elevation data over multiple water year types is an essential component of identifying ISWs. The use of data from one point in time does not reflect the temporal (seasonal and interannual) variability inherent in California’s climate.

On the map of stream reaches in the basin (Figure 4.17 Regional Surface Water Features), the reaches are not labeled as interconnected and disconnected, nor are areas with data gaps noted.

Therefore, potential ISWs are not being identified, described, nor managed in the GSP. Until a disconnection can be proven, include all potential ISWs in the GSP. This is necessary to assess whether surface water depletions caused by groundwater use are having an adverse impact on environmental beneficial users of surface water.

RECOMMENDATIONS
● Provide a map showing all the stream reaches in the basin, with reaches clearly labeled as interconnected (gaining/losing) or disconnected. Consider any segments with data gaps as potential ISWs and clearly mark them as such on maps provided in the GSP.
● Use seasonal data over multiple water year types to capture the variability in environmental conditions inherent in California’s climate, when mapping ISWs. We recommend the 10-year pre-SGMA baseline period of 2005 to 2015. ● Provide depth-to-groundwater contour maps using the best practices presented in Attachment D, to aid in the determination of ISWs. Specifically, ensure that the first step is contouring groundwater elevations, and then subtracting this layer from land surface elevations from a digital elevation model (DEM) to estimate depth to groundwater contours across the landscape. This will provide accurate contours of depth-to-groundwater along streams and other land surface depressions where GDEs are commonly found.
● Reconcile ISW data gaps with specific measures (shallow monitoring wells, stream gauges, and nested/clustered wells) along surface water features in the Monitoring Network section of the GSP.

Groundwater Dependent Ecosystems
The identification of Groundwater Dependent Ecosystems (GDEs) is incomplete. The GSP took initial steps to identify and map GDEs using the Natural Communities Commonly Associated with Groundwater dataset (NC dataset). However, the GDE section of the GSP could be improved by more clearly describing and mapping the basin’s GDEs to show the data sources and areas of data gaps. Figure 4-21(Critical Habitat Areas) shows a map layer called “NCCAG riparian vegetation,” however based on the description in the text, it is not clear if this is the entire NC dataset or if any screening criteria were used to modify the mapped potential GDEs. The GSP Temescal Basin Draft GSP Page 4 of 13

text (p. 4-17) discusses the corridor of dense riparian trees and shrubs along the bedrock reach of Temescal Wash between the Bedford-Coldwater Subbasin and the Temescal Basin, but does not explicitly state the data source (i.e., field verification) or whether this vegetation is included in the set of potential GDEs. Data gaps are described in the text, but the areas of data gaps are not clearly labeled on the map.

The GSP discusses trends in groundwater elevations over the period 2010 to 2020 and plots a limited set of hydrographs over this period in Figure 4-23. However, the only depth to groundwater contours show are from Spring 2017. The GSP could be improved by mapping depth to groundwater contours over multiple years and seasons to illustrate the temporal (seasonal and interannual) variability inherent in California’s climate.

RECOMMENDATIONS
● Provide a comprehensive set of maps for the basin’s GDEs. For example, provide a map of the NC Dataset. On the map, label polygons retained, removed, or added to/from the NC dataset (include the removal reason if polygons are not considered potential GDEs, or include the data source if polygons are added). Discuss how local groundwater data was used to verify whether polygons in the NC Dataset are supported by groundwater in an aquifer. Refer to Attachment D of this letter for best practices for using local groundwater data to verify whether polygons in the NC Dataset are supported by groundwater in an aquifer.
● Provide depth-to-groundwater contour maps from multiple seasons and water year types (e.g., wet, dry, average, drought), noting the best practices presented in Attachment D. Specifically, ensure that the first step is contouring groundwater elevations, and then subtracting this layer from land surface elevations from a DEM to estimate depth-to-groundwater contours across the landscape. We recommend that a baseline period (10 years from 2005 to 2015) be established to characterize groundwater conditions over multiple water year types.
● If insufficient data are available to describe groundwater conditions within or near polygons from the NC dataset, include those polygons as “Potential GDEs” in the GSP until data gaps are reconciled in the monitoring network.

Native Vegetation and Managed Wetlands
Native vegetation and managed wetlands are water use sectors that are required to be included into the water budget.
, The integration of these ecosystems into the water budget is 2 3
insufficient. Appendix I (Temescal Groundwater Sustainability Plan Numerical Groundwater Model Documentation Report) that accompanies the water budget section of the GSP was not included in the published version of the Draft GSP. Without this Appendix of the GSP, which documents the water budgets, we could not evaluate whether the water budget includes the current, historical, and projected demands of native vegetation. Inclusion of the explicit demands for native vegetation is essential so that key environmental uses of groundwater are being accounted for as water supply decisions are made using this budget and considered in project 3 “The water budget shall quantify the following, either through direct measurements or estimates based on data: (3) Outflows from the groundwater system by water use sector, including evapotranspiration, groundwater extraction, groundwater discharge to surface water sources, and subsurface groundwater outflow.” [23 CCR §354.18] 2 “’Water use sector’ refers to categories of water demand based on the general land uses to which the water is applied, including urban, industrial, agricultural, managed wetlands, managed recharge, and native vegetation.” [23 CCR §351(al)]
Temescal Basin Draft GSP Page 5 of 13

and management actions. Managed wetlands are not mentioned in the GSP, so it is not known whether or not they are present in the basin.

RECOMMENDATIONS
● Quantify and present all water use sector demands in the historical, current, and projected water budgets with individual line items for each water use sector, including native vegetation.
● State whether or not there are managed wetlands in the basin. If there are, ensure that their groundwater demands are included as separate line items in the historical, current, and projected water budgets.

B. Engaging Stakeholders
Stakeholder Engagement During GSP Development
Stakeholder engagement during GSP development is insufficient. SGMA’s requirement for public notice and engagement of stakeholders is not fully met by the description in the Outreach and Stakeholder Involvement Communications Plan (Appendix D). 4 The GSP documents targeted outreach to DACs, including distribution of SGMA Fact Sheets through local churches and community centers; Spanish translation of materials and interpretation at events; and meetings with community leaders, community action organizations, and elected officials. However, we note the following deficiencies with the overall stakeholder engagement process:
● The GSA’s Technical Advisory Committee fails to include representation from DACs and environmental stakeholders in the basin.
● Aside from the details of the Technical Advisory Committee, the GSP documents opportunities for public involvement and engagement in general terms. These include communication and engagement through the GSP webpage, outreach materials, communication through social media, websites, and email, and public workshops. The plan lacks specific details of outreach and engagement targeted to environmental stakeholders.
● The plan fails to document the outcome of the outreach and engagement conducted, nor does it document how information obtained from beneficial users was incorporated into the GSP development process.
● The GSP describes plans for Technical Advisory Committee meetings to continue during the implementation phase of the GSP. However, the GSP does not include a detailed plan for continual opportunities for engagement outside of these meetings through the implementation phase of the GSP that is specifically directed to DACs, domestic well owners, and environmental stakeholders within the basin. 4 “A communication section of the Plan shall include a requirement that the GSP identify how it encourages the active involvement of diverse social, cultural, and economic elements of the population within the basin.” [23 CCR §354.10(d)(3)]
Temescal Basin Draft GSP Page 6 of 13

RECOMMENDATIONS
● In the Outreach and Stakeholder Involvement Communications Plan, describe active and targeted outreach to engage all stakeholders throughout the GSP development and implementation phases. Refer to Attachment B for specific recommendations on how to actively engage stakeholders during all phases of the GSP process. ● Utilize DWR’s tribal engagement guidance to comprehensively identify, involve, and address all tribes and tribal interests that may be present in the basin. 5 C. Considering Beneficial Uses and Users When Establishing Sustainable Management Criteria and Analyzing Impacts on Beneficial Uses and Users The consideration of beneficial uses and users when establishing sustainable management criteria (SMC) is insufficient. The consideration of potential impacts on all beneficial users of groundwater in the basin are required when defining undesirable results and establishing minimum thresholds. 6,7,8
Disadvantaged Communities and Drinking Water Users
For chronic lowering of groundwater levels, minimum thresholds are defined at each representative well as historical groundwater low levels. The GSP discounts private domestic wells when establishing SMC, based on the following rationale (6-6): “There are very few active private wells in the Basin (see Section 2.3.2.1). The owners and operators of those wells are known and they have not reported any adverse effects to those wells in the past; None of the existing private well owners report that their wells went dry or were otherwise affected during the recent drought. Because of this, some flexibility exists for purposes of analysis; Responsibility for potential undesirable results to shallow wells is shared between a GSA and a well owner; there is a reasonable expectation that a well owner would construct, maintain, and operate the well to provide its expected yield over the well’s life span, including droughts; As discussed below, MTs are initially set at historical groundwater level lows and then adjusted upward to be protective.” No further details are provided regarding the minimum threshold impacts on domestic wells. The GSP does not sufficiently describe whether minimum thresholds will avoid significant and unreasonable loss of drinking water to domestic well users that are not protected by the minimum threshold. In addition, the GSP does not sufficiently describe or analyze direct or indirect impacts on DACs or drinking water users when defining undesirable results, nor does it describe how the groundwater levels minimum thresholds are consistent with Human Right to Water policy. 9 9 California Water Code §106.3. Available at:
https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?lawCode=WAT&sectionNum=106.3 8 “The description of minimum thresholds shall include [...] how state, federal, or local standards relate to the relevant sustainability indicator. If the minimum threshold differs from other regulatory standards, the agency shall explain the nature of and the basis for the difference.” [23 CCR §354.28(b)(5)] 7 “The description of minimum thresholds shall include [...] how minimum thresholds may affect the interests of beneficial uses and users of groundwater or land uses and property interests.” [23 CCR §354.28(b)(4)] 6 “The description of undesirable results shall include [...] potential effects on the beneficial uses and users of groundwater, on land uses and property interests, and other potential effects that may occur or are occurring from undesirable results.” [23 CCR §354.26(b)(3)]
5 Engagement with Tribal Governments Guidance Document. Available at:
https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Groundwater-Management/Sustainable-Groundwat er-Management/Best-Management-Practices-and-Guidance-Documents/Files/Guidance-Doc-for-SGM-Engagementwith-Tribal-Govt_ay_19.pdf Temescal Basin Draft GSP Page 7 of 13

For degraded water quality, constituents of concern (COCs) are total dissolved solids (TDS) and nitrate. The minimum threshold for nitrate is defined as the percentage of wells with concentrations exceeding the nitrate MCL (45 mg/L) based on current conditions (2015-2019), which is 50% of wells. The minimum threshold for TDS is defined as the percentage of wells with concentrations exceeding the TDS value of 1,000 mg/L based on current conditions (2015-2019), which is 26 percent of wells. However, according to the state’s anti-degradation policy, water 10 quality should be protected and is only allowed to worsen if a finding is made that it is in the best interest of the people of the State of California. No analysis has been done and no such finding has been made.

The GSP states (p. 6-25): “Other constituents have been documented (see Groundwater Conditions Section 4.8) but occurrences of these are either under regulation by RWQCB (e.g., perchlorate) or are naturally occurring with no recent exceedances of MCLs and limited potential for mobilization due to management actions (e.g., arsenic, chromium, iron, and manganese).” However, all COCs in the basin that may be impacted or exacerbated by groundwater use and/or management should be included in the SMC, in addition to coordinating with water quality regulatory programs.

RECOMMENDATIONS
Chronic Lowering of Groundwater Levels
● Describe direct and indirect impacts on drinking water users and DACs when describing undesirable results and defining minimum thresholds for chronic lowering of groundwater levels.
● Consider and evaluate the impacts of selected minimum thresholds and measurable objectives on drinking water users and DACs within the basin. Further describe the impact of passing the minimum threshold for these users. For example, provide the number of domestic wells that would be fully or partially de-watered at the minimum threshold.

Degraded Water Quality
● Describe direct and indirect impacts on drinking water users and DACs when defining undesirable results for degraded water quality. For specific guidance on how to 11 consider these users, refer to “Guide to Protecting Water Quality Under the Sustainable Groundwater Management Act.”
12 ● Evaluate the cumulative or indirect impacts of proposed minimum thresholds for degraded water quality on drinking water users and DACs. 12 Guide to Protecting Water Quality under the Sustainable Groundwater Management Act https://d3n8a8pro7vhmx.cloudfront.net/communitywatercenter/pages/293/attachments/original/1559328858/Guide_to _Protecting_Drinking_Water_Quality_Under_the_Sustainable_Groundwater_Management_Act.pdf?1559328858. 11 “Degraded Water Quality [...] collect sufficient spatial and temporal data from each applicable principal aquifer to determine groundwater quality trends for water quality indicators, as determined by the Agency, to address known water quality issues.” [23 CCR §354.34(c)(4)]
10 Anti-degradation Policy
https://www.waterboards.ca.gov/board_decisions/adopted_orders/resolutions/1968/rs68_016.pdf Temescal Basin Draft GSP Page 8 of 13

● Set minimum thresholds and measurable objectives for all water quality constituents within the basin that can be impacted and/or exacerbated as a result of groundwater use or groundwater management.
● Set minimum thresholds that do not allow water quality to degrade to levels at or above the MCL trigger level.

Groundwater Dependent Ecosystems and Interconnected Surface Waters Sustainable management criteria for chronic lowering of groundwater levels provided in the GSP do not consider potential impacts to environmental beneficial users. The GSP neither describes nor analyzes direct or indirect impacts on environmental users of groundwater when defining undesirable results. This is problematic because without identifying potential impacts on GDEs, minimum thresholds may compromise, or even destroy, these environmental beneficial users.

Since GDEs are present in the basin, they must be considered when developing SMC.

For depletion of interconnected surface waters, SMC are only established for the Prado Wetlands area. Our comments above in the ISW section of this letter note that interconnected surface waters have not been sufficiently identified and mapped in the basin. Therefore, SMC for depletion of interconnected surface waters may disregard some of the ISWs in the basin.

For the Prado Wetlands area, SMC are established as follows (p. 6-34): “The Minimum Threshold for depletion of interconnected surface water is the amount of depletion that occurs when the depth to the water along the southern edge of the Prado Wetlands is greater than 15 feet for a period exceeding one year. This threshold corresponds approximately to the maximum depth to water measured in shallow monitoring wells in the northern part of the Prado Wetlands.” However, if minimum thresholds are set to historic low groundwater levels and the basin is allowed to operate at or close to those levels over many years, there is a risk of causing catastrophic damage to ecosystems that are more adverse than what was occurring at the height of the 2012-2016 drought. This is because California ecosystems, which are adapted to our Mediterranean climate, have some drought strategies that they can utilize to deal with short-term water stress. However, if the drought conditions are prolonged, the ecosystem can collapse. No analysis or discussion is presented to describe how the SMC will affect beneficial users, and more specifically GDEs, or the impact of these minimum thresholds on GDEs in the basin.

Furthermore, the GSP makes no attempt to evaluate how the proposed minimum thresholds and measurable objectives avoid significant and unreasonable effects on surface water beneficial users in the basin (see Attachment C for a list of environmental users in the basin), such as increased mortality and inability to perform key life processes (e.g., reproduction, migration).

RECOMMENDATIONS
● Evaluate impacts on GDEs when establishing SMC for chronic lowering of groundwater levels. When defining undesirable results, provide specifics on what biological responses (e.g., extent of habitat, growth, recruitment rates) would best characterize a significant and unreasonable impact to GDEs. Undesirable results to environmental users occur when ‘significant and unreasonable’ effects on beneficial users are caused by one of the sustainability indicators (i.e., chronic lowering of groundwater levels, degraded water quality, or depletion of interconnected surface water). Thus, potential impacts on environmental beneficial uses and users need to be Temescal Basin Draft GSP Page 9 of 13

considered when defining undesirable results in the basin. Defining undesirable 13 results is the crucial first step before the minimum thresholds can be determined. 14 ● Re-evaluate the extent of ISWs in the basin. When defining undesirable results for depletion of interconnected surface water, include a description of potential impacts on instream habitats within ISWs when minimum thresholds in the basin are reached. The GSP should confirm that minimum thresholds for ISWs avoid adverse 15 impacts on environmental beneficial users of interconnected surface waters as these environmental users could be left unprotected by the GSP. These recommendations apply especially to environmental beneficial users that are already protected under pre-existing state or federal law.
6,16
● When establishing SMC for the basin, consider that the SGMA statute [Water Code §10727.4(l)] specifically calls out that GSPs shall include “impacts on groundwater dependent ecosystems.”

2. Climate Change
The SGMA statute identifies climate change as a significant threat to groundwater resources and one that must be examined and incorporated in the GSPs. The GSP Regulations require integration of climate change into the projected water budget to ensure that projects and management actions sufficiently account for the range of potential climate futures. The effects of climate change will intensify the impacts 17 of water stress on GDEs, making available shallow groundwater resources especially critical to their survival. Condon et al. (2020) shows that GDEs are more likely to succumb to water stress and rely more on groundwater during times of drought. When shallow groundwater is unavailable, riparian forests can 18 die off and key life processes (e.g., migration and spawning) for aquatic organisms, such as steelhead, can be impeded.

The integration of climate change into the projected water budget is insufficient. The GSP does incorporate climate change into the projected water budget using DWR change factors for 2070.

However, the plan does not consider multiple climate scenarios (e.g., the 2070 extremely wet and extremely dry climate scenarios) in the projected water budget. The GSP would benefit from clearly and transparently incorporating the extremely wet and dry scenarios provided by DWR into projected water budgets or select more appropriate extreme scenarios for the basin. While these extreme scenarios may 18 Condon et al. 2020. Evapotranspiration depletes groundwater under warming over the contiguous United States.

Nature Communications. Available at: https://www.nature.com/articles/s41467-020-14688-0 17 “Each Plan shall rely on the best available information and best available science to quantify the water budget for the basin in order to provide an understanding of historical and projected hydrology, water demand, water supply, land use, population, climate change, sea level rise, groundwater and surface water interaction, and subsurface groundwater flow.” [23 CCR §354.18(e)]
16 Rohde MM, Seapy B, Rogers R, Castañeda X, editors. 2019. Critical Species LookBook: A compendium of California’s threatened and endangered species for sustainable groundwater management. The Nature Conservancy, San Francisco, California. Available at:
https://groundwaterresourcehub.org/public/uploads/pdfs/Critical_Species_LookBook_91819.pdf 15 “The minimum threshold for depletions of interconnected surface water shall be the rate or volume of surface water depletions caused by groundwater use that has adverse impacts on beneficial uses of the surface water and may lead to undesirable results.” [23 CCR §354.28(c)(6)]
14 The description of minimum thresholds shall include [...] how minimum thresholds may affect the interests of beneficial uses and users of groundwater or land uses and property interests.” [23 CCR §354.28(b)(4)] 13 “The description of undesirable results shall include [...] potential effects on the beneficial uses and users of groundwater, on land uses and property interests, and other potential effects that may occur or are occurring from undesirable results”. [23 CCR §354.26(b)(3)]
Temescal Basin Draft GSP Page 10 of 13

have a lower likelihood of occurring, their consequences could be significant and their inclusion can help identify important vulnerabilities in the basin's approach to groundwater management.

The GSP appears to integrate climate change into key inputs (e.g., changes in precipitation and evapotranspiration) of the rainfall-runoff-recharge model. However, this could not be confirmed since the details of the described rainfall-runoff-recharge model included in Appendix I were not included for review in the Draft GSP. Furthermore, water is imported into the basin, but these inputs are not quantified and included in the surface water flow volumes of the water budget tables and it is unclear if these inputs are adjusted for climate change.

The sustainable yield is calculated based on the projected water budget with climate change incorporated. However, if the water budgets are incomplete, including the omission of extreme climate scenarios as well as the omission of projected climate change effects on key inputs (e.g., precipitation, evapotranspiration, imported water flows), then there is increased uncertainty in virtually every subsequent calculation used to plan for projects, derive measurable objectives, and set minimum thresholds. Plans that do not adequately include climate change projections may underestimate future impacts on vulnerable beneficial users of groundwater such as ecosystems, DACs, and domestic well owners.

RECOMMENDATIONS
● Ensure that Appendix I, including a description of the rainfall-runoff-recharge model, is included in the GSP.
● Integrate climate change, including extreme climate scenarios, into all elements of the projected water budget to form the basis for development of sustainable management criteria and projects and management actions.
● Integrate climate change into precipitation and evapotranspiration inputs and include the values in the projected water budget tables.
● Integrate climate change into surface water flow inputs, including imported water, for the projected water budget.
● Incorporate climate change scenarios into projects and management actions.

3. Data Gaps
The consideration of beneficial users when establishing monitoring networks is insufficient, due to lack of specific plans to increase the Representative Monitoring Wells (RMWs) in the monitoring network that represent water quality conditions and shallow groundwater elevations around domestic wells, GDEs, and ISWs in the basin. These beneficial users may remain unprotected by the GSP without adequate monitoring and identification of data gaps in the shallow aquifer. The Plan therefore fails to meet SGMA’s requirements for the monitoring network.
19 Figure 7-1 (Groundwater Level Monitoring Wells) shows insufficient representation of GDEs and drinking water users for groundwater elevation monitoring. Figure 7-2 (Water Quality Monitoring Wells) shows 19 “The monitoring network objectives shall be implemented to accomplish the following: [...] (2) Monitor impacts to the beneficial uses or users of groundwater.” [23 CCR §354.34(b)(2)] Temescal Basin Draft GSP Page 11 of 13

insufficient representation of drinking water users for water quality monitoring. Refer to Attachment E for maps of these monitoring sites in relation to key beneficial users of groundwater.

The GSP includes plans to install three shallow monitoring wells near the Prado Wetlands to monitor GDEs in this area. However, our comments above note that since this is the only area of the basin where SMC to protect ecosystems have been established, the GSP disregards other areas of the basin where GDEs and ISW may exist. Additional monitoring may be needed to adequately assess the presence of GDEs and ISWs and to monitor the impact of SMC on these ecosystems.

RECOMMENDATIONS
● Provide maps that overlay current and proposed monitoring well locations with the locations of DACs, domestic wells, and GDEs to clearly identify monitored areas. ● Increase the number of RMWs in the shallow aquifer across the basin as needed to map ISWs and adequately monitor all groundwater condition indicators across the basin and at appropriate depths for all beneficial users. Prioritize proximity to DACs, domestic wells, GDEs, and ISWs when identifying new RMWs. ● Ensure groundwater elevation and water quality RMWs are monitoring groundwater conditions spatially and at the correct depth for all beneficial users - especially DACs, domestic wells, and GDEs.
● Further describe biological monitoring that can be used to assess the potential for significant and unreasonable impacts to GDEs or ISWs due to groundwater conditions in the basin.

4. Addressing Beneficial Users in Projects and Management Actions The consideration of beneficial users when developing projects and management actions is insufficient, due to the failure to completely identify benefits or impacts of identified projects and management actions, including water quality impacts, to key beneficial users of groundwater such as GDEs, aquatic habitats, surface water users, DACs, and drinking water users. Therefore, potential project and management actions may not protect these beneficial users. Groundwater sustainability under SGMA is defined not just by sustainable yield, but by the avoidance of undesirable results for all beneficial users.

RECOMMENDATIONS
● For DACs and domestic well owners, include a drinking water well impact mitigation program to proactively monitor and protect drinking water wells through GSP implementation. Refer to Attachment B for specific recommendations on how to implement a drinking water well mitigation program.
● For DACs and domestic well owners, include a discussion of whether potential impacts to water quality from projects and management actions could occur and how the GSA plans to mitigate such impacts.

Temescal Basin Draft GSP Page 12 of 13

● Recharge ponds, reservoirs, and facilities for managed aquifer recharge can be designed as multiple-benefit projects to include elements that act functionally as wetlands and provide a benefit for wildlife and aquatic species. For guidance on how to integrate multi-benefit recharge projects into your GSP, refer to the “Multi-Benefit Recharge Project Methodology Guidance Document.”
20 ● Develop management actions that incorporate climate and water delivery uncertainties to address future water demand and prevent future undesirable results. 20 The Nature Conservancy. 2021. Multi-Benefit Recharge Project Methodology for Inclusion in Groundwater Sustainability Plans. Sacramento. Available at:
https://groundwaterresourcehub.org/sgma-tools/multi-benefit-recharge-project-methodology-guidance/ Temescal Basin Draft GSP Page 13 of 13

Attachment B
SGMA Tools to address DAC, drinking water, and
environmental beneficial uses and users
Stakeholder Engagement and Outreach
Clean Water Action, Community Water Center and Union of Concerned Scientists developed a guidance document
called Collaborating for success: Stakeholder engagement for Sustainable Groundwater Management Act
Implementation. It provides details on how to conduct
targeted and broad outreach and engagement during
Groundwater Sustainability Plan (GSP) development and
implementation. Conducting a targeted outreach involves:
• Developing a robust Stakeholder Communication and Engagement plan that includes outreach at frequented locations (schools, farmers markets, religious settings, events) across the plan area to increase the involvement and participation of disadvantaged communities, drinking water users and the environmental stakeholders. • Providing translation services during meetings and technical assistance to enable easy participation for non-English speaking stakeholders.
• GSP should adequately describe the process for requesting input from beneficial users and provide details on how input is incorporated into the GSP.

The Human Right to Water
The Human Right to Water Scorecard was developed
by Community Water Center, Leadership Counsel for
Justice and Accountability and Self Help Enterprises to aid Groundwater Sustainability Agencies (GSAs) in
prioritizing drinking water needs in SGMA. The
scorecard identifies elements that must exist in GSPs
to adequately protect the Human Right to Drinking
water.

Drinking Water Well Impact Mitigation Framework
The Drinking Water Well Impact Mitigation
Framework was developed by Community Water
Center, Leadership Counsel for Justice and
Accountability and Self Help Enterprises to aid
GSAs in the development and implementation of
their GSPs. The framework provides a clear
roadmap for how a GSA can best structure its
data gathering, monitoring network and
management actions to proactively monitor and
protect drinking water wells and mitigate impacts
should they occur.

Groundwater Resource Hub
The Nature Conservancy has
developed a suite of tools based on
best available science to help GSAs,
consultants, and stakeholders
efficiently incorporate nature into
GSPs. These tools and resources are
available online at
GroundwaterResourceHub.org. The
Nature Conservancy’s tools and
resources are intended to reduce
costs, shorten timelines, and increase
benefits for both people and nature.

Rooting Depth Database
The Plant Rooting Depth Database provides information that can help assess whether groundwater-dependent vegetation are accessing groundwater. Actual rooting depths will depend on the plant species and site-specific conditions, such as soil type and

availability of other water sources. Site-specific knowledge of depth to groundwater combined with rooting depths will help provide an understanding of the potential groundwater levels are needed to sustain GDEs.

How to use the database
The maximum rooting depth information in the Plant Rooting Depth Database is useful when verifying whether vegetation in the Natural Communities Commonly Associated with Groundwater (NC Dataset) are connected to groundwater. A 30 ft depth-togroundwater threshold, which is based on averaged global rooting depth data for phreatophytes1, is relevant for most plants identified in the NC Dataset since most plants have a max rooting depth of less than 30 feet. However, it is important to note that deeper thresholds are necessary for other plants that have reported maximum root depths that exceed the averaged 30 feet threshold, such as valley oak (Quercus lobata), Euphrates poplar (Populus euphratica), salt cedar (Tamarix spp.), and shadescale (Atriplex confertifolia). The Nature Conservancy advises that the reported max rooting depth for these deeper-rooted plants be used. For example, a depth-to groundwater threshold of 80 feet should be used instead of the 30 ft threshold, when verifying whether valley oak polygons from the NC Dataset are connected to groundwater. It is important to re-emphasize that actual rooting depth data are limited and will depend on the plant species and site-specific conditions such as soil and aquifer types, and availability to other water sources.

The Plant Rooting Depth Database is an Excel workbook composed of four worksheets:

1. California phreatophyte rooting depth data (included in the NC Dataset)

2. Global phreatophyte rooting depth data

3. Metadata

4. References
How the database was compiled
The Plant Rooting Depth Database is a compilation of rooting depth information for the groundwater-dependent plant species identified in the NC Dataset. Rooting depth data were compiled from published scientific literature and expert opinion through a crowdsourcing campaign. As more information becomes available, the database of rooting depths will be updated. Please Contact Us if you have additional rooting depth data for California phreatophytes.
1 Canadell, J., Jackson, R.B., Ehleringer, J.B. et al. 1996. Maximum rooting depth of vegetation types at the global scale. Oecologia 108, 583–595. https://doi.org/10.1007/BF00329030

GDE Pulse
GDE Pulse is a free online tool that allows Groundwater Sustainability Agencies to assess changes in groundwater dependent ecosystem (GDE) health using satellite, rainfall, and groundwater data. Remote sensing data from satellites has been used to monitor the health of vegetation all over the planet. GDE pulse has compiled 35 years of satellite imagery from NASA’s Landsat mission for every polygon in the Natural Communities Commonly Associated with Groundwater Dataset. The following datasets are available for downloading:

Normalized Difference Vegetation Index (NDVI) is a satellite-derived index that represents the greenness of vegetation. Healthy green vegetation tends to have a higher NDVI, while dead leaves have a lower NDVI. We calculated the average NDVI during the driest part of the year (July - Sept) to estimate vegetation health when the plants are most likely dependent on groundwater.

Normalized Difference Moisture Index (NDMI) is a satellite-derived index that represents water content in vegetation. NDMI is derived from the Near-Infrared (NIR) and Short-Wave Infrared (SWIR) channels. Vegetation with adequate access to water tends to have higher NDMI, while vegetation that is water stressed tends to have lower NDMI. We calculated the average NDVI during the driest part of the year (July– September) to estimate vegetation health when the plants are most likely dependent on groundwater.

Annual Precipitation is the total precipitation for the water year (October 1st – September 30th) from the PRISM dataset. The amount of local precipitation can affect vegetation with more precipitation generally leading to higher NDVI and NDMI.

Depth to Groundwater measurements provide an indication of the groundwater levels and changes over time for the surrounding area. We used groundwater well measurements from nearby (<1km) wells to estimate the depth to groundwater below the GDE based on the average elevation of the GDE (using a digital elevation model) minus the measured groundwater surface elevation.

ICONOS Mapper
Interconnected Surface Water in the Central Valley
ICONS maps the likely presence of interconnected surface water (ISW) in the Central Valley using depth to groundwater data. Using data from 2011-2018, the ISW dataset represents the likely connection between surface water and groundwater for rivers and streams in California’s Central Valley. It includes information on the mean, maximum, and minimum depth to groundwater for each stream segment over the years with available data, as well as the likely presence of ISW based on the minimum depth to groundwater. The Nature Conservancy developed this database, with guidance and input from expert academics, consultants, and state agencies.

We developed this dataset using groundwater elevation data available online from the California Department of Water Resources (DWR). DWR only provides this data for the Central Valley. For GSAs outside of the valley, who have groundwater well measurements, we recommend following our methods to determine likely ISW in your region. The Nature Conservancy’s ISW dataset should be used as a first step in reviewing ISW and should be supplemented with local or more recent groundwater depth data.

Attachment C
Freshwater Species Located in the Temescal Basin
To assist in identifying the beneficial users of surface water necessary to assess the undesirable result “depletion of interconnected surface waters”, Attachment C provides a list of freshwater species located in the Temescal Basin. To produce the freshwater species list, we used ArcGIS to select features within the California Freshwater Species Database version 2.0.9 within the basin boundary. This database contains information on ~4,000 vertebrates, macroinvertebrates and vascular plants that depend on fresh water for at least one stage of their life cycle. The methods used to compile the California Freshwater Species Database can be found in Howard et al. 20151. The spatial database contains locality observations and/or distribution information from ~400 data sources. The database is housed in the California Department of Fish and Wildlife’s BIOS2 as well as on The Nature Conservancy’s science website3.

Scientific Name Common Name
Legal Protected Status
Federal State Other
BIRDS
Aechmophorus clarkii Clark's Grebe
Aix sponsa Wood Duck
Anas acuta Northern Pintail
Anas americana American Wigeon
Anas clypeata Northern Shoveler
Anas crecca Green-winged Teal
Anas cyanoptera Cinnamon Teal
Anas platyrhynchos Mallard
Anas strepera Gadwall
Ardea alba Great Egret
Ardea herodias Great Blue Heron
Aythya affinis Lesser Scaup
Aythya americana Redhead Special
Concern
BSSC -
Third priority
Bucephala albeola Bufflehead
Bucephala clangula Common Goldeneye
Butorides virescens Green Heron
Calidris mauri Western Sandpiper
Calidris minutilla Least Sandpiper
Chroicocephalus philadelphia Bonaparte's Gull
Cistothorus palustris palustris Marsh Wren
1 Howard, J.K. et al. 2015. Patterns of Freshwater Species Richness, Endemism, and Vulnerability in California.

PLoSONE, 11(7). Available at: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0130710 2 California Department of Fish and Wildlife BIOS: https://www.wildlife.ca.gov/data/BIOS 3 Science for Conservation: https://www.scienceforconservation.org/products/california-freshwater-speciesdatabase

Coccyzus americanus occidentalis Western Yellow-billed
Cuckoo
Candidate -
Threatened Endangered
Egretta thula Snowy Egret
Fulica americana American Coot
Gallinago delicata Wilson's Snipe
Haliaeetus leucocephalus Bald Eagle
Bird of
Conservation
Concern
Endangered
Himantopus mexicanus Black-necked Stilt
Icteria virens Yellow-breasted Chat Special
Concern
BSSC -
Third priority
Limnodromus scolopaceus Long-billed Dowitcher
Megaceryle alcyon Belted Kingfisher
Nycticorax nycticorax Black-crowned NightHeron
Oxyura jamaicensis Ruddy Duck
Phalacrocorax auritus Double-crested
Cormorant
Plegadis chihi White-faced Ibis Watch list
Podiceps nigricollis Eared Grebe
Recurvirostra americana American Avocet
Setophaga petechia Yellow Warbler
BSSC -
Second
priority
Tachycineta bicolor Tree Swallow
Vireo bellii Bell's Vireo
Vireo bellii pusillus Least Bell's Vireo Endangered Endangered

CRUSTACEANS
Hyalella spp. Hyalella spp.

FISH
Catostomus santaanae Santa Ana sucker Threatened Special Concern
Endangered
- Moyle 2013
HERPS
Actinemys marmorata marmorata Western Pond Turtle Special Concern ARSSC
Anaxyrus boreas boreas Boreal Toad
Anaxyrus californicus Arroyo Toad Endangered Special
Concern ARSSC
Pseudacris cadaverina California Treefrog ARSSC
Rana draytonii California Red-legged
Frog Threatened Special Concern ARSSC
Spea hammondii Western Spadefoot
Under
Review in the
Candidate or
Petition
Process
Special
Concern ARSSC
Taricha torosa Coast Range Newt Special
Concern ARSSC

Thamnophis hammondii hammondii Two-striped
Gartersnake
Special
Concern ARSSC
Thamnophis sirtalis sirtalis Common Gartersnake

INSECTS & OTHER INVERTS
Apedilum spp. Apedilum spp.

Chaoboridae fam. Chaoboridae fam.

Chironomus spp. Chironomus spp.

Corixidae fam. Corixidae fam.

Cricotopus spp. Cricotopus spp.

Cryptochironomus spp. Cryptochironomus spp.

Dicrotendipes spp. Dicrotendipes spp.

Ephydridae fam. Ephydridae fam.

Fallceon spp. Fallceon spp.

Hydroptila spp. Hydroptila spp.

Hydroptilidae fam. Hydroptilidae fam.

Nanocladius spp. Nanocladius spp.

Phaenopsectra spp. Phaenopsectra spp.

Polypedilum spp. Polypedilum spp.

Pseudochironomus spp. Pseudochironomus
spp.

Psychodidae fam. Psychodidae fam.

Simulium spp. Simulium spp.

Thienemannimyia spp. Thienemannimyia spp.

MOLLUSKS
Gyraulus spp. Gyraulus spp.

Physa spp. Physa spp.

Pisidium spp. Pisidium spp.

PLANTS
Arundo donax NA
Baccharis salicina Not on any
status lists
Marsilea vestita vestita NA Not on any
status lists

1 July 2019
IDENTIFYING GDEs UNDER SGMA
Best Practices for using the NC Dataset
The Sustainable Groundwater Management Act (SGMA) requires that groundwater dependent ecosystems (GDEs) be identified in Groundwater Sustainability Plans (GSPs). As a starting point, the Department of Water Resources (DWR) is providing the Natural Communities Commonly Associated with Groundwater Dataset (NC Dataset) online1 to help Groundwater Sustainability Agencies (GSAs), consultants, and stakeholders identify GDEs within individual groundwater basins. To apply information from the NC Dataset to local areas, GSAs should combine it with the best available science on local hydrology, geology, and groundwater levels to verify whether polygons in the NC dataset are likely supported by groundwater in an aquifer (Figure 1)2. This document highlights six best practices for using local groundwater data to confirm whether mapped features in the NC dataset are supported by groundwater.
1 NC Dataset Online Viewer: https://gis.water.ca.gov/app/NCDatasetViewer/ 2 California Department of Water Resources (DWR). 2018. Summary of the “Natural Communities Commonly Associated with Groundwater” Dataset and Online Web Viewer. Available at: https://water.ca.gov/-/media/DWR-Website/WebPages/Programs/Groundwater-Management/Data-and-Tools/Files/Statewide-Reports/Natural-Communities-DatasetSummary-Document.pdf Figure 1. Considerations for GDE identification.

Source: DWR2
Attachment D

2 The NC Dataset identifies vegetation and wetland features that are good indicators of a GDE. The dataset is comprised of 48 publicly available state and federal datasets that map vegetation, wetlands, springs, and seeps commonly associated with groundwater in California3. It was developed through a collaboration between DWR, the Department of Fish and Wildlife, and The Nature Conservancy (TNC).

TNC has also provided detailed guidance on identifying GDEs from the NC dataset4 on the Groundwater Resource Hub5, a website dedicated to GDEs.

BEST PRACTICE #1. Establishing a Connection to Groundwater Groundwater basins can be comprised of one continuous aquifer (Figure 2a) or multiple aquifers stacked on top of each other (Figure 2b). In unconfined aquifers (Figure 2a), using the depth-to-groundwater and the rooting depth of the vegetation is a reasonable method to infer groundwater dependence for GDEs. If groundwater is well below the rooting (and capillary) zone of the plants and any wetland features, the ecosystem is considered disconnected and groundwater management is not likely to affect the ecosystem (Figure 2d). However, it is important to consider local conditions (e.g., soil type, groundwater flow gradients, and aquifer parameters) and to review groundwater depth data from multiple seasons and water year types (wet and dry) because intermittent periods of high groundwater levels can replenish perched clay lenses that serve as the water source for GDEs (Figure 2c). Maintaining these natural groundwater fluctuations are important to sustaining GDE health.

Basins with a stacked series of aquifers (Figure 2b) may have varying levels of pumping across aquifers in the basin, depending on the production capacity or water quality associated with each aquifer. If pumping is concentrated in deeper aquifers, SGMA still requires GSAs to sustainably manage groundwater resources in shallow aquifers, such as perched aquifers, that support springs, surface water, domestic wells, and GDEs (Figure 2). This is because vertical groundwater gradients across aquifers may result in pumping from deeper aquifers to cause adverse impacts onto beneficial users reliant on shallow aquifers or interconnected surface water. The goal of SGMA is to sustainably manage groundwater resources for current and future social, economic, and environmental benefits. While groundwater pumping may not be currently occurring in a shallower aquifer, use of this water may become more appealing and economically viable in future years as pumping restrictions are placed on the deeper production aquifers in the basin to meet the sustainable yield and criteria. Thus, identifying GDEs in the basin should done irrespective to the amount of current pumping occurring in a particular aquifer, so that future impacts on GDEs due to new production can be avoided. A good rule of thumb to follow is: if groundwater can be pumped from a well - it’s an aquifer. 3 For more details on the mapping methods, refer to: Klausmeyer, K., J. Howard, T. Keeler-Wolf, K. Davis-Fadtke, R. Hull, A. Lyons. 2018. Mapping Indicators of Groundwater Dependent Ecosystems in California: Methods Report. San Francisco, California. Available at: https://groundwaterresourcehub.org/public/uploads/pdfs/iGDE_data_paper_20180423.pdf 4 “Groundwater Dependent Ecosystems under the Sustainable Groundwater Management Act: Guidance for Preparing Groundwater Sustainability Plans” is available at: https://groundwaterresourcehub.org/gde-tools/gsp-guidance-document/ 5 The Groundwater Resource Hub: www.GroundwaterResourceHub.org

3 Figure 2. Confirming whether an ecosystem is connected to groundwater. Top: (a) Under the ecosystem is an unconfined aquifer with depth-to-groundwater fluctuating seasonally and interannually within 30 feet from land surface. (b) Depth-to-groundwater in the shallow aquifer is connected to overlying ecosystem. Pumping predominately occurs in the confined aquifer, but pumping is possible in the shallow aquifer. Bottom: (c) Depthto-groundwater fluctuations are seasonally and interannually large, however, clay layers in the near surface prolong the ecosystem’s connection to groundwater. (d) Groundwater is disconnected from surface water, and any water in the vadose (unsaturated) zone is due to direct recharge from precipitation and indirect recharge under the surface water feature. These areas are not connected to groundwater and typically support species that do not require access to groundwater to survive.

4 BEST PRACTICE #2. Characterize Seasonal and Interannual Groundwater Conditions SGMA requires GSAs to describe current and historical groundwater conditions when identifying GDEs [23 CCR §354.16(g)]. Relying solely on the SGMA benchmark date (January 1, 2015) or any other single point in time to characterize groundwater conditions (e.g., depth-to-groundwater) is inadequate because managing groundwater conditions with data from one time point fails to capture the seasonal and interannual variability typical of California’s climate. DWR’s Best Management Practices document on water budgets6 recommends using 10 years of water supply and water budget information to describe how historical conditions have impacted the operation of the basin within sustainable yield, implying that a baseline7 could be determined based on data between 2005 and 2015. Using this or a similar time period, depending on data availability, is recommended for determining the depth-to-groundwater.

GDEs depend on groundwater levels being close enough to the land surface to interconnect with surface water systems or plant rooting networks. The most practical approach8 for a GSA to assess whether polygons in the NC dataset are connected to groundwater is to rely on groundwater elevation data. As detailed in TNC’s GDE guidance document4, one of the key factors to consider when mapping GDEs is to contour depth-to-groundwater in the aquifer that is supporting the ecosystem (see Best Practice #5).

Groundwater levels fluctuate over time and space due to California’s Mediterranean climate (dry summers and wet winters), climate change (flood and drought years), and subsurface heterogeneity in the subsurface (Figure 3). Many of California’s GDEs have adapted to dealing with intermittent periods of water stress, however if these groundwater conditions are prolonged, adverse impacts to GDEs can result. While depth-to-groundwater levels within 30 feet4 of the land surface are generally accepted as being a proxy for confirming that polygons in the NC dataset are supported by groundwater, it is highly advised that fluctuations in the groundwater regime be characterized to understand the seasonal and interannual groundwater variability in GDEs. Utilizing groundwater data from one point in time can misrepresent groundwater levels required by GDEs, and inadvertently result in adverse impacts to the GDEs. Time series data on groundwater elevations and depths are available on the SGMA Data Viewer9.

However, if insufficient data are available to describe groundwater conditions within or near polygons from the NC dataset, include those polygons in the GSP until data gaps are reconciled in the monitoring network (see Best Practice #6).

Figure 3. Example seasonality
and interannual variability in
depth-to-groundwater over
time. Selecting one point in time,
such as Spring 2018, to
characterize groundwater
conditions in GDEs fails to capture
what groundwater conditions are
necessary to maintain the
ecosystem status into the future so
adverse impacts are avoided.
6 DWR. 2016. Water Budget Best Management Practice. Available at:
https://water.ca.gov/LegacyFiles/groundwater/sgm/pdfs/BMP_Water_Budget_Final_2016-12-23.pdf 7 Baseline is defined under the GSP regulations as “historic information used to project future conditions for hydrology, water demand, and availability of surface water and to evaluate potential sustainable management practices of a basin.” [23 CCR §351(e)] 8 Groundwater reliance can also be confirmed via stable isotope analysis and geophysical surveys. For more information see The GDE Assessment Toolbox (Appendix IV, GDE Guidance Document for GSPs4). 9 SGMA Data Viewer: https://sgma.water.ca.gov/webgis/?appid=SGMADataViewer

5 BEST PRACTICE #3. Ecosystems Often Rely on Both Groundwater and Surface Water GDEs are plants and animals that rely on groundwater for all or some of its water needs, and thus can be supported by multiple water sources. The presence of non-groundwater sources (e.g., surface water, soil moisture in the vadose zone, applied water, treated wastewater effluent, urban stormwater, irrigated return flow) within and around a GDE does not preclude the possibility that it is supported by groundwater, too. SGMA defines GDEs as "ecological communities and species that depend on groundwater emerging from aquifers or on groundwater occurring near the ground surface" [23 CCR §351(m)]. Hence, depth-to-groundwater data should be used to identify whether NC polygons are supported by groundwater and should be considered GDEs. In addition, SGMA requires that significant and undesirable adverse impacts to beneficial users of surface water be avoided. Beneficial users of surface water include environmental users such as plants or animals10, which therefore must be considered when developing minimum thresholds for depletions of interconnected surface water.

GSAs are only responsible for impacts to GDEs resulting from groundwater conditions in the basin, so if adverse impacts to GDEs result from the diversion of applied water, treated wastewater, or irrigation return flow away from the GDE, then those impacts will be evaluated by other permitting requirements (e.g., CEQA) and may not be the responsibility of the GSA. However, if adverse impacts occur to the GDE due to changing groundwater conditions resulting from pumping or groundwater management activities, then the GSA would be responsible (Figure 4).

Figure 4. Ecosystems often depend on multiple sources of water. Top: (Left) Surface water and groundwater are interconnected, meaning that the GDE is supported by both groundwater and surface water. (Right) Ecosystems that are only reliant on non-groundwater sources are not groundwater-dependent. Bottom: (Left) An ecosystem that was once dependent on an interconnected surface water, but loses access to groundwater solely due to surface water diversions may not be the GSA’s responsibility. (Right) Groundwater dependent ecosystems once dependent on an interconnected surface water system, but loses that access due to groundwater pumping is the GSA’s responsibility.
10 For a list of environmental beneficial users of surface water by basin, visit: https://groundwaterresourcehub.org/gdetools/environmental-surface-water-beneficiaries/

6 BEST PRACTICE #4. Select Representative Groundwater Wells Identifying GDEs in a basin requires that groundwater conditions are characterized to confirm whether polygons in the NC dataset are supported by the underlying aquifer. To do this, proximate groundwater wells should be identified to characterize groundwater conditions (Figure 5). When selecting representative wells, it is particularly important to consider the subsurface heterogeneity around NC polygons, especially near surface water features where groundwater and surface water interactions occur around heterogeneous stratigraphic units or aquitards formed by fluvial deposits. The following selection criteria can help ensure groundwater levels are representative of conditions within the GDE area:
● Choose wells that are within 5 kilometers (3.1 miles) of each NC Dataset polygons because they are more likely to reflect the local conditions relevant to the ecosystem. If there are no wells within 5km of the center of a NC dataset polygon, then there is insufficient information to remove the polygon based on groundwater depth. Instead, it should be retained as a potential GDE until there are sufficient data to determine whether or not the NC Dataset polygon is supported by groundwater.
● Choose wells that are screened within the surficial unconfined aquifer and capable of measuring the true water table.
● Avoid relying on wells that have insufficient information on the screened well depth interval for excluding GDEs because they could be providing data on the wrong aquifer. This type of well data should not be used to remove any NC polygons.

Figure 5. Selecting representative wells to characterize groundwater conditions near GDEs.

7 BEST PRACTICE #5. Contouring Groundwater Elevations
The common practice to contour depth-to-groundwater over a large area by interpolating measurements at monitoring wells is unsuitable for assessing whether an ecosystem is supported by groundwater. This practice causes errors when the land surface contains features like stream and wetland depressions because it assumes the land surface is constant across the landscape and depth-to-groundwater is constant below these low-lying areas (Figure 6a). A more accurate approach is to interpolate groundwater elevations at monitoring wells to get groundwater elevation contours across the landscape. This layer can then be subtracted from land surface elevations from a Digital Elevation Model (DEM)11 to estimate depth-to-groundwater contours across the landscape (Figure b; Figure 7). This will provide a much more accurate contours of depth-to-groundwater along streams and other land surface depressions where GDEs are commonly found.

Figure 6. Contouring depth-to-groundwater around surface water features and GDEs. (a) Groundwater level interpolation using depth-to-groundwater data from monitoring wells. (b) Groundwater level interpolation using groundwater elevation data from monitoring wells and DEM data.

Figure 7. Depth-to-groundwater contours in Northern California. (Left) Contours were interpolated using depth-to-groundwater measurements determined at each well. (Right) Contours were determined by interpolating groundwater elevation measurements at each well and superimposing ground surface elevation from DEM spatial data to generate depth-to-groundwater contours. The image on the right shows a more accurate depth-togroundwater estimate because it takes the local topography and elevation changes into account. 11 USGS Digital Elevation Model data products are described at: https://www.usgs.gov/core-sciencesystems/ngp/3dep/about-3dep-products-services and can be downloaded at: https://iewer.nationalmap.gov/basic/

8 BEST PRACTICE #6. Best Available Science
Adaptive management is embedded within SGMA and provides a process to work toward sustainability over time by beginning with the best available information to make initial decisions, monitoring the results of those decisions, and using the data collected through monitoring programs to revise decisions in the future. In many situations, the hydrologic connection of NC dataset polygons will not initially be clearly understood if site-specific groundwater monitoring data are not available. If sufficient data are not available in time for the 2020/2022 plan, The Nature Conservancy strongly advises that questionable polygons from the NC dataset be included in the GSP until data gaps are reconciled in the monitoring network. Erring on the side of caution will help minimize inadvertent impacts to GDEs as a result of groundwater use and management actions during SGMA implementation.

ABOUT US
The Nature Conservancy is a science-based nonprofit organization whose mission is to conserve the lands and waters on which all life depends. To support successful SGMA implementation that meets the future needs of people, the economy, and the environment, TNC has developed tools and resources (www.groundwaterresourcehub.org) intended to reduce costs, shorten timelines, and increase benefits for both people and nature.

KEY DEFINITIONS
Groundwater basin is an aquifer or stacked series of aquifers with reasonably welldefined boundaries in a lateral direction, based on features that significantly impede groundwater flow, and a definable bottom. 23 CCR §341(g)(1) Groundwater dependent ecosystem (GDE) are ecological communities or species that depend on groundwater emerging from aquifers or on groundwater occurring near the ground surface. 23 CCR §351(m)
Interconnected surface water (ISW) surface water that is hydraulically connected at any point by a continuous saturated zone to the underlying aquifer and the overlying surface water is not completely depleted. 23 CCR §351(o) Principal aquifers are aquifers or aquifer systems that store, transmit, and yield significant or economic quantities of groundwater to wells, springs, or surface water systems. 23 CCR §351(aa)

Attachment E
Maps of representative monitoring sites in
relation to key beneficial users
Figure 1. Groundwater elevation representative monitoring sites in relation to key beneficial users: a) Groundwater Dependent Ecosystems (GDEs), b) Drinking Water users, c) Disadvantaged Communities (DACs), and d) Tribes.

Figure 2. Groundwater quality representative monitoring sites in relation to key beneficial users: a) Groundwater Dependent Ecosystems (GDEs), b) Drinking Water users, c) Disadvantaged Communities (DACs), and d) Tribes.

1 | Page
City of Corona
Utilities Department
“Protecting Public Health”
755 Public Safety Way
Corona, CA 92878 – www.CoronaCA.gov
Office: 951.736.2234
Fax: 951.735.3786
January 13, 2021
To: Ngoro Atume – Clean Water Action/Clean Water Fund
Dr. J. Pablo Ortiz-Partida – Union of Concerned Scientists Samantha Arthur – Audubon California
Danielle V. Dolan – Local Government Commission
E.J. Remson – The Nature Conservancy (TNC)
Melissa M. Rohde – The Nature Conservancy
RE: Public Comment Letter for the Temescal Basin Draft GSP Dated December 4, 2021
The Temescal Groundwater Sustainability Agency (GSA) appreciates your thorough review of our Groundwater Sustainability Plan (GSP). Throughout the process, the Temescal GSA has encouraged and welcomed public input, including the comment letter you submitted on December 14, 2021. We have reviewed your comments.

Detailed responses to your comments, including identification of edits to the GSP, are provided below.

Responses are organized according to the Specific Comments in Attachment A of your December 14, 2021 comment letter, which is attached for reference.

COMMENT 1. Beneficial uses and users are not sufficiently considered in GSP development.

COMMENT 1a. Human Right to Water considerations are not sufficiently incorporated.

Multiple topics were included in this comment; these are presented with responses by topic below.

Comment:

The identification of Disadvantaged Communities (DACs) and drinking water users is insufficient.

Response:

The text in Section 2.8.4 discusses DACs within the Basin and the DACs are delineated in Figure 2-13. This section specifically addresses the fact that the DAC and SDAC areas of the Basin are within the service areas of the City of Corona (Corona) and the Home Gardens County Water District (HGCWD) and that there are no active private wells within these areas.

Comment:

The identification of Interconnected Surface Waters (ISWs) is insufficient, due to lack of supporting information provided for the ISW analysis. The GSP describes the use of aerial photos to analyze stream reaches and presents analysis of stream gage and groundwater elevation data. The spring 2017 depth to water data are the only data discussed when referring

to depth to water. However, using seasonal groundwater elevation data over multiple water year types is an essential component of identifying ISWs. The use of data from one point in time does not reflect the temporal (seasonal and interannual) variability inherent in California’s climate.

On the map of stream reaches in the basin (Figure 4.17 Regional Surface Water Features), the reaches are not labeled as interconnected and disconnected, nor are areas with data gaps noted. Therefore, potential ISWs are not being identified, described, nor managed in the GSP. Until a disconnection can be proven, include all potential ISWs in the GSP. This is necessary to assess whether surface water depletions caused by groundwater use are having an adverse impact on environmental beneficial users of surface water.

Response:

We have modified Figure 4-21 so that all stream reaches are designated “mostly interconnected”, “mostly not interconnected” or “disconnected”. Interconnection can vary over time and over short distances along a stream. The “mostly interconnected” category is one where the water table is shallow enough to allow phreatophytic riparian vegetation to become established and develop tall stature and dense canopy. The “mostly not interconnected” category is assigned to reaches that might have shallow water table conditions or gaining stream flow under wet conditions, but not with enough frequency or duration to develop dense riparian vegetation.
“Disconnected” stream reaches are ones where the water table of the regional groundwater system is almost always 30 or more feet below the stream bed elevation. This can occur where surface flow from tributary watersheds enters the Basin. The regional water table is far below the stream bed at those locations, but persistent base flow in the tributary streams can create enough soil saturation in the stream banks along a short reach of stream to support lush riparian vegetation. Examples include the dense riparian vegetation along Temescal Wash for about 1 mile downstream of the Basin boundary and several NCCAG riparian vegetation polygons along the western margin of the Basin.

The depth to water in the three wells closest to the Temescal Wash vegetation (south of well Corona 13 in Figure 4-1) was historically 50-150 feet below the ground surface. Depth to water increases from the center of the Channel Aquifer area toward the margins of the Basin because the ground slope (0.04-0.13 ft/ft from Figure 3-1) is four or more times steeper than the water table slope (about

0.01 ft/ft from Figures 4-10 and 4-11). Thus, the depth to water at the Temescal Wash riparian vegetation is likely greater than the 50-150 foot range of the three wells farther north. Groundwater elevations at the vegetation location is not considered a data gap because available data indicate that the regional water table could not plausibly be less than 30 ft below the ground surface.

Water level data for the four wells midway along the western margin (see Figure 4-1) are available from the 1930s to 1970s. Depths to water were generally 100- 180 feet, and 185-220 feet in the southernmost of those wells. Water levels could not plausibly have risen 70-150 feet in that region in the past 60 years. Thus, there is no data gap with respect to water table elevation at those NCCAG

polygons. The vegetation is clearly disconnected from the regional groundwater system that is the focus of this GSP.

Three wells near the north end of the western Basin margin (near State Highway 91) also have historical water levels from the 1930s to 1960s, which were generally 85-105 feet below the ground surface. The ground elevation at the wells is roughly the same as the bed elevation at the nearest point along Wardlow Wash. Riparian tree cover is fairly dense along much of the Wash, and the NCCAG mapping indicates that cottonwood (an obligate phreatophyte) and sycamore (a facultative phreatophyte) are the most common tree species.

Intervening hydrogeology—such as the faults shown in Figure 3-6—could result in a higher water table at Wardlow Wash than at the wells, but by the same token that hydrogeology would block the spread of pumping drawdown from the wells to the wash. Wardlow Wash is conservatively classified as “mostly interconnected” in Figure 4-17.

The comment requests additional contour maps of groundwater elevation, apparently on the assumption that seasonal or annual fluctuations in groundwater elevation change the lengths or locations of interconnected stream reaches. The GSP presents contour maps for two dates (fall 2015 in Figure 4-10 and spring 2017 in Figures 4-11 and 4-21), not just one date as implied by the comment. Transient changes in depth to water are also shown in the hydrographs in Figures 4-2 through 4-9, each of which indicates the ground surface elevation at the well. More importantly, long-term variations in spring groundwater elevation have been on the order of 20 feet even in the heavily pumped Channel Aquifer area (Figures 4-2 through 4-9). Variations are likely smaller in the alluvial aquifer area where pumping stresses are much smaller.

That magnitude of variation is not large enough to fundamentally alter the designation of stream interconnection status or riparian vegetation interconnection status.

Comment:

The identification of Groundwater Dependent Ecosystems (GDEs) is incomplete. The GSP took initial steps to identify and map GDEs using the Natural Communities Commonly Associated with Groundwater dataset (NC dataset). However, the GDE section of the GSP could be improved by more clearly describing and mapping the basin’s GDEs to show the data sources and areas of data gaps. Figure 4-21 (Critical Habitat Areas) shows a map layer called “NCCAG riparian vegetation,” however based on the description in the text, it is not clear if this is the entire NC dataset or if any screening criteria were used to modify the mapped potential GDEs. The GSP text (p. 4-17) discusses the corridor of dense riparian trees and shrubs along the bedrock reach of Temescal Wash between the Bedford-Coldwater Subbasin and the Temescal Basin, but does not explicitly state the data source (i.e., field verification) or whether this vegetation is included in the set of potential GDEs. Data gaps are described in the text, but the areas of data gaps are not clearly labeled on the map.

The GSP discusses trends in groundwater elevations over the period 2010 to 2020 and plots a limited set of hydrographs over this period in Figure 4-

23. However, the only depth to groundwater contours show are from Spring

2017. The GSP could be improved by mapping depth to groundwater contours over multiple years and seasons to illustrate the temporal (seasonal and interannual) variability inherent in California’s climate.

Response:

The comment inquired about the source of the vegetation mapping. The area mapped as “dense riparian vegetation” along Temescal Wash was delineated for this GSP by outlining areas of dense riparian vegetation visible in recent highresolution aerial photographs (Google Earth©). The riparian vegetation polygons shown in Figure 4- 21 include all of the NCCAG mapped polygons. The Prado Wetlands boundary is from the Prado Basin Habitat Sustainability Program (WEI 2020). The figure does not show NCCAG wetland polygons. Almost all of those polygons are within or immediately adjacent to NCCAG riparian vegetation polygons, and conclusions regarding depth to groundwater and interconnection are consequently the same. One small polygon is mapped where a canyon enters the Basin near the Main Street stormwater detention basin. That polygon is plausibly supported by groundwater leaking from fractured bedrock in the tributary watershed. That source is not affected by Basin management, and the depth of the regional water table at the wetland location is greater than the few feet required by wetland vegetation, for the reasons explained above. A couple of tiny mapped wetland polygons in the middle of the stormwater detention basin are bare dirt in aerial photographs and were at best transient patches of mesic vegetation that appeared after a storm event.

Comment:

Native vegetation and managed wetlands are water use sectors that are required to be included into the water budget. The integration of these ecosystems into the water budget is insufficient. Appendix I (Temescal Groundwater Sustainability Plan Numerical Groundwater Model Documentation Report) that accompanies the water budget section of the GSP was not included in the published version of the Draft GSP. Without this Appendix of the GSP, which documents the water budgets, we could not evaluate whether the water budget includes the current, historical, and projected demands of native vegetation. Inclusion of the explicit demands for native vegetation is essential so that key environmental uses of groundwater are being accounted for as water supply decisions are made using this budget and considered in project and management actions.

Managed wetlands are not mentioned in the GSP, so it is not known whether or not they are present in the basin.

Response:

The comment stated that the GSP does not discuss managed wetlands. While the exact phrase “management wetlands” was not used, the text clearly indicates that Prado wetlands are managed: “Prado Dam impounds the river to regulate flood flows in winter and sustain a perennial wetland…. Surface water behind the dam is maintained at a specified elevation” (Section 4.10.4). We have inserted the word “managed” in the text for clarity.

COMMENT 1b. Public trust resources are not sufficiently considered. Stakeholder engagement during GSP development is insufficient.

Response:

The Temescal GSA encouraged stakeholder engagement throughout the GSP process. Outreach efforts included website updates, individual phone calls, email, and postal mail. Domestic well owners, environmental stakeholders, and the community at large were invited to participate through these communication mediums, and there was participation in GSP development from private pumpers and other interested parties. Local DAC representatives and environmental stakeholders were contacted early in GSP development and invited to participate, including receiving invitations to attend and participate in Technical Advisory Committee (TAC) meetings. However, these individuals and agencies declined the invitation to be on the TAC and generally did not participate either the well-advertised public workshops or the public TAC meetings. Information regarding targeted outreach to DACs was included in the draft GSP.

The GSP does document the outcome of the outreach and engagement through presentation of notes from all public meetings in Appendices F, G, and H of the draft GSP. Very little feedback was received during these public meetings.

However, all feedback was considered in development of the GSP.

The comment criticizes the plan for ongoing engagement in GSP implementation.

The draft GSP indicated that TAC meetings will continue during GSP implementation. These are public meetings open to all interested parties, including DACs, domestic well owners, and environmental stakeholders. So, it is the opinion of the GSA that continued TAC meetings does constitute a plan for continual opportunities for engagement for all interested parties.

There are no known tribal lands within the Temescal Basin.

COMMENT 1c. Impacts of Minimum Thresholds, Measurable Objectives and Undesirable Results on beneficial uses and users are not sufficiently analyzed.

Multiple topics were included in this comment; these are presented with responses by topic below.

Comment:

For chronic lowering of groundwater levels, minimum thresholds are defined at each representative well as historical groundwater low levels.

The GSP discounts private domestic wells when establishing SMC, based on the following rationale (6-6): “There are very few active private wells in the Basin (see Section 2.3.2.1). The owners and operators of those wells are known and they have not reported any adverse effects to those wells in the past; None of the existing private well owners report that their wells went dry or were otherwise affected during the recent drought. Because of this, some flexibility exists for purposes of analysis; Responsibility for potential undesirable results to shallow wells is shared between a GSA and a well owner; there is a reasonable expectation that a well owner would construct, maintain, and operate the well to provide its expected yield over the well’s life span, including droughts; As discussed below, MTs are initially set at historical groundwater level lows and then adjusted upward to be protective.” No further details are provided regarding the minimum threshold impacts on domestic wells. The GSP does not sufficiently describe whether minimum thresholds will avoid significant and unreasonable loss of drinking water to domestic well users that are not

protected by the minimum threshold. In addition, the GSP does not sufficiently describe or analyze direct or indirect impacts on DACs or drinking water users when defining undesirable results, nor does it describe how the groundwater levels minimum thresholds are consistent with Human Right to Water policy.

Response:

The quoted text from the GSP specifically addresses all private wells and does not discount domestic wells. The quoted text also references an earlier section of the GSP which states the following:

The GSA agencies searched for existing active wells within the Basin. This search included reviewing water use records and contacting owners of large private properties (domestic, commercial, and industrial), inquiring about private wells in discussions with knowledgeable local residents and community leaders, and polling interested parties during public meetings.

This effort indicated that the only private pumpers in the Basin are All American Asphalt, Dart Corporation, and 3M. No active private domestic wells were identified in this search.

The comment ignores these earlier statements that clearly indicate that there are no known active domestic wells in the Basin. The GSA believes that all domestic water use in the Basin is supplied by the municipal water purveyors that make up the GSA (the cities of Corona and Norco and the Home Gardens County Water District). This includes the DAC areas of the Basin. The known private wells in the Basin are operated by industrial water users who have reported no previous problems with water shortages. Additionally, no private wells have been reported to have water shortages for the Basin in the DWR led Household Water Supply Shortage Reporting System. Therefore, it is the opinion of the GSA that the use of historical minimum groundwater elevations as the minimum threshold for water levels is protective of all groundwater users.

Comment:

For degraded water quality, constituents of concern (COCs) are total dissolved solids (TDS) and nitrate. The minimum threshold for nitrate is defined as the percentage of wells with concentrations exceeding the nitrate MCL (45 mg/L) based on current conditions (2015-2019), which is 50% of wells. The minimum threshold for TDS is defined as the percentage of wells with concentrations exceeding the TDS value of 1,000 mg/L based on current conditions (2015-2019), which is 26 percent of wells. However, according to the state’s anti-degradation policy,water quality should be protected and is only allowed to worsen if a finding is made that it is in the best interest of the people of the State of California. No analysis has been done and no such finding has been made.

Response:

There is no indication that water quality will worsen in the Basin due to groundwater management. The minimum thresholds for water quality are designed to recognize baseline conditions and monitor the Basin for potential changes related to groundwater management. The minimum threshold would be exceeded If water quality is significantly degraded and additional projects and

management actions would be triggered. In addition, groundwater in the Temescal Basin is treated before being delivered to municipal customers, there are no known domestic wells that use untreated groundwater for drinking water or any other potable uses.

Comment:

Sustainable management criteria for chronic lowering of groundwater levels provided in the GSP do not consider potential impacts to environmental beneficial users. The GSP neither describes nor analyzes direct or indirect impacts on environmental users of groundwater when defining undesirable results. This is problematic because without identifying potential impacts on GDEs, minimum thresholds may compromise, or even destroy, these environmental beneficial users. Since GDEs are present in the basin, they must be considered when developing SMC.

Response:

The comment correctly notes that the short paragraph on impacts of the waterlevel minimum threshold on beneficial (Section 6.2.4) uses does not mention interconnected surface water or GDEs at all. We have added a sentence directing the reader to the interconnected surface water sustainable management criteria section (specifically section 6.7.6.1), where the consistency of the interconnected surface water minimum threshold and water-level minimum threshold is explicitly discussed, and to Sections 4.10.4 and 6.7.4 which lay out the evidence that changes in Prado Wetlands vegetation have not historically correlated with changes in water levels in production wells north or south of the Wetlands. Noticeable vegetation die-back along Temescal Wash where it enters the Prado Wetlands started around 2009 and has been attributed to decreased base flows in Temescal Wash (McMichael 2021). No shallow water table information is available for that area and time period. This is an important data gap that will be filled by installing shallow monitoring wells in the southern Prado Wetlands area.

COMMENT 2. Climate change is not sufficiently considered.

Response:

The comment states that “the GSP does not consider multiple climate scenarios (e.g., the 2070 extremely wet and extremely dry climate scenarios) in the projected water budget.” The comment appears to be referring to two alternative sets of monthly climate multipliers provided in the files of climate change factors downloadable from the SGMA Data Portal. Those sets of factors are labeled Drier/Extreme-Warming (DEW) and Wetter/Moderate-Warming (WMW). There is no requirement to use anything but the expected factors. The California Department of Water Resources (DWR) document “Guidance for Climate Change Data Use during Groundwater Sustainability Plan Development” does not even mention the alternative data sets. Rather, Section 4.5 of the guidance document states that uncertainty in climate change predictions is represented by inter-annual variability in the 50-year future simulations. It also states that the evaluation of sustainability will be based on the “central tendency” of the climate change factors, which is represented by the primary climate factor data set. The

DEW and WMW data sets are for optional research purposes. Therefore, the climate change analysis in the GSP is adequate.

COMMENT 3. Data gaps are not sufficiently identified and the GSP does not have a plan to eliminate them.

Response:

In the context of SGMA, a data gap is “a lack of information that significantly affects the understanding of basin setting or evaluation of the efficacy of the Plan implementation and could limit the ability to assess whether a basin is being sustainably managed.” Data gaps were identified in the monitoring network (Chapter 7), and construction of new shallow monitoring wells for interconnected surface water monitoring are included in a project in Chapter 8 of the draft GSP.

The comment asserts that the wells in the monitoring network do not adequately represent GDEs and drinking water users. The new shallow monitoring well project is specifically designed to represent GDE and interconnected surface water conditions, and as noted in previous responses, drinking water users in the Basin receive water supply directly from the agencies that make up the GSA. The wells these agencies use are adequately monitored in the existing monitoring network. The comment also asserts that the monitoring network does not adequately identify data gaps in the shallow aquifer. However, there is not a shallow aquifer in the Temescal Basin that is separate from either the principal or secondary aquifer, as described in Chapter 3.

COMMENT 4. Projects and Management Actions do not sufficiently consider potential impacts or benefits to beneficial uses and users.

Response:

The comment claims that the projects and management actions in the GSP do not completely identify benefits or impacts to all beneficial users. The projects and management actions in the GSP do assess effects on beneficial uses and users consistent with SGMA. The recommendations related to this comment specifically request the addition of discussion and mitigation for DAC and domestic well owners. As noted in previous responses, there are no known domestic well users within the Basin. The agencies that make up the GSA are municipal water purveyors who provide service to domestic, municipal, commercial, and industrial users throughout the Basin, including the DAC areas.

Climate and water delivery uncertainties have been included in the water budget analysis (Chapter 5).

OCWD Comments on: Draft Temescal Basin Groundwater Sustainability Plan, September 2021.

The Orange County Water District (OCWD) is responsible for managing the Orange County Groundwater Basin, which is part of the Coastal Plain of Orange County Groundwater Basin (DWR Basin 8-1). In 2017, OCWD, along with partner agencies, submitted an Alternative to a Groundwater Sustainability Plan (Alternative) to the Department of Water Resources (DWR) to show that Basin 8-1 had been sustainably managed over the last 10 years. DWR approved the Alternative on July 17, 2019.

OCWD thanks the Temescal Basin Groundwater Sustainability Agency (GSA) for reaching out and including OCWD in the public participation process of developing the Temescal Basin Groundwater Sustainability Plan (GSP or Plan). Not only is the Temescal Basin adjacent to Basin 8-1, Coastal Plain of Orange County Groundwater Basin, OCWD owns and manages wetlands and a large area of riparian habitat behind Prado Dam that is within the Temescal Basin. The health of this riparian habitat is dependent on surface flows of the Santa Ana River (SAR) and its tributaries as well as rising groundwater. There are data gaps within this area that need to be filled, particularly with respect to interconnected surface water. A number of studies are ongoing by multiple agencies to better understand the impacts of increased recycling of SAR water and groundwater pumping on SAR flows and rising groundwater and their potential impacts to interconnected surface water and riparian vegetation.

Our detailed comments are presented below, but in general, our comments can be summarized as follows:

1. Additional data needs to be obtained in the north end of the Temescal Basin and Prado Basin to adequately characterize the interconnection of surface water and groundwater and establish the appropriate sustainability criteria and minimum thresholds.

2. The Temescal Basin GSA needs to closely coordinate with adjacent upstream groundwater basins, such as the Chino Basin, to ensure that their groundwater management activities do not create undesirable results in the Temescal Basin, particularly in the area of interconnected surface water and groundwater.

OCWD provided prior comments on the Temescal Basin GSP, Draft Plan Area, Sept. 2020. These comments are presented in Attachment A for completeness.

OCWD is providing initial comments on the Draft GSP in this letter and may follow up with additional comments during the public review period after the GSP is submitted to DWR.

The Regional Water Quality Control Board’s Basin Plan defines the ‘Prado Basin Management Zone’. It is defined as follows in the Regional Board’s Basin Plan:

The Prado Basin Management Zone is generally defined by the 566-foot elevation above mean sea level. It extends from Prado Dam up Chino Creek, Reach 1A and 1B to the concrete lined portion near the road crossing at Old Central Avenue, up the channel of Mill Creek (Prado Area) to where Mill Creek becomes named as Cucamonga Creek and the concrete-lined portion near the crossing at Hellman Road, up what was formerly identified as Temescal Creek, Reach 1A (from the confluence with the Santa Ana River upstream of Lincoln Avenue) (this area is indistinguishable because of shifting topography and is now considered a part of the Prado Basin Management Zone), and up the Santa Ana River, Reach 3 to the 566-foot elevation (just west of Hamner Avenue). The Prado Basin Management Zone encompasses the Prado Flood Control

Basin, which is a created wetlands as defined in this Plan (see the discussion of wetlands elsewhere in this Chapter). Orange County Water District’s wetlands ponds are also located within the Prado Basin Management Zone.

OCWD, the Army Corps of Engineers and the United States Fish and Wildlife Service commonly refer to the reservoir area behind Prado Dam, up to elevation 566 feet, as ‘Prado Basin’. In these comments, we will refer to the area using the terminology in the Regional Board’s Basin Plan – as the ‘Prado Basin Management Zone’ or PBMZ.

Page ES-8, Sustainable Management Criteria. In this section, “The Minimum Threshold for depletion of interconnected surface water is the amount of depletion that occurs when the depth to the water along the southern edge of the Prado Wetlands is greater than 15 feet for a period exceeding one year.” Later in the GSP, it is stated that this is an initial value subject to change as more data is collected. Based on the limited data available, this should be considered a starting point to be refined as the large existing data gaps in the northern part of the Temescal Basin and the PBMZ are filled. Additionally, the minimum threshold should be linked to impacts on interconnected surface water and groundwater, such as adverse impacts on riparian vegetation.

included in coordination and data sharing. The Chino Basin is adjacent to the Temescal Basin, and thus it is important that the Temescal Basin GSA monitor conditions in the Chino Basin to ensure they do not cause undesirable results in the Temescal Basin, particularly with respect to the interconnection of surface water and groundwater. OCWD provided comments on this issue and others during the public participation process, which are documented on pages 627-628.

included in the PBMZ to the Monitoring Network. OCWD is in the process of installing more than a dozen shallow monitoring wells in the PBMZ to provide more information on shallow groundwater conditions. These wells will be critical in understanding how groundwater management of the Chino and Temescal Basins impacts interconnected surface water and groundwater dependent ecosystems in the PBMZ.

measurements on Temescal Creek as measured by the USGS gage at Main Street in Corona, CA (Gage No. 11072100), only recent data from 2012 to present is shown (Figure 4-18). In contrast, historical data for the outflow from Prado Dam from 1949 to present is shown. All available historical data from 1980 to present for the Temescal Gage at Main Street is shown in Figure 21 in Attachment B. It is important to note that a noticeable decline in minimum flows is noted in the mid-2000s and has been very low over the past decade. These reductions in flow are due to reduced discharges of treated wastewater and perhaps other factors and has adversely impacted the quantity and quality of riparian habitat in Temescal Creek. OCWD staff biologists reviewed historical aerial photographs and have prepared a summary of the changes noted since 1939, which is presented in Attachment B. This historical summary indicates that the quantity and quality of habitat in the lower reaches of Temescal Creek rapidly declined starting in approximately the mid-2000s. This area remains a good candidate to restore habitat and ecological health provided there was sufficient water supplies.

data are of limited use for this purpose due to insufficient vertical and geographic coverage.” As mentioned above, OCWD has installed multiple new shallow monitoring wells and is the process of constructing additional monitoring wells in the PBMZ that will be critical to understanding interconnected surface water and groundwater in the north end of the Temescal Basin. We look forward to further development by the Temescal Basin GSA of the understanding of the interconnection of surface water and groundwater using data from OCWD wells, wells installed by the Temescal Basin GSA, and well installed by other agencies.

reach of Temescal Creek. This lower reach was very productive until the mid-2000s, when surface flows were reduced. See comments on Section 4.10.1, Stream Flow Measurements and Attachment B for more information on the history of vegetation in the lower reach of Temescal Creek. Section 4.10.3 also needs additional discussion of riparian vegetation in the PBMZ, in the area in and around the Corona Airport. Section 4.10.3 should identify the number of acres of riparian vegetation within the PBMZ (generally speaking the area below ground elevation 566 feet above mean sea level). A key source of data that should be included and shown below (Figure 1) is data from a joint effort of CA DWR and the Nature Conservancy to map areas referred to by DWR as ‘natural communities commonly associated with groundwater (NC Dataset Viewer (ca.gov)).

Figure 1. NC Dataset Viewer for PMBZ

be temporarily impounded behind Prado Dam up to elevation 505 ft msl during the flood and non-flood seasons. This change was approved by the US Army Corps of Engineers in April 2021. Please make the appropriate changes to the text of this section.

This section addresses the wetlands/riparian habitat and interconnected surface water in the PBMZ and the north end of the Temescal Basin. Within this section, multiple indirect lines of evidence are

presented in an attempt to show that the wetlands are sustained by surface flows and not groundwater.

Clearly there are critical data gaps that need to be filled to adequately understand the interconnection of surface water and groundwater in the north end of the Temescal Basin and the PBMZ. The additional monitoring wells OCWD has and is installing will provide key data and should be utilized by the Temescal Basin GSA as it becomes available. In addition, the Integrated Santa Ana River Model (ISARM), that was used as part of the Habitat Conservation Plan (HCP), cannot be relied on in its current configuration within the PBMZ. Although the ISARM is a useful tool to evaluate watershed wide conditions, there are data gaps within the PBMZ that need to be filled in order to adequately calibrate the model to make it useful in understanding interconnected surface water and groundwater in the area. This may require the development of a PBMZ Sub-model that would include portions of the Temescal Basin The draft GSP states:
“The correlation of precipitation and river flow with Prado groundwater levels and the lack of correlation with groundwater pumping north and south of the wetlands indicates that the wetlands are primarily sustained by surface inflows.”
Additionally, the draft GSP states:
“The low importance of groundwater as a factor in managing Prado Wetlands is also implicit in the Upper Santa Ana River Habitat Conservation Plan (HCP) (ICF 2020).” OCWD disagrees with these statements. The statement about the perceived lack of correlation with Prado groundwater levels and groundwater pumping north and south of the wetlands should be revised to account for the following factors:
• Historical pumping north of the PBMZ in the Chino Basin may not be of sufficient magnitude to induce observable historic changes in groundwater elevation in the PBMZ. This is particularly the case since there have been some changes in overall pumping in the southern portion of the Chino Basin as agricultural-related groundwater production has declined (see for example the report ‘2013 Chino Basin Groundwater Model Update and Recalculation of Safe Yield Pursuant to the Peace Agreement’, prepared by Wildermuth Environmental Inc for Chino Basin Watermaster, October 2015).
• The ability to observe changes in groundwater elevations in the PBMZ and relate changes to a particular factor like pumping can be obscured or made more complicated to detect due to recharge of groundwater from surface water.
• There is an overall lack of sufficient historical groundwater elevation data on the eastern side of the PBMZ, such as near the Corona Airport; this lack of sufficient data restricts or limits the ability to reach definitive conclusions regarding the impacts of pumping on groundwater levels. • Surface water and groundwater are connected in the PBMZ and also along the Santa Ana River upstream of the PBMZ. Surface water flow in the SAR and its tributaries upstream of the PBMZ and surface flow into the PBMZ help support groundwater levels as surface water flows recharge the shallow groundwater system.
• The flow rate in the Santa Ana River and its tributaries such as Temescal Creek and Chino Creek are projected to decline in the future due to water recycling, stormwater capture, and other factors. As described in the Draft EIR for the Upper Santa Ana Habitat Conservation Plan (HCP), flows in the SAR reaching the PBMZ are estimated to decline in the low flow period (summer to early Fall) to approximately 35 to 40 cubic feet per second. With decreased flow in the Santa

Ana River and its tributaries, groundwater levels in the PBMZ are anticipated to decline in the future. Under these future conditions, groundwater pumping may have a greater impact on groundwater levels in the PBMZ.

Regarding the state perceived low importance of groundwater as a factor in managing Prado Wetlands and information in the Upper Santa Ana River HCP, OCWD submitted extensive comments on the draft EIR for the HCP. These comments have not yet been addressed in writing and the EIR for the Upper Santa Ana HCP has not been certified. OCWD’s comments on the draft EIR included several issues related to groundwater levels and riparian vegetation in the PBMZ. Additionally, the Upper SAR HCP public review draft document (May 2021) states:
“The Upper Santa Ana River Sustainable Resources Alliance (Alliance) would create an account within SARCCUP, or other conjunctive use program, to purchase water that would be used to supply environmental flow. Alternatively, additional discharge from the aforementioned WWTPs could be purchased by the Alliance to provide supplemental flow.” Provision for supplemental flows to sustain environmental resources is included in the proposed HCP to account for uncertainty in model projections and potential impacts that may occur. The supplemental flows, if needed, would help recharge groundwater and support groundwater levels. It is not appropriate to state or imply that the HCP contains evidence that groundwater is of low importance in managing environmental resources such as riparian vegetation in the PBMZ.

A portion of the PBMZ is within the Temescal Basin. The GSP should identify the number of acres of the PBMZ that are within the Temescal Basin.

applied at model cells within the Prado Wetlands where aerial photographs indicate the presence of potential riparian vegetation.” The word ‘potential’ should be removed from this sentence. The number of acres of riparian habitat should be identified. There is clear evidence of riparian habitat in the portion of Temescal Basin within the PBMZ and there is also evidence that the riparian vegetation is dependent on interconnected surface water and groundwater. Attachment B discusses how riparian vegetation was negatively impacted when surface flows declined in the lower portion of Temescal Creek, within the PBMZ.

threshold for depletion of interconnected surface water is defined as the historical minimum water levels (maximum depth to water) in shallow monitoring wells in the southern Prado area, where these shallow water level declines are correlated with Temescal Basin pumping and/or water levels.” First of all, this minimum threshold does not meet the definition established in Section 354.28(c)(6) of the Regulations, which states that “The minimum threshold for depletions of interconnected surface water shall be the rate or volume of surface water depletions caused by groundwater use that has adverse impacts on beneficial uses of the surface water and may lead to undesirable results” (CCR, 2016). Minimum thresholds only apply to the interconnected stream reaches.

The PBMZ has Beneficial Uses identified in the Regional Water Quality Control Board’s Basin Plan. The Beneficial Uses identified for the PBMZ include ‘WILD’ and ‘RARE’. ‘WILD’ is defined in the Regional Board’s Basin Plan as:

Wildlife Habitat (WILD) waters support wildlife habitats that may include, but are not limited to, the preservation and enhancement of vegetation and prey species used by waterfowl and other wildlife.
‘RARE’ is defined as:

Rare, Threatened or Endangered Species (RARE) waters support the habitats necessary for the survival and successful maintenance of plant or animal species designated under state or federal law as rare, threatened or endangered.

The PBMZ contains wildlife habitat for the least Bell’s vireo, which is an endangered species identified by the US Fish and Wildlife Service. The US Fish and Wildlife Service has also identified critical habitat for the least Bell’s vireo, and the PBMZ includes area with this designated critical habitat. Additionally, the RARE beneficial use occurs in the PBMZ by virtue of nest locations of the least Bell’s vireo occurring in the PBMZ. As a result, the minimum threshold needs to consider not only the potential changes in surface water depletions caused by groundwater pumping and or management actions, but also impacts to the beneficial uses in the PBMZ.

Secondly, no data are presented to document what the “historical minimum levels” are. As noted in prior comments, there are data gaps in the northern Temescal Basin and PBMZ that need to be filled. As such, it is not possible to establish a minimum threshold until these data gaps are filled.

Finally, because the Temescal Basin is hydrologically connected to the Chino Basin and other basins, it is not possible to isolate the impacts of pumping in adjacent basins from those in the Temescal Basin. This is why it is critical that the Temescal Basin GSA monitor and coordinate with adjacent basins to ensure that their actions do not cause or contribute to undesirable results in the Temescal Basin.

for TDS and Nitrate. While we appreciate that the TDS of basin outflows is included, other constituents of concern should also be included. There are some constituents of concern that may have Maximum Contaminant Levels (MCLs) established in the future that may require additional actions to reduce concentrations in outflow to the Orange County Groundwater Basin, including stormwater.

indirect evidence are used to imply that groundwater discharge to the PBMZ is not significant and not expected to decrease significantly in the future. As stated above, additional data needs to be collected and the ISARM needs to be further developed to better understand interconnected surface and groundwater interactions in the north end of the Temescal Basin and the PBMZ.

sustained by surface water and not groundwater is based on Section 6.6.2.1. See comments on this section above.

impacts to groundwater dependent ecosystems or natural communities commonly associated with groundwater. OCWD has been managing riparian vegetation in the PBMZ for many years to support the endangered least Bell’s Vireo. Through these actions, the PBMZ has become one of the most heavily

populated areas of least Bell’s Vireo in California. Any reduction in the quality of riparian habitat caused by impacts to surface/groundwater interactions could also affect least Bell’s Vireo habitat.

are data gaps and uncertainties regarding establishing the minimum threshold of depth to water exceeding 15 feet for more than one year. This threshold should be refined based on Section 354.28(c)(6) of the Regulations and OCWD comments on Section 6.1 above.

source of water supplies to the Coastal Plain of Orange County Groundwater Basin (Basin 8-1) is the baseflow of the Santa Ana River (SAR). SAR baseflow is a combination of surface flow and rising groundwater within the PBMZ. Any reduction in rising groundwater will result in reduced baseflow to Basin 8-1. It is unclear what evidence would be used to quantify the “historical minimum” of groundwater discharge to the SAR in the PBMZ. As mentioned in prior comments, there are significant data gaps with respect to the interconnection surface water and groundwater that need to be filled.

installation of additional wells and filling data gaps. We look forward to coordinating with the Temescal Basin GSA in this effort and further refining the Minimum Threshold for the interconnection of surface water and groundwater (see prior comments).

Attachment A
OCWD Comments on: Temescal Subbasin Groundwater Sustainability Plan, Draft Plan Area, September 2020.

OCWD Comments on: Temescal Subbasin Groundwater Sustainability Plan, Draft Plan Area, September 2020.

The Orange County Water District (OCWD) is responsible for managing the Orange County Groundwater Basin, which is part of the Coastal Plain of Orange County Groundwater Basin (DWR Basin 8-1). In 2017, OCWD, along with partner agencies, submitted an Alternative to a Groundwater Sustainability Plan (Alternative) to the Department of Water Resources (DWR) to show that Basin 8-1 had been sustainably managed over the last 10 years. DWR approved the Alternative on July 17, 2019.

OCWD thanks the City of Corona for reaching out and including OCWD in the process of developing the Temescal Subbasin Groundwater Sustainability Plan (GSP or Plan). Not only is the Temescal Subbasin adjacent to Basin 8-1, Coastal Plain of Orange County Groundwater Basin, OCWD owns and manages wetlands and a large area of riparian habitat behind Prado Dam. The health of this riparian habitat is dependent on surface flows of the Santa Ana River (SAR) and its tributaries as well as rising groundwater. A number of studies are ongoing by multiple agencies to better understand the impacts of increased recycling of SAR water and groundwater pumping on SAR flows and rising groundwater with respect to potential impacts upon riparian vegetation. We have listed some of these studies in our comments.

Specific Comments on the Temescal Subbasin GSP, Draft Plan Area, Sept. 2020 are as follows:

that manage groundwater in adjacent groundwater basins. Even though they do not have jurisdiction within the Temescal Subbasin, the Chino Basin (Upper Santa Ana Valley Basin 8-002.01) is adjacent and upgradient of the Temescal Subbasin. Groundwater management actions taken by the Chino Basin Watermaster and other agencies upgradient of the Temescal Subbasin can affect groundwater conditions within the subbasin. Although a formal coordination agreement is not required, it is important that these agencies know that they must carefully consider any actions that could affect groundwater conditions in the Temescal Subbasin.

groundwater (18%). Please include a description of this water use and location as a separate subsection in Section 2.4. Also provide information on how the 18% figure was calculated, including assumptions, acreages, etc.

Given the significant water use by native vegetation and significant natural resources present in the Prado Basin, we suggest that you add Natural Resources to the list of water resources monitoring categories.

We suggest that you add the following monitoring programs in the appropriate category. We have put a suggested category (or categories) in parentheses.

1. Groundwater Monitoring in Prado Basin by Western Riverside County Regional Wastewater Authority. (Groundwater Levels)

2. Natural resources and groundwater monitoring in Prado Basin by OCWD. (Groundwater Levels, Groundwater Quality, Surface Water Quality, Natural Resources (new category))

3. Chino Basin Watermaster/Inland Empire Utilities Agency Monitoring as part of Prado Basin Habitat Sustainability Committee. (Groundwater Levels, Groundwater Quality, Natural Resources (new category))

4. Upper Santa Ana River Habitat Conservation Plan (HCP) and associated monitoring plans being managed by the San Bernardino Valley Municipal Water District (Valley District) with participation from multiple other agencies including OCWD. (Groundwater Levels, Surface Water Flow, Surface Water Quality, Natural Resources (new category)). Please note that to support the HCP, an Integrated Santa Ana River Model (ISARM) has been developed.

groundwater dependent ecosystems present in Prado Basin and the Santa Ana River and tributaries to the SAR upstream of Prado Basin. Prado Basin is generally defined as the area behind Prado Dam up to elevation 566 feet mean sea level. The groundwater dependent ecosystems present within Prado Basin and Santa Ana River and its tributaries need to be carefully considered in the Plan.

Attachment B
Temescal Creek Riparian Habitat Timeline
Prepared by David McMichael, Biologist
Orange County Water District
December 2021

Temescal Creek Riparian Habitat Timeline
Prepared by David McMichael, Biologist
Orange County Water District
December 2021
Temescal Creek has long shown a tendency to grow a riparian belt especially as it nears its confluence with the Santa Ana River. Temescal Creek was protected from the scouring effect of the river hidden behind the Norco bluffs, which means riparian habitat would have been allowed to mature creating a perennial haven for wildlife. Aerials taken in 1939 and 1953 clearly show riparian trees growing thick along Temescal Creeks (Figures 1-2).

Figure 1. Temescal Creek and the Prado Basin in 1939

Figure 2. Temescal Creek and the Prado Basin 1953
This timelapse habitat study focuses on a portion of Temescal Creek that is not channelized. This natural setting is confined to a roughly 2 mile stretch from Lincoln Ave. downstream past to where it meets the Santa Ana River. Upstream of Lincoln Ave the creek is channelized. This concrete channel originates at Temescal Canyon Lake in the city of Corona. Two segments will be studied using aerial imaging, and observations from the ground. The first segment begins at Lincoln Ave. and continues down to Auburndale Street. The next segment will be area of the creek immediately upstream and downstream of Rincon St.

The segment immediately downstream of Auburndale St. will not be looked at due to the proximity of an earthen dike project which involved the removal of habitat. Multiple fires in this area also made habitat comparisons difficult. It should be noted that the habitat along this stretch has not regrown following the completion of this project and no recruitment has been noted here.

The area of Temescal Creek which joins with the Santa Ana River flood plain will also not be discussed due to the proximity of water from the river as well as several fire events. The Willow Forest in this area seems to be in optimum health and any alteration of species composition seems to be from the shifting nature of the river. When the river moves away (west) from the creek it’s possible to find more Mulefat recruitment, but when the river flows south its common for there to be wetland species’ such as Cattails. Water will often form ponds near the end of the Corona Airport runway.

1994
Habitat along Temescal Creek was generally good where water was available. Figure 3 indicates that creek bottom between Lincoln Ave. and Auburndale St. was lined with riparian trees with clumps of Mulefat immediately upslope of the creek. Ground observations from this period indicate that the Mulefat was established right up to the farmed fields on the south side of the creek. The darker clumps were Black Willows, Cottonwoods, and Arroyo Willows. There were and still are a few clumps of Eucalyptus on the site and Arundo donax is also present during this time mixed in the riparian.

The aerial at figure 4 indicates that the thick Willow Forest seen behind the Prado Dam has creeped up to Rincon Street an extended into the Corydon St. corner. Mulefat was common in the upland areas adjacent to and well away from the creek bed. Besides seasonal invasives such as Black Mustard other perennial non-natives such as Tamarisk and Arundo were seen only sporadically. They were not abundant along this segment of the creek.

Figure 3. 1994 Temescal Creek – Lincoln Ave. to Auburndale St.

Figure 4. 1994 Temescal Creek at Rincon St.
2003
Habitat conditions remain relatively unchanged in these 2003 aerials but key invasives such as Tamarisk are beginning to spread in Temescal as well as the entire Prado Basin (Figures 5-6). Tamarisk is a more xeric riparian species with deep root systems able to tap into deeper ground water sources. Mulefat is also becoming more common along the banks of the creek. Mulefat is a transitional riparian species which can exist at different elevational zones often growing alongside sage scrub patches upslope of riparian settings. Mulefat is more tolerant of dry conditions as it can monopolize lower ground water levels than Black Willows.

Figure 5. 2003 Temescal Creek – Lincoln Ave to Auburndale St.

Figure 6. 2003 Temescal Creek at Rincon St.
2007
Figure 7 shows that little changed along Temescal from 2003 to the end of 2007. There were some significant weather events in the basin such as the historic flood in 2004-2005 where 33.94” of rain fell over a relatively short period of time. This event caused destructive flooding in Prado Basin and resulted

in a long pool retention time behind the dam. The rain year of 2005-2006 was a relatively average rain year with 11.53” of rain but the bulk of this came late in the season resulting in another long inundation period behind the dam. These subsequent inundation events should have had little impact on the habitat along Temescal Creek except in the lowest portion of the creek near the Santa Ana River. The final important weather impact happened in 2006-2007 when the area saw historic low rainfall at 3.37”.

Drought conditions can be very damaging to riparian habitat but any sort of impactful damage to the habitat was not seen until 2009.

Figure 7. 2007 Temescal Creek at Rincon St.
2009
The aerials taken in 2009 showed a drastic shift in the disposition of the riparian habitat along Temescal Creek. The segment of Temescal Creek from Lincoln Ave. down to Auburndale St. showed significant loss of both Willow and Mulefat habitat (Figure 8). Much of the creek bottom banks still shows some Black Willows stands but many of the historic patches are now gone. Upslope of the creek where the Mulefat used to reside was also vacant save for some weedy annual species.

Figure 9 indicates that the Rincon St. segment of Temescal Creek is dramatically altered from previous aerials. The Black Willow Forest has retreated downstream and in its place non-native such as Perennial Pepperweed, and Tamarisk has begun to form monocultural stands. Mulefat is still present but competing with pepperweed in the upland sites. Channel scouring is evident as well which could have removed some of the riparian habitat during the previous year’s big rain events.

Figure 8. 2009 Temescal Creek- Lincoln Ave. to Auburndale St.

Figure 9. 2009 Temescal Creek at Rincon St.

2009-2021 Temescal Creek (Lincoln Ave.-Auburndale St.)
These aerials indicated that the habitat downstream of Lincoln Ave. further degraded during the 2009- 2021 period. Further Black Willows disappeared and Mulefat continued to be uncommon over the entire site. No recruitment of native riparian species was noted during this period. Some Eucalyptus continue to reside at this site but most of the non-native species’ seen further downstream are also absent from this stretch.

Figure 10. 2012 Temescal Creek – Lincoln Ave. to Auburndale St.

Figure 11. 2018 Temescal Creek – Lincoln to Auburndale St.

Figure 12. 2020 Temescal Creek – Lincoln Ave. to Auburndale St. 2014 Temescal Creek at Rincon St.

Figure 13 shows the continual retreat of Black Willows away from the Rincon St. area. Some patches continue to remain around the Corydon -Rincon curve but it’s clear the Willows are disappearing from the area. Perennial Pepperweed and Tamarisk continue to plague the site and some of the upland patches of Mulefat appear to be in decline.

Figure 13. 2014 Temescal Creek at Rincon St.
2016-2017 Temescal Creek at Rincon St.

The invasive species were removed from the Rincon St. area in 2015-2016 and treatment continued until 2019. This included the physical removal of Arundo and Tamarisk but also the spraying of Perennial Pepperweed. The removal of the exotics facilitated the recruitment of Mulefat into this area Recruitment was primarily in those areas where Perennial Pepperweed was treated (Figures 14-15).

There was little evidence of lasting recruitment by young Willows even during average rainfall years.

Mulefat recruitment and habit transition to more xeric riparian species has been seen in other parts of the basin where drought stress and low ground water levels create conditions unfavorable to Black Willow growth. Mulefat have deeper root structure than Black Willows and other water dependent species. Mulefat continued to thin out in those areas that do not get flood irrigated during the winter.

Figure 14. 2016 Temescal Creek at Rincon St.

Figure 15. 2017 Temescal Creek at Rincon St.
2020-2021 Temescal Creek at Rincon St.

During 2020 and 2021 the encouraging growth and recruitment of Mulefat in the Rincon St. area has been halted and instead the evidence suggests that many Mulefat plants have begun disappearing and those that remain appear stunted and sporadic. The remaining mature Black Willow trees are persisting but there is still no recruitment of young willows and very little recruitment of Mulefat plants (Figures 16-17). There appears to be an influence from the Santa Ana River on the remaining Willow Forest where the constantly shifting course of the river often favors the Temescal creek area. The larger willows must be able to tap into ground water replenished by the river.

Figure 16. 2020 Temescal Creek at Rincon St.

Figure 17. 2021 Temescal Creek at Rincon St.

Least Bell’s Vireo Distribution
Figure 18 suggests that least Bell’s Vireo were previously widely distributed below Lincoln Ave, but Figure 19 suggests that in recent years Vireo have not stayed in those areas where the habitat is unsuitable for nesting success. Figure 20 shows the distribution of least Bell’s Vireo in 2021. least Bell’s Vireo is currently completely absent along Temescal Creek from Lincoln Ave. to Auburndale St.

Figure 18. Least Bell’s Vireo Distribution 2007-2021
Figure 19. Least Bell’s Vireo Territories 2019-2021

Figure 20. Least Bell’s Vireo Territories 2021
Stream Flow Measurements
Figure 21 shows all the available daily measurements of streamflow in Temescal Creek at Main Street in Corona, CA, which is USGS Gage 11072100. What is important to note is the decline in the daily minimum flows that started in the mid-2000s and have been persistently at 1 cfs or less since 2012.

These changes in flows correlate with the changes in vegetation described above.

Figure 21 Daily Flow in Temescal Creek at Main Street, Corona, CA Conclusion
Aerial imagery of the downstream portion of Temescal Creek below Lincoln Ave. indicated that this portion of the Santa Ana River Watershed saw many decades of optimum growth and health of its riparian species’, mainly Black Willow and Mulefat. These important riparian species provided refuge to many species of birds and wildlife including least Bell’s Vireo and Southwestern Willow Flycatcher.

There is evidence that this portion of the watershed saw considerable dieback around 2009 but it’s possible that this began earlier with subtle habitat transitional changes noted as early as 2003. The 2009 event is substantial but the inability of the habitat to recover and establish new trees and shrubs in later years shows a shift in the water regime. This condition can no longer sustain water dependent riparian species and may even exclude more xeric species. The further decline of Temescal Creek habitat will eventually exclude riparian nesting bird species entirely if the structure of the habitat cannot repair itself.
0 1
10 100
1,000
10,000
Jan-80 Jan-90 Jan-00 Jan-10 Jan-20
Discharge Rate (cfs)
Daily Flow in Temescal Creek Above Main Street, Corona, CA (USGS Gage: 11072100)

City of Corona
Utilities Department
“Protecting Public Health”
755 Public Safety Way
Corona, CA 92878 – www.CoronaCA.gov
Office: 951.736.2234
Fax: 951.735.3786
January 13, 2021
To: Adam Hutchinson, Orange County Water District
RE: OWCD Comments on Draft Temescal Basin GSP 12.14.2021 The Temescal Basin Groundwater Sustainability Agency (GSA) appreciates your thorough review of our Groundwater Sustainability Plan (GSP). Throughout the process, the Temescal Basin GSA has encouraged and welcomed public input, including the comment letter you submitted December 14, 2021. We have reviewed your comments.

Detailed responses to your comments, including identification of edits to the GSP, are provided below.

Responses are organized to be consistent with your comments on specific pages and sections of the Draft GSP.

Comment on Page ES-8, Sustainable Management Criteria
Response:

The minimum threshold for interconnected surface water is a starting point and may be revised in future periodic evaluations as additional data and understanding of interconnected surface water and groundwater become available. This minimum threshold is discussed in greater detail in Chapter 6.

Comment on Page 2-25, Section 2.8.5, Neighboring Basin Coordination.

Response:

Corona is aware of future potential items of concerns and Corona representatives actively attend and engage in local inter-agency working groups / committees This comment also referenced comments OCWD submitted previously that included suggestions for additional information to be included in Chapter 2 of the GSP. Thank you for reminding us of these comments; text related to your comments has been added to Chapter 2.

Comment on Page 4-12. Section 4.6.4 Monitoring Networks.

Response:

The GSP used data for OCWD monitoring wells in the PBMZ, including PDHQ, PD3A, PD3B, PD4, PD6A, PD6B, PD9, PD10 through PD19, PD14R, PD22 through PD25. In fact, water levels from those wells were the basis for defining the depth to water minimum threshold for riparian vegetation. However, all of those wells are in the northern part of PBMZ. Thus, the data gap in the southern part identified in the Temescal Basin GSP still stands. We are aware that there are newly installed shallow monitoring wells in the southern portion of the PBMZ.

However, those wells were installed during preparation of this GSP and any water level data collected from them does not correlate to the time period assessed in the Temescal GSP, which includes data through 2018. Data and

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information from these newer wells will be used in future updates to the Temescal GSP.

Comment on Page 4-15. Section 4.10.1, Stream Flow Measurements.

Response:

The historical vegetation analysis along the lower reach of Temescal Wash by OCWD staff biologist David McMichael (attachment B of the comment letter) is excellent and is similar to analysis Todd Groundwater completed for reaches of Temescal Wash in the Bedford-Coldwater and Elsinore Basins. We concur with the finding that riparian vegetation canopy extent and density along Temescal Wash from North Livermore Avenue to below West Rincon Street decreased greatly beginning around 2009. OCWD’s analysis concluded that decreased base flow in Temescal Wash was a likely cause of the die-back. Flows at the gage near Main Street do show a decrease in base flow beginning around 2009.

The analysis attributed the decrease in flows to decreased discharges from Corona water reclamation facilities. However, we note that discharges at that time were from WRF-1 to Butterfield Drain, which enters Prado Wetlands downstream of the gage and impacted reach. Also, those discharges were not decreased until a State Water Resources Control Board decision was made in 2011.

Declining groundwater elevations might have contributed to the vegetation dieback, but historical data are insufficient to verify that hypothesis. The nearest well with water level data was the Butterfield Park well, which had a period of record only for 2011-2016. Water levels declined steadily during that period, which coincided with drought conditions and small water-level declines in other Temescal Basin wells. However, water levels had been higher in 2009, and had also been lower than the 2011-2016 range in the late 1990s and early 2000s, when there was no vegetation die-back. So available water level data do not entirely correlate with the timing of changes in vegetation.

The GSP affirms the possibility that declines in water table elevation in the southern part of the PBMZ could impact riparian vegetation, and the sustainable management criteria for interconnected surface water are designed to prevent such impacts in the future.

Finally, it should be emphasized that SGMA does not require that GSPs restore groundwater conditions (including GDEs) to any state they were in prior to 2015.

Comment on Page 4-15. Section 4.10.2, Depth to Groundwater.

Response:

See response to above comment regarding Page 4-12. Section 4.6.4 Monitoring Networks.

Comment on Page 4-17. Section 4.10.3, Riparian Vegetation.

Response:

With respect to vegetation die-back along the lowermost reach of Temescal Wash, see response to above comment Page 4-15. Section 4.10.1, Stream Flow Measurements.

With respect to NCCAG vegetation mapping, the NCCAG riparian vegetation and wetland maps were both evaluated during the analysis of interconnected surface water and GDEs. The vegetation polygons are included in Figure 4-21 and

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Section 4.10.3 “Riparian Vegetation” includes a page of discussion of the mapping and the related NDVI and NDMI information.

Comment on Page 4-18 to-21. Section 4.10.4, Wetlands and Interconnected Surface Water.

Response:

The text has been changed to state that the target impoundment elevation behind Prado Dam is 505 feet in all seasons.

The temporal correlation of factors potentially associated with vegetation in Prado Wetlands is not flawed due to “critical data gaps that need to be filled to adequately understand the interconnection of surface water and groundwater”.

Additional data are always useful. Nevertheless, the analysis stands on its own as making good use of available data. Specifically, it includes information from OCWD wells in the Prado Wetlands. The GSP furthermore concurs that there is a data gap regarding water table elevations in the southern part of the wetlands.

That data gap does not undermine the conclusions drawn from the rest of the data. The data gap will be filled primarily to confirm relationships of pumping and water levels farther south in the Temescal Basin with water table depths along the southern edge of the wetlands.

If regional groundwater models used for the Prado Basin Adaptive Management Plan, the 2019 Annual Report of the Prado Basin Habitat Sustainability Committee, and the Upper Santa Ana River Habitat Conservation Plan are found to be flawed based on OCWD’s review, revised modeling results can be considered during the 5-year update of the GSP.

We agree that the Santa Ana River and the pool of water impounded behind Prado Dam are almost certainly hydraulically connected with a shallow water table at least most of the time. However, the shallow water table is not strongly connected to deeper aquifers and pumping stresses due to the relative abundance of fine-grained sediments in the Prado Wetlands part of the ChinoTemescal Basins region.1 The draft GSP does not assert that Prado Wetlands are entirely disconnected from groundwater, but that as a practical matter the water supply for the wetlands is almost entirely derived from surface inflows.

If surface inflows to Prado are expected to decline in the future as the comment suggests, OCWD is free to intervene with the wastewater dischargers who currently supply those flows. It is worth noting that the City of Corona continues to comply with an agreement adopted in 1968 and modified in 2011 to provide reclaimed water inflow to the Prado Wetlands. Furthermore, as long as the pool behind Prado Dam is held at a constant elevation, groundwater discharge into that area will remain essentially constant.
1 “The distinction between aquifer systems is most pronounced within the west-southwest portions of the Chino Basin. This is likely because of the relative abundance of fine-grained sediments in the southwest (multiple layers of clays and silts). Groundwater flowing from high-elevation forebay areas in the north and east become confined beneath these fine-grained sediments in the west-southwest, and these sediments effectively isolate the shallow aquifer system from the deep aquifer system(s).” (from page 2.9, last paragraph, of Wildermuth Environmental, Inc. October 2015. 2013 Chino Basin groundwater model update and recalculation of safe yield pursuant to the peace agreement. Prepared for Chino Basin Watermaster).

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The comment states that supplemental surface inflows to the Prado Wetlands “would help recharge groundwater and support groundwater levels”. Percolation from the river would likely maintain the shallow water table described above. But with respect to the overall Temescal Basin, the Prado Wetlands cannot simultaneously be a recharge boundary and a discharge boundary.

Comment on Page 5-19. Section 5.7.2.4, Riparian Evapotranspiration.

Response:

We have removed the mention of “potential riparian vegetation” in Prado Wetlands from the text.

Comment on Page 6-4. Section 6.1, Summary of Sustainable Management Criteria.

Response:

The comment asserts that sustainable management criteria for interconnected surface water must be expressed as a rate of stream flow depletion. The GSP regulations do include a clause to that effect. However, the very next clause (§354.28(d)) states “An Agency may establish a representative minimum threshold for groundwater elevation to serve as the value for multiple sustainability indicators, where the Agency can demonstrate that the representative value is a reasonable proxy for multiple individual minimum thresholds as supported by adequate evidence.” This is explicitly discussed in section 6.7.5 of the GSP. If the primary beneficial use of interconnected surface water is riparian vegetation—as it is in the Prado Wetlands—then water table depth is an appropriate metric for defining sustainable management criteria.

Plants do not care about the rate of flow across a stream bed, but they do care about the depth of the water table.

The Basin Plan’s inclusion of WILD and RARE as beneficial uses of surface water in Prado Wetlands simply corroborates the GDE discussion in the GSP, which covers arroyo chub, least Bell’s vireo and southwestern willow flycatcher (section 4.10.5), among other species.

The comment objects that minimum historical groundwater levels are used in the sustainable management criteria definitions for interconnected surface water.

This statement was in the introductory summary of sustainable management criteria at the beginning of chapter 6. It was an editorial oversight. The statement has been changed to “The Minimum Threshold for depletion of interconnected surface water is defined as a depth to water of 15 feet in shallow monitoring wells in the southern Prado area, where declines to lower water levels are correlated with Temescal Basin pumping and/or water levels.” The revision is consistent with the actual minimum threshold definition presented in section 6.7.

The last paragraph of the comment implies that the GSP is responsible for ensuring that groundwater management in the Chino Basin does not impact Prado Wetlands or downstream water users. The GSP must show that actions taken to achieve sustainability in the Temescal Basin do not impair the ability of adjacent basins to achieve sustainability. Potential effects of sustainable management criteria on adjacent basins are discussed in sections 6.2.6.3,

6.3.6.2, 6.5.4.2, 6.6.5.4, and 6.7.6.2. In contrast, the Chino Basin appeared to adopt its goal of “hydraulic control” (that is, zero groundwater outflow to Prado

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Wetlands) without consideration of impact on Temescal Basin sustainability. The Temescal Basin GSA has no authority relating to subsurface outflows from the Chino subbasin.

Comment on Page 6-31. Section 6.6.6.1, Description of Measurable Objectives.

Response:

The GSP includes description and analysis of several constituents of concern.

Nitrate and TDS were selected because objectives are set in the Basin Plan, there are sufficient baseline data, and they serve as indicators for both natural and anthropogenic impacts. Additional water quality minimum threshold components may be considered in future periodic evaluations of the plan. In addition, the monitoring network will continue to track and record water quality for the minimum threshold and for all constituents of concerns.

Comment on Page 6-32 to 6-33. Section 6.6.2.1, Surface Water Users.

Response:

This comment basically reiterates the previous comment on Page 4-18 to-21.

Section 4.10.4, and the response to that comment applies here. Furthermore, the comment presents no information to refute the analysis in the GSP nor indicates which additional data specifically need to be collected to refute or improve the analysis.

Comment on Page 6-33, Section 6.7.2.3, Riparian Vegetation.

Response:

This comment also reiterates concerns presented in the comment on Page 4-18 to-21. Section 4.10.4, and the response to that comment applies here.

Comment on Page 6-33. Section 6.7.3, Definition of Undesirable Results.

Response:

The comment notes that OCWD has been working to expand and improve wetland habitat in Prado Wetlands for many years, primarily to provide habitat for the endangered least Bell’s vireo, a bird that resides in the wetlands. The GSP does not interfere with that objective and program. The interconnected surface water sustainable management criteria are designed to avoid future impacts to Prado Wetlands vegetation (and by extension to the vireo) due to groundwater management.

Comment on Page 6-34. Section 6.7.6, Minimum Threshold.

Response:

There are no “data gaps and uncertainties regarding establishing the minimum threshold” for interconnected surface water. The data gaps relate to implementing the minimum threshold. We expect the minimum threshold to continue to be adequate. Shallow monitoring wells will be installed in the southern Prado Wetlands area during the first five-year GSP implementation period. During the five-year GSP update, the appropriateness of the 15-foot depth to water criterion used in the minimum threshold will be reviewed in light of data from the new wells. This could potentially lead to a revised minimum threshold definition.

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Comment on Page 6-35. Section 6.7.6.2, Effect of Minimum Threshold on Sustainability of Adjacent Areas.

Response:

It is a simple matter of groundwater hydraulics that if groundwater levels within the Temescal Basin do not drop below historical minimum water levels, the discharge at the Basin boundary (to Prado Wetlands, the Santa Ana River and/or Chino Basin) will not drop below its historical minimum.

The comment expresses a concern about "any reduction in rising groundwater” that contributes to Prado Dam outflow. However, nothing in the GSP contemplates a reduction in groundwater outflow from the Temescal Basin.

Comment on Page 6-36. Section 6.7.6.5, How the Minimum Threshold will be Monitored.

Response:

The Temescal Basin GSA likewise appreciates OCWD’s efforts to monitor groundwater levels in the Prado Wetlands area. The information for OCWD’s shallow wells in the northern Prado area was extremely useful for preparing the GSP. The Temescal Basin GSA intends to continue its cooperation with OCWD and share monitoring data from the new shallow wells to be installed in the southern part of the Prado Wetlands.

Temescal Basin GSP

APPENDIX J
Temescal Groundwater Sustainability Plan
Numerical Groundwater Model
Documentation Report

TEMESCAL BASIN

GROUNDWATER MODEL

DOCUMENTATION
REPORT

PREPARED FOR THE TEMESCAL

GROUNDWATER

SUSTAINABILITY PLAN
December 2021
2490 Mariner Square Loop, Suite 215
Alameda, CA 94501
510.747.6920
www.toddgroundwater.com

SIGNATURE PAGE
_____________________________
Gus Yates, PG, CHg
Senior Hydrologist
Todd Groundwater, Alameda, CA
_____________________________
Michael P. Maley, PG, CHg, CEG, PE
Principal Hydrogeologist
Todd Groundwater, Alameda, CA

Temescal Basin GSP
Groundwater Model Report i TODD GROUNDWATER
Table of Contents

1. Introduction ............................................................................................................................. 1

1.1. Scope and Objective......................................................................................................... 1

1.2. Summary of Previous Models.......................................................................................... 1

2. Basin Geology and Structure ................................................................................................... 2

2.1. Temescal Basin................................................................................................................. 2

2.1.1. Groundwater Basin .................................................................................................. 2

2.1.2. Physiography............................................................................................................ 2

2.1.3. Hydrology................................................................................................................. 2

2.2. Regional Geology ............................................................................................................. 3

2.2.1. Geologic Units.......................................................................................................... 3

2.2.2. Faults........................................................................................................................ 3

2.2.3. Definable Basin Bottom ........................................................................................... 3

2.3. Groundwater conditions.................................................................................................. 4

2.3.1. Basin Aquifer............................................................................................................ 4

2.3.2. Basin Boundaries...................................................................................................... 5

2.3.3. Recharge and Discharge Areas................................................................................. 5

2.3.4. Primary Groundwater Uses...................................................................................... 6

3. Rainfall-Runoff-Recharge Model ............................................................................................. 7

3.1. Approach.......................................................................................................................... 7

3.2. Land Use and Recharge Zones......................................................................................... 8

3.3. Rainfall ............................................................................................................................. 8

3.4. Interception ..................................................................................................................... 9

3.5. Runoff and Infiltration ..................................................................................................... 9

3.6. Root Zone Depth and Moisture Content ....................................................................... 10

3.7. Evapotranspiration......................................................................................................... 10

3.8. Irrigation......................................................................................................................... 11

3.9. Deep Percolation from Root Zone to Shallow Groundwater......................................... 11

3.10. Movement of Shallow Groundwater to Deep Recharge and Stream Base Flow........... 12

3.11. Evapotranspiration by Riparian Vegetation................................................................... 12

3.12. Groundwater Inflow....................................................................................................... 13

3.13. Calibration of Rainfall-Runoff-Recharge Model............................................................. 13

4. Numerical Groundwater Model Development...................................................................... 15

4.1. General Approach .......................................................................................................... 15

4.2. Model Setup................................................................................................................... 15

4.2.1. Model Code Selection ............................................................................................ 15

4.2.2. Base Period ............................................................................................................ 16

4.2.3. Model Domain and Grid......................................................................................... 16

4.2.4. Model Layers.......................................................................................................... 17

4.2.5. Faults...................................................................................................................... 18

4.2.6. Aquifer Conditions ................................................................................................. 18

4.3. Boundary Conditions...................................................................................................... 19

4.3.1. Surface Recharge ................................................................................................... 19

4.3.2. Streams .................................................................................................................. 19

4.3.3. Mountain Front Recharge...................................................................................... 21

4.3.4. Evapotranspiration................................................................................................. 21

4.3.5. Groundwater Pumping........................................................................................... 22

Temescal Basin GSP
Groundwater Model Report ii TODD GROUNDWATER

4.3.6. Recycled Water Recharge Ponds ........................................................................... 22

4.3.7. Subsurface Flow with Adjacent Groundwater Basins............................................ 23

4.4. Aquifer Properties.......................................................................................................... 24

4.4.1. Aquifer Characteristics........................................................................................... 24

4.4.2. Zone Approach....................................................................................................... 24

4.4.3. Hydraulic Conductivity ........................................................................................... 25

4.4.4. Vertical Conductance ............................................................................................. 25

4.4.5. Specific Yield and Specific Storage......................................................................... 25

4.5. Initial Condition.............................................................................................................. 25

5. Historical Model Results........................................................................................................ 26

5.1. Calibration Methodology ............................................................................................... 26

5.1.1. Approach................................................................................................................ 26

5.1.2. Calibration Methodology ....................................................................................... 26

5.2. Sensitivity Analysis......................................................................................................... 27

5.3. Statistical Calibration ..................................................................................................... 27

5.4. Groundwater Level Trends............................................................................................. 28

5.5. Evaluation Of Groundwater Flow .................................................................................. 29

5.6. Model-Based Hydrologic Budget ................................................................................... 31

6. Simulation of Future Conditions............................................................................................ 32

7. SGMA Requirements.............................................................................................................. 33

7.1. Model Data Gaps............................................................................................................ 33

7.2. Model Accuracy.............................................................................................................. 34

7.3. Limitations to Calibration............................................................................................... 34

8. References Cited.................................................................................................................... 36

Temescal Basin GSP
Groundwater Model Report iii TODD GROUNDWATER
List of Tables (all tables follow text)
Table 1 Average Annual Groundwater Pumping
Table 2 Aquifer Properties
Table 3 Summary of Primary Calibration for The Temescal Basin Model (in Text) Table 4 Statistical Calibration by Well
List of Figures (all figures follow text)
Figure 1. Location and Topography of the Temescal Basin Figure 2 Rivers, Streams, Wetlands and Tributary Watersheds Figure 3 Land Use in 1990, 2018 and 2068
Figure 4 Average Annual Rainfall
Figure 5 Relationship of Rainfall to Infiltration
Figure 6 Rainfall to Runoff Calibration
Figure 7 Extent of MODFLOW Model Domain
Figure 8 Location of Channel Aquifer
Figure 9 Topographic Elevation of the Top of Model Layer 1 Figure 10 Bottom Elevation Distribution for Model Layer 1 Figure 11 Bottom Elevation Distribution for Model Layer 2 Figure 12 Bottom Elevation Distribution for Model Layer 3 Figure 13 Schematic Cross Sections to Illustrate Relative Model Layer Thicknesses Figure 14 Location of Faults Applied in MODFLOW Model
Figure 15 Location of Streams Applied in MODFLOW Model
Figure 16 Distribution of Specified Flux Boundary for Mountain Front Recharge Figure 17 Distribution of Evapotranspiration (ET) Zones Figure 18 Locations of Metered Municipal or Industrial Well Applied in MODFLOW Model Figure 19 Approximated Locations of Agricultural Pumping Wells Applied in MODFLOW Model Figure 20 Annual Groundwater Pumping Applied in MODFLOW Model Figure 21 Location of Recycled Water Recharge Ponds
Figure 22 Boundary Conditions Applied at the Basin Margins in the MODFLOW Model Figure 23 Distribution of Aquifer Property Zones for Layers 1, 2 and 3 Figure 24 Initial Groundwater Elevations Applied in the MODFLOW Model Figure 25 Location of Monitoring Wells Used for Calibration Figure 26 Scatter Plot Comparing Simulated to Measured Groundwater Levels Figure 27 Calibration Hydrographs Corona Wells 6, 7 and 7A HG-01 Figure 28 Calibration Hydrographs Corona Wells 8, 8A, 9, 9A Figure 29 Calibration Hydrographs Corona Wells 11, 12, 12A and 13 Figure 30 Calibration Hydrographs Corona Wells 14, 15, 16 and 17 Figure 31 Calibration Hydrographs Corona Wells 17A, 19, 22 and 23 Figure 32 Calibration Hydrographs Corona Wells 24, 25, 27 and 28 Figure 33 Calibration Hydrographs Corona Wells 29 and 31, Butterfield and 3S7W-27F6

Temescal Basin GSP
Groundwater Model Report iv TODD GROUNDWATER
Figure 34 Layer 1 Groundwater Elevations End of Simulation September 2018 Figure 35 Layer 3 Groundwater Elevations End of Simulation September 2018 Figure 36 Groundwater Elevations near Highest Levels in Layer 1 January 1997 Figure 37 Groundwater Elevations near Highest Levels in Layer 3 January 1997 Figure 38 Groundwater Elevations near Lowest Levels in Layer 1 August 2014 Figure 39 Groundwater Elevations near Lowest Levels in Layer 3 August 2014

Temescal Basin GSP
Groundwater Model Report 1 TODD GROUNDWATER

1. INTRODUCTION
The groundwater model was developed to support the Groundwater Sustainability Plan (GSP) for the Temescal Subbasin (Basin) of the Upper Santa Ana Valley Basin (Department of Water Resource [DWR] Groundwater Basin 8-002.09) and is prepared in accordance with Sustainable Groundwater Management Act (SGMA). For convenience, DWR Basin 8-002.09 (DWR 2016b) will be referred to as the Temescal Basin (Basin) in this memo.

1.1. SCOPE AND OBJECTIVE
SGMA effectively requires that groundwater modeling be used to demonstrate that a GSP will achieve sustainable basin operation. A previous model of the Temescal Basin model was updated and refined to simulate surface water and groundwater conditions for the entire Basin, update key parameters, match the DWR Basin boundary, and improve discretization, geologic layering and aquifer parameter distribution to reflect new information. The resulting model focuses on applicability to SGMA GSP regulations, including consistency with DWR Best Management Practices for surface water and groundwater modeling (DWR 2016a). This comprehensive groundwater model serves as a quantitative tool for computing Basin-wide water budgets and the effects of sustainability criteria and management actions.

1.2. SUMMARY OF PREVIOUS MODELS
A groundwater model of the Temescal Basin was previously developed to support development of the 2008 City of Corona Groundwater Management Plan (Todd and AKM, 2008). That model did not cover the entire extent of the Basin as defined by DWR and simulated the period from 1990 through 2004.

However, information regarding bedrock elevations, aquifer characteristics, layering, inflows and outflows served as a starting point for constructing the new model.

The development of the Temescal Basin Model utilized information from groundwater model from adjacent basins. The Model developed for this GSP also includes a portion of the Chino Basin to better simulate the interactions with the adjoining basin and assess conditions in the Prado wetlands area.

Information for the northern part of the model flow domain in the Chino Basin was based on documentation of the Chino Basin groundwater model o developed in 2015 and has been updated several times since then (WEI, 2015 2020). In the vicinity of the Arlington Gap, a groundwater model of the Riverside-Arlington Basin, which simulates conditions on the eastern side of the Arlington Gap, was used to help understand groundwater flow through Arlington Gap into the Temescal Basin (Geoscience, 2009, WRIME, 2010, 2011).

Temescal Basin GSP
Groundwater Model Report 2 TODD GROUNDWATER

2. BASIN GEOLOGY AND STRUCTURE
The following summarizes the hydrogeologic conceptual model (HCM) and groundwater conditions from the main body of the Temescal Basin GSP report. The HCM and groundwater conditions create a foundation for the technical aspects of the Basin’s hydrogeology necessary for model development. This section references figures and text from GSP Sections 3 and 4.

2.1. TEMESCAL BASIN
The Temescal Basin covers approximately 23,500 acres or 37 square miles of the southwest part of upper Santa Ana Valley in western Riverside County as shown on Figure 1. The following summarizes the physical description of the Temescal Basin and surrounding areas as described in the GSP.

2.1.1. Groundwater Basin
The Basin covers approximately 23,500 acres or 37 square miles of the southwest part of upper Santa Ana Valley in western Riverside County (Figure 1). The Basin is located between the Santa Ana Mountains to the west and a lower, parallel range of hills to the east.

The Basin has connection four other groundwater basins or subbasins defined by DWR Bulletin 118 (DWR 2016b). The Basin is separated from the Bedford-Coldwater Subbasin of the Elsinore Groundwater Basin (DWR Basin No. 8-004.02) to the south by a groundwater divide Eagle Canyon and Bedford Canyon. The Basin is connected to the east by a narrow body of alluvium through Arlington Gap to the Riverside-Arlington Subbasin of the Santa Ana Valley Groundwater Basin (DWR Basin No. 8-002.03). To the north, the Basin adjoins the Chino Subbasin of the Santa Ana Valley Groundwater Basin (DWR Basin No. 8-002.01) along a line that approximately follows the Santa Ana River. This boundary is permeable, and groundwater can flow in either direction between the two basins depending on their respective groundwater levels. To the west, the Basin has a narrow connection along the Santa Ana River with the Coastal Plain of Orange County Groundwater Basin (DWR Basin No. 8 001).

SGMA allows a groundwater basin to be subdivided into management areas if it facilitates sustainable management in areas within the Basin where groundwater conditions or water use and supplies are distinctly different. The Temescal Basin has not been divided into management areas; the GSP treats it as a single unit.

2.1.2. Physiography
Ground surface elevations at the surface of the Basin slope northward toward the Santa Ana River at a slope of 200-300 feet per mile. Elevations range from approximately 470 feet above mean sea level (msl) at the base of Prado Dam to approximately 1,500 feet above msl to the south (see GSP Figure 3-1). The tributary watersheds reach up to approximately 4,000 feet msl at the highest peak in the Santa Ana Mountain watersheds west of the Basin. Watersheds east of the Basin are significantly lower in elevation and rise only to about 1,600 feet.

2.1.3. Hydrology
The Basin covers a portion of the Santa Ana River watershed. Figure 2 shows the locations of the Santa Ana River, Temescal Wash, Prado wetlands area, minor streams and tributary watersheds to the

Temescal Basin GSP
Groundwater Model Report 3 TODD GROUNDWATER
Temescal Basin. The Santa Ana River roughly follows the northern edge of the Basin, flowing from east to west. Prado Dam impounds the river near the western edge of the Basin, where the river enters a canyon that passes through the Santa Ana Mountains to the coastal plain in Orange County. The largest surface waterway in the interior of the Basin is Temescal Wash, which originates near Lake Elsinore and flows through the Bedford-Coldwater Subbasin before entering the southern edge of the Temescal Basin. It continues north through the City of Corona and discharges into the Prado wetlands. A number of small streams enter the Basin from watersheds in the Santa Ana Mountains. Flow in all of them is ephemeral, and with the exception of Wardlow Wash at the northwest corner of the Basin, all of the small creek channels are lined beginning 1,500 to 6,100 feet downstream of the point where they enter the Basin. The Temescal Wash channel is also cement lined along about half of its length between the Basin boundary and the Santa Ana River.

2.2. REGIONAL GEOLOGY
The Basin is located within one of the structural blocks of the Peninsular Ranges of Southern California.

The Basin occurs in a linear low-lying block, referred to as the Elsinore-Temecula trough, between the Santa Ana Mountains on the west and the Perris Plain on the east (Todd and AKM 2008). The trough extends from Corona southeast approximately 30 miles and was formed along an extensive northwestsoutheast trending fault zone including the Elsinore, Chino, and related faults.

2.2.1. Geologic Units
The oldest rocks in the Basin crop out at the foot of the Santa Ana Mountains. These units are composed principally of volcanic (including the Santiago Peak Volcanics) and metamorphic rocks (including the Bedford Canyon Formation) of Jurassic and Cretaceous age. A thin rim of younger sedimentary units of Tertiary age also crops out along the mountain front generally lying between the Elsinore and Chino faults. This zone of sedimentary units broadens to the north and contains numerous mapped formations of Cretaceous and Tertiary age. The northeastern side of the valley is flanked primarily by granitic rocks of Cretaceous age. Erosion of these units has filled in the trough over time resulting in quaternary-age alluvial fan, channel, and other deposits making up the permeable portions of the Basin (USGS 2004, 2006).

The geologic map (see GSP Figure 3-5) shows the distribution of these units in the Basin (USGS 2004, 2006). The main surficial deposits on the floor of the Basin include younger and older alluvial fans deposited from the erosion of volcanic rocks and Bedford Canyon Formation to the west. These units prograde across the Basin to the northeast and are truncated by channel deposits along Temescal Wash.

2.2.2. Faults
The Basin was formed along an extensive northwest-southeast trending fault zone including the Elsinore, Chino, and related faults. The Elsinore and Chino fault zones bound the Basin on the west and trend along the mountain fronts. The Elsinore Fault Zone extends approximately 200 km from Baja California north to the Corona area. It passes through the western margin of the Basin. Some fault traces are inside the Basin and may function as partial barriers to groundwater flow (see GSP Figure 3-5).

2.2.3. Definable Basin Bottom
The Basin bottom is defined by bedrock, which is shallow around the perimeter and deep in the center.

Depth to bedrock ranges in depth from 10 feet to approximately over 1,000 feet (see GSP Figure 3-11).

Temescal Basin GSP
Groundwater Model Report 4 TODD GROUNDWATER
The greatest Basin thickness is in the central-west part of the Basin (see GSP Figure 3-7). The formation of a trough along the Elsinore-Chino Fault zone is indicated by the asymmetric basin geometry.

Bedrock is much shallower in the eastern portion of the Basin, however there is a slight deepening near the Arlington Gap (see GSP Figure 3-8). Here, unconsolidated sediments are approximately 250 feet thick. This area is interpreted to have been eroded by a branch of the ancestral Santa Ana River. The Basin is only about 100 feet thick in the Norco area but over 1,000 feet thick beneath the Santa Ana River, where the Basin adjoins the Chino Basin.

2.3. GROUNDWATER CONDITIONS
Understanding the groundwater conditions is important in development of the surface water and groundwater models. A summary of the discussion of the groundwater conditions and water balance based on the model results is provided in GSP Sections 4 and 5 is provided below.

2.3.1. Basin Aquifer
Three aquifers provide water supply to wells within the Basin. These include the Channel Aquifer, the Alluvial Fan aquifers and the Sandstone Aquifer (Todd and AKM 2008). Of these three aquifers, the Channel Aquifer is the only principal aquifer as it the most productive aquifer and provides most of the groundwater supply in the Basin. The Alluvial Fan and Sandstone Aquifers have historically been used to a lesser extent than the principal aquifer. The combined Alluvial Fan and Sandstone Aquifers are referred to as the Secondary Aquifers within the Basin.

The Channel Aquifer is the principal aquifer in the Basin. This aquifer is a package of relatively homogeneous and highly permeable sands up to 200 feet thick that have been encountered in many of the Corona wells in the northern half of Basin. This sand package is interpreted as channel deposits of an ancestral arm of the Santa Ana River and, as such, has been referred to as the Channel Aquifer (Todd and AKM 2008). The alignment of the aquifer suggests that an ancestral river channel had entered the Basin at Arlington Gap, eroding the sedimentary units and possibly older alluvial fan deposits in the area.

Permeable channel sands were deposited in the eroded channel over time. From the Arlington Gap, the Channel Aquifer trends northwest toward Prado Dam.

The Alluvial Fan Aquifer is composed of both older and recent alluvial fans that have been deposited through time along the mountain front on the western edge of the Basin. These fans have prograded across the Basin from west to east (see GSP Figure 3-5). Although these deposits are relatively thick, the entire unit is heterogeneous and cannot be considered one single aquifer. Rather, sand lenses within the deposits collectively form the Alluvial Fan Aquifers. Lithologic data from wells are insufficient to map out the extent of the aquifers or characterize the deposits. Limited data indicate relatively fine-grained textures throughout much of the area, especially with depth (Todd and AKM 2008).

The Sandstone Aquifer is composed of the older sedimentary units underlying the alluvial Basin that provide sufficient well yields to categorize them as aquifers. Although generally grouped with other bedrock units, the subsurface sedimentary rocks of Tertiary age in the northeast Basin area contain sandstone layers that are screened in several Corona wells. Due to the limited production, small areal extent, increasing depths, and relatively low permeability in most areas, the Sandstone Aquifer is not considered a primary source of water supply.

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2.3.2. Basin Boundaries
The Temescal Basin as defined by DWR is bounded on the west by the Santa Ana Mountains and the east by low-lying El Sobrante de San Jacinto and La Sierra hills. The northeastern arm of the Temescal Basin, referred to as the Norco area, consists of relatively low permeability alluvium and bedrock residuum flanked on the east and west by bedrock outcrops. The Basin is connected to four adjacent groundwater basins (Figure 1) defined by DWR Bulletin 118 (DWR 2016b). These include:
• The boundary with the Chino Basin (DWR Basin No. 8-002.01) to the north is generally marked by the Santa Ana River and a series of low-lying hills in the Norco area. • Groundwater from the Riverside-Arlington Basin (DWR Basin No. 8-002.03) flows into the Basin through the Arlington Gap. The Arlington Gap is a narrow restriction along the eastern side of the Basin north of the Temescal Wash.
• The boundary with the Coastal Plain of Orange County Basin (DWR Basin No. 8-001) is a narrow canyon where the Santa Ana River exits the Temescal Basin to the Coastal Plain of Orange County Basin.
• The southern boundary of the Basin is the Bedford-Coldwater Subbasin of the Elsinore Groundwater Basin (DWR Basin No. 8-004.02). The boundary is located at the Bedford Canyon.

Generally, there is little to no groundwater flow along this boundary at the eastern portion where it borders the alluvium along the Temescal Wash.

The remaining lateral boundaries of the Basin are formed by contacts with bedrock units. The entire western Basin boundary and much of the eastern boundary of the Basin are contacts between Basin sedimentary units and upland bedrock outcrops (Todd and AKM 2008).

2.3.3. Recharge and Discharge Areas
Recharge to the Basin occurs primarily from stream percolation, wastewater discharge and deep percolation of rainfall and irrigation water, and to a lesser extent from pipe leaks and subsurface inflow from bedrock areas and other basins, as shown in GSP Table 5-4. Recharge from streams occurs along the unlined reach of Temescal Wash above Temescal Lake and the unlined reaches of tributary streams along the western edge of the Basin. Recharge associated with wastewater occurs when treated wastewater is discharged to ponds. Deep percolation from irrigation includes historical agricultural irrigation as well as current urban irrigation.

Large amounts of runoff from the mountains flows into channels and the shallow subsurface at the edges of the Basin and then into and through the Basin. Stream flows are flashy, and during brief highflow events, the amount of stream recharge is limited by the percolation capacities of the unlined channel reaches upstream of the stormwater detention basins. The creek channels are lined with cement downstream of the detention basins.

Return flows are those portions of applied water (e.g., landscape irrigation) that are not consumed by evapotranspiration and hence return to the groundwater system through deep percolation or infiltration. Return flows associated with urban, industrial, and agricultural water uses all have the potential to contribute to recharge to the Basin (Todd and AKM 2008).

Discharge from the Basin is primarily from groundwater pumping. Smaller outflows are to the Santa Ana River near the Prado wetlands, evapotranspiration in the wetlands, and subsurface outflow to the Chino Basin (see GSP Table 5-4).

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2.3.4. Primary Groundwater Uses
The primary groundwater uses in the Basin are municipal pumping, with limited private pumping for small water system, commercial, industrial and residential users. Groundwater use estimates are included in GSP Section 5 (Water Budget). The Channel Aquifer is primarily used for municipal water supply. Most of the pumping in this area is from wells owned and operated by the City of Corona, with some additional pumping by small community water system, small commercial users and aggregate mines. Until the 1990s, there was significant agricultural pumping to irrigate citrus orchards.

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3. RAINFALL-RUNOFF-RECHARGE MODEL
A rainfall-runoff-recharge model developed by Todd Groundwater was used to prepare estimates of groundwater recharge from rainfall, irrigation, bedrock inflow, and pipe leaks. It also generated the estimates of groundwater use for agricultural irrigation and flows in ungauged streams tributary to or within the basin. Several commercially available software programs were used to prepare model input and evaluate model output, such as Microsoft Excel and ArcGIS. Finally, the rainfall-runoff-recharge model and several pre-processing utility programs were developed in the Fortran 90 programming language by Todd Groundwater.

3.1. APPROACH
The rainfall-runoff-recharge model is built around a soil moisture balance of the root zone, which is simulated continuously using daily time steps for the 29-year calibration period. Numerous variables are involved in the physical processes of rainfall, interception, runoff, infiltration, root zone soil moisture storage, evapotranspiration, irrigation, shallow groundwater storage, recharge of deeper regional aquifers from shallow groundwater, and lateral flow of shallow groundwater into streams. Accordingly, the groundwater basin and tributary watersheds were divided into small recharge zones over which the most influential variables were relatively homogeneous. The daily water balance was then simulated for each zone, and the results aggregated geographically to cells in the groundwater model grid and temporally to the model stress periods.

The rainfall-runoff-recharge model provides several benefits to the groundwater modeling effort:
• It represents the hydrological processes with governing equations that reflect the actual physical processes, at least in a simplified way. This allows sensitivity or suspected errors to be traced to specific assumptions and processes.
• It enforces the principle of conservation of mass on the recharge and stream flow values.

Beginning with rainfall, all water mass is accounted for as it moves through the hydrological system.
• It allows additional data sets to be included in model calibration. In tributary watersheds with gauged stream flow data, measured flows can be compared with simulated flows, which consist of the sum of direct runoff and shallow-groundwater seepage to streams. Simulated irrigation frequency can be compared with actual grower practices, and applied irrigation amounts can be compared with water delivery data recorded by the District. Simulated urban irrigation amounts can be compared with seasonal variations in measured urban water use, which are primarily related to urban irrigation.
• It provides estimates of stream flow in ungauged tributary streams, as well as runoff from valley floor areas within the active model domain.
• It provides estimates of inflow from bedrock and/or upland areas adjacent to the active model domain and constrains the amounts of inflow according to the water balance for each tributary watershed.
• It simulates the effects of runoff from impervious surfaces in urban areas, either to storm drainage systems or to adjacent pervious soils.
• It simulates changes in land use over the 29-year calibration period and the resulting changes in recharge and irrigation demand.

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• It combines and parses all of these flows—plus estimated recharge from leaky water and sewer pipes—into recharge values by model cell and stress period in the format required by MODFLOW.

The following sections describe the input data sets and the assumptions and governing equations used to simulate each hydrologic process included in the rainfall-runoff-recharge model.

3.2. LAND USE AND RECHARGE ZONES
Recharge zones were developed by intersecting and editing numerous maps in GIS. The starting point was a map of the Temescal Basin and the boundaries of all surrounding watersheds that flow into it. The Basin and tributary watersheds were then divided into numerous polygons reflecting land use as of 1990 and changes in land use since then. Land use was delineated into 13 categories based on DWR land use maps for Riverside County from 1993 and 2000, a statewide crop map developed by LandIQ for DWR in 2014 and Google Earth historical aerial imagery available for 1990-2018. The primary change in land use has been urbanization of undeveloped (natural vegetation) areas. Polygons were delineated to represent the locations of changes in land use so that a single, fixed set of polygons could accurately represent the evolution of land use by changing the use type of a polygon beginning in the year that land use changed. Additional divisions of polygons were made on the basis of soil texture, annual rainfall and watershed. This resulted in a total of 224 polygons ranging in size from 2 to 4,529 acres. A map of the zones and their land uses in 1990 and 2018 is shown in Figure 3.

Land use in each zone was assigned to one of sixteen categories (see GSP Table 5-2). Each land use category is further divided into irrigated, non-irrigated and impervious subareas. These are not explicitly mapped but are expressed as percentages of total zone area. Citrus orchards irrigated with groundwater were common in the Basin in the early 1990s, but except for one small grove those have all been replaced by urban development. Natural land cover categories are grassland, shrubs/trees, dense riparian, sparse riparian and open water. Developed land uses are residential, low-density residential, turf, commercial, industrial and vacant. The natural and developed land uses were mapped by inspection of Google Earth aerial photography. The categories are listed in GSP Table 5-2 along with their total acreages in 1990, 2018 and 2068 (estimated) in the groundwater basin management areas and tributary watersheds.

3.3. RAINFALL
The distribution of average annual rainfall over the basin and tributary watersheds was obtained from PRISM climate modeling (http://www.prism.oregonstate.edu/ ). Annual precipitation varies from 11 inches in the Norco area to about 14 inches at the south end of the Basin. It increases to about 21 inches at the top of the highest tributary watershed in the Santa Ana Mountains to the west. Figure 4 shows the average annual rainfall distribution across the Temescal Basin and its surrounding watersheds. Each recharge zone was assigned an average annual rainfall value based on its location.

The surface hydrology model requires daily rainfall as one of two transient inputs. Daily rainfall for the Elsinore station was used for this purpose, with missing values supplied by correlation with rainfall at the Riverside Fire Station and Claremont-Pomona Stations, both of which also have long periods of record. Daily rainfall for each recharge zone was calculated as Elsinore daily rainfall multiplied by the ratio of zonal average-annual rainfall to Elsinore average-annual rainfall.

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3.4. INTERCEPTION
Plant leaves intercept some of the rain that falls from the sky, and the amount is roughly proportional to the total leaf area of the vegetation canopy. The estimated interception on each day of rain ranged from zero for industrial, idle and vacant land uses, to 0.03 inch for turf and 0.06 inch for trees in full leaf.

These estimates were inferred from published results of interception studies (Viessman and others, 1977). For each day of the simulation, rainfall reaching the land surface (throughfall) is calculated as rainfall minus interception. Interception storage is assumed to completely evaporate each day and is not carried over from one day to the next.

3.5. RUNOFF AND INFILTRATION
Most throughfall infiltrates into the soil, but direct runoff occurs when net rainfall exceeds a certain threshold. The threshold at which runoff commences and the percent of additional rainfall that runs off are significantly influenced by a number of variables, including soil texture, soil compaction, leaf litter, ground slope, and antecedent moisture. These factors can be highly variable within a recharge zone, and data are not normally available for them. Also, the intercept and slope of the rainfall-runoff relationship depend on the time increment of analysis. Most analytical equations for infiltration and runoff apply to spatial scales of a few square meters over periods of minutes to hours (Viessman and others, 1977).

They are suitable for detailed analysis of individual storm events. The curve number approach to estimating runoff also applies to single, large storm events. It is not suitable for continuous simulation of runoff over the complete range of rainfall intensities (Van Mullen and others, 2002). The approach used in the rainfall-runoff-recharge model is similar but less complex than the approach used in popular watershed models such as HSPF (Bicknell and others, 1997).

In the rainfall-runoff-recharge model, daily infiltration is simulated as a three-segment linear function of throughfall, and throughfall in excess of infiltration is assumed to become runoff. The general shape of the relationship of daily infiltration to daily net rainfall is shown in Figure 5 (upper graph). Below a specified runoff threshold, all daily throughfall is assumed to infiltrate. Above that amount, a fixed percentage of throughfall is assumed to infiltrate, which is the slope of the second segment of the infiltration function. Finally, an upper limit is imposed that represents the maximum infiltration capacity of the soil. The runoff threshold, the percentage of excess net rainfall that infiltrates, and the maximum daily infiltration capacity were assumed to vary by land use and were among the variables adjusted for model calibration. The runoff threshold ranged from 0.2 inches per day (in/d) for unpaved areas in industrial and commercial zones to 1.0 in/d for turf and natural vegetation areas. The infiltration percentage for excess rainfall ranged from 60 percent in commercial and industrial areas to 94 percent in areas of natural vegetation. The maximum daily infiltration was set to 2.5 in/d in upland tributary areas and 4 in/d for zones overlying the Basin. These values were selected on the basis of calibration, although results were not very sensitive to this parameter.

The above parameter values are for soils that are relatively dry. Infiltration rates decrease as soils become more saturated. This phenomenon led to the development of the Antecedent Runoff Condition adjustment factor for rainfall-runoff equations (Rawls and others, 1993). However, application of the concept has been focused on individual storm events. For the purpose of the rainfall-runoff-recharge model, the adjustment provides a means of simulating empirical observations that a given amount of rainfall produces less runoff at the beginning of the rainy season when soils are relatively dry than at the end of the rainy season when soils are relatively wet. This effect is included in the recharge model as a multiplier that decreases the estimated infiltration as soil saturation increases. This multiplier is applied to the runoff threshold, the infiltration slope and the maximum infiltration rate. The multiplier

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decreases from 1.0 when the soil is dry to a user-selected value between 1.0 and 0.60 when the soil is fully saturated (lower graph in Figure 5). A low value has the effect of decreasing infiltration (and potential groundwater recharge) toward the end of the rainy season or in very wet years, and also to increase simulated peak runoff during large storm events. The multiplier under saturated conditions was assumed to be 0.75 for the Temescal rainfall-runoff-recharge model.

Runoff from impervious surfaces was assumed to equal 100 percent of rainfall. Runoff that flows into a storm drain system (known as “connected impervious runoff”) contributes to stream flow but not groundwater recharge. However, runoff from some impervious surfaces flows onto adjacent areas of pervious soils (“disconnected impervious runoff”). The surface hydrology model treats this type of runoff as if it were a large increment of additional rainfall where it flows over or ponds on the pervious soils. The excess water can quickly saturate the soil and initiate deep percolation. The model incorporates this process by means of a variable representing the fraction of impervious runoff that becomes deep percolation. Data and literature values are not available for this variable. It was estimated to be 20 percent in residential, commercial and industrial areas and 80 percent in low-density residential areas.

3.6. ROOT ZONE DEPTH AND MOISTURE CONTENT
The storage capacity of the root zone equals the product of the vegetation root depth and the available water capacity of the soil. The available water capacity for each recharge zone was a depth-weighted average for the dominant soil type, as reported in the soil survey (NRCS, 2015). Root depth is a complex variable. Except for cropland, vegetation cover typically consists of a mix of species with different root depths. At a very local scale, roots are deepest directly beneath a plant and shallower between plants.

Root density and water extraction also typically decrease with depth within the root zone. To complicate matters, root depth is somewhat facultative for some plants, which means that roots will tend to grow deeper in soils with low available water capacity, such as sands. Finally, root depth in upland watershed areas can be restricted by shallow bedrock.

The root depth selected for each recharge zone essentially represents an average of all these factors.

Simulated recharge and stream base flow are both quite sensitive to vegetation root depth, and values were adjusted during the joint calibration of the rainfall-runoff-recharge model and the groundwater flow model. Separate root depths were specified for irrigated and non-irrigated vegetation in each recharge zone. Root depths for turf and crops were required to be the same in all zones. In upland watersheds root depth can be affected by the depth to bedrock, which is often shallow. Outflow from individual tributaries flowing into the basin is not gaged, and uniform rooting depths for grass and shrubs/trees were used throughout all of the watersheds.

3.7. EVAPOTRANSPIRATION
Evapotranspiration is affected by meteorologic conditions, plant type, plant maturity, and soil moisture availability. All of these factors are included in the rainfall-runoff-recharge model. The evaporative demand created by meteorological conditions is represented by reference evapotranspiration (ETo).

Numerous equations have been developed over the years relating ETo to solar radiation, air temperature, relative humidity and wind speed. For the purposes of this study, daily values of ETo were obtained from a microclimate station in Temecula (about 20 miles south of the Basin) that is part of the California Irrigation Management Information System (CIMIS) network.

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Vegetation factors are lumped into multipliers called crop coefficients. Reference ET is the amount of water evapotranspired from a broad expanse of turf mowed to a height of 4-6 inches with ample irrigation. ETo is multiplied by a crop coefficient to obtain the actual ET of a different crop or vegetation type at a particular stage in its growth and development. Although primarily used for agricultural crops, crop coefficients can also be applied to urban landscape plants and natural vegetation. The only agricultural crop in the Basin is citrus trees, which have a crop coefficient that ranges from 0.5 in winter to 0.91 in mid-summer (U.N. Food and Agriculture Organization, 2006). Irrigated landscaping was assumed to consist primarily of turf, for which a crop coefficient of 0.8 was used in all months (Snyder and others, 2007). Non-irrigated natural grassland consists of annual grasses that go dormant in summer once soil moisture has been depleted. A crop coefficient of 1.0 was assigned in all months, but actual ET decreases to zero as the grasses lower soil moisture to the wilting point in summer. Natural shrubs/trees were assigned a crop coefficient of 0.8 year-round. Those perennial species have deeper roots and do not tend to fully deplete root zone soil moisture during a single dry season (Blaney and others, 1963).

Many riparian phreatophytes are deciduous, and a crop coefficient of 0.75 was assigned for winter months to reflect a reduced leaf area index. Their tall stature and linear distribution within an arid landscape raises the crop coefficient in summer months, and a coefficient of 1.10 was assigned to reflect those factors.

3.8. IRRIGATION
Evapotranspiration gradually depletes soil moisture, and for irrigated areas the rainfall-runoff-recharge model triggers an irrigation event whenever soil moisture falls below a specified threshold. The amount of applied irrigation water is equal to the volume required to refill soil moisture storage to field capacity, divided by the assumed irrigation efficiency. An irrigation threshold equal to 70 percent of maximum soil moisture storage was used for citrus, and a threshold of 0.8 was used for urban landscaping. This variable primarily affects the frequency of irrigation; a higher threshold results in more frequent irrigation but approximately the same total amount of water applied annually. Ten percent of water applied to citrus was assumed to percolate past the root zone, and 15 percent was assumed for urban irrigation. This reflects nonuniformity of applied water, such as uneven overlap of sprinkler spray areas.

There are additional sources of irrigation inefficiency, such as evaporation of sprinkler spray mist and sprinkler overspray or runoff onto impervious surfaces in urban areas. Thus, total irrigation efficiency is less than 90 percent for citrus and 85 percent for urban landscaping. Total efficiency was used to estimate applied water, but only the deep percolation component was used to estimate deep percolation. Urban irrigation in the Basin is supplied by municipal water purveyors, and irrigation use is included in their metered deliveries.

The rainfall-runoff-recharge model was only used to estimate groundwater pumping for citrus irrigation.

Because irrigation is assumed to completely refill soil moisture storage and is less than 100 percent efficient, simulated soil moisture exceeds capacity immediately following an irrigation event. The excess is assumed to become deep percolation beneath the root zone.

3.9. DEEP PERCOLATION FROM ROOT ZONE TO SHALLOW GROUNDWATER The surface hydrology model updates soil moisture storage each day to reflect inflows and outflows.

Rainfall infiltration and applied irrigation water are added to the ending storage of the previous day, and ET is subtracted. If the resulting soil moisture storage exceeds the root zone storage capacity, all of the excess is assumed to percolate down from the root zone to shallow groundwater on that day. In

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modeling parlance, this is known as a “bathtub model”; vertical unsaturated flow and preferential flow through cracks and root tubes in the soil are not considered.

3.10. MOVEMENT OF SHALLOW GROUNDWATER TO DEEP RECHARGE AND

STREAM BASE FLOW
A shallow groundwater storage component may not be part of all groundwater systems, but its presence is sometimes indicated by groundwater hydrographs and stream base flow. In upland watersheds, for example, the shallow groundwater reservoir is what supplies base flow to streams.

Without it, simulated stream flow consists of large flows occurring only on rainy days. Physically, it represents the overall permeability and storage capacity of deep soil horizons and bedrock fractures beneath hillsides bordering a gaining stream. It allows the integration of shallow and deep, fast and slow flow paths between the point of rainfall infiltration and the stream. In valley floor areas with flat terrain and deep deposits of unconsolidated basin fill, the presence of a shallow groundwater system is sometimes evident in a lack of response of deep well hydrographs to rainfall recharge events or even wet versus dry years. The shallow zone in that case attenuates the pulses of recharge percolating beneath the root zone into a relatively steady recharge flux, and there may be little outflow to streams.

In the surface hydrology model, the only inflow to shallow groundwater storage is deep percolation from the root zone. There are two outflows: laterally to a nearby creek and downward to the regional groundwater flow system. Outflow to streams is specified as a certain percentage of current groundwater storage, which results in a first-order logarithmic recession of stream base flow, consistent with gaged stream flows. Outflow to the regional groundwater system is simulated as a constant downward flux. This is consistent with flow across confining layers in which the vertical head gradient is near unity. Both outflows are calculated and subtracted from shallow groundwater storage each day.

They continue until the storage has been exhausted, resuming whenever a new influx of deep percolation from the root zone arrives. There is no assumed maximum capacity of shallow groundwater storage.

The two parameters defining shallow groundwater flow are the recession constant for flow to streams and the constant downward flow rate for deep recharge. Both of these are obtained by calibration. The recession constant can generally be calibrated by matching simulated to measured stream base flow in gaged watersheds. The deep recharge rate can be used to adjust the long-term partitioning of shallow groundwater mass into base flow versus recharge.

The shallow groundwater component of the surface hydrology model is simple but adequate to capture the fundamental behaviors of logarithmic stream base flow recession and attenuated deep recharge.

Other watershed models invoke more complex systems of storage and flow to simulate these processes.

For example, the Precipitation and Runoff Modeling System (PRMS) developed by the U.S. Geological Survey includes a total of seven storage components between the point where a raindrop reaches the ground and the stream into which it ultimately flows (Markstrom and others, 2015). This larger number of components and parameters enables relatively detailed matching of observed stream flow hydrographs but is unnecessarily complex for the purposes of groundwater modeling.

3.11. EVAPOTRANSPIRATION BY RIPARIAN VEGETATION
In locations where the water table is shallow, some plants (phreatophytes) can extract water directly from the water table to meet evaporative demand. The rainfall-runoff-recharge model was used to estimate the amount that would be drawn from the water table if a shallow water table were present.

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The potential use of groundwater by phreatophytes was assumed to equal the ET demand of the vegetation minus the amount that could be supplied by soil moisture. In practice, this was accomplished by temporarily simulating the vegetation as if it were irrigated using the rainfall-runoff-recharge model, then using the simulated irrigation rates as the maximum rate of withdrawal by roots from the water table. This rate of groundwater use is thought to decrease with increasing depth to the water table because fewer shrub and tree roots are able to reach the water table and the energetics of withdrawing the water become less favorable. The use of groundwater decreases from the maximum rate when the water table is at the land surface to zero when the water table is 20 feet or more below the ground surface. These calculations are applied at model cells where aerial photographs indicate the presence of dense, lush riparian vegetation, which is a sign of phreatophytic water use. These calculations were also made using the MODFLOW evapotranspiration (EVT) module.

3.12. GROUNDWATER INFLOW
Groundwater inflow into the basin from adjacent uplands—also called mountain front recharge—is difficult to estimate. If the basin is bounded by igneous or metamorphic rocks with very limited groundwater flow through fractures, it can be reasonable to assume that inflow from bedrock is negligibly small. If the bedrock is fractured, the total amount of inflow across the long “no-flow” boundaries on the east and west sides of the Basin can be cumulatively significant. Subsurface inflow across those boundaries was estimated using the rainfall-runoff-model results for the tributary watersheds. By this method, the estimates must be consistent with conservation of mass in the watersheds; that is, with the estimates of rainfall, ET, and surface outflow. The resulting estimates are still highly uncertain, however, because groundwater outflow from the watersheds—and surface outflow, too, for that matter—are both small compared to the two largest flows in the watershed water balances: rainfall and evapotranspiration. Thus, a small error in the estimate of either of those flows can result in a large error in groundwater outflow.

Ultimately, groundwater flows produced by the rainfall-runoff-recharge model were calibrated based on their effects on simulated groundwater levels at nearby wells within the basin and on the simulated amount of stream base flow exiting the watersheds. The initial groundwater inflow estimates were generally too high. The estimates were lowered primarily by increasing the estimated root depth of natural vegetation in the watersheds, which is highly uncertain due to the effects of shallow bedrock on rooting depth.

Groundwater inflow from tributary watersheds was smoothed over time to reflect attenuation of recharge pulses that occur during wet months and wet years as they gradually flow through long, relatively slow flow pathways. Smoothing was accomplished by a moving average of simulated groundwater recharge in the tributary areas over the preceding 2 to 10 years. This range represents local variability that was indicated by rates of recession in stream base flow and groundwater levels near the basin boundary during prolonged droughts. The final estimate of average annual groundwater inflow during the calibration period was 5,400 to 7,200 AFY under normal climatic conditions.

3.13. CALIBRATION OF RAINFALL-RUNOFF-RECHARGE MODEL
Parameters in the rainfall-runoff-recharge model were jointly calibrated with the groundwater model.

The total amount of dispersed recharge and annual variations in recharge influence simulated groundwater levels, and parameters in the rainfall-runoff-recharge model were adjusted to improve the fit between measured and simulated groundwater hydrographs. The rainfall-runoff-recharge model was also calibrated based on a comparison of measured and simulated daily stream flow at two stream

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gages on Temescal Wash, one below Lee Lake at the upstream end of the Bedford-Coldwater Subbasin (Temescal Wash at Corona Lake; USGS 11071900) and one at Main Street downstream of the wastewater treatment plant in Corona (Temescal Creek above Main Street at Corona; USGS 11072100).

Characteristics and model parameters for that watershed were assumed to also apply to similar watersheds along the western edge of the Basin. Unfortunately, the gage began operation in 2019, which is after the 1990-2018 model simulation period. Nevertheless, the general pattern of flow peaks and base flow recession simulated in prior years was similar to the gaged pattern in 2019-2020, as shown in Figure 6.

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4. NUMERICAL GROUNDWATER MODEL DEVELOPMENT
The numerical model incorporated the hydrogeological data from the basin and hydrologic model and is capable of simulating historical and future conditions. The following section describes the development of each of the components in the MODFLOW model.

4.1. GENERAL APPROACH
The Temescal Basin Model is a numerical groundwater model, which is a mathematical description of the hydrogeological conceptual model (Bear and Verruijt, 1987). The advantage of a numerical model is that, once in a mathematical format, the model quantitatively combines data on basin geometry, aquifer properties, recharge, and discharge to simulate changes in groundwater elevations and calculate the water balance over time.

The Temescal Basin Model is setup to represent the physical features that influence groundwater flow including the geology, hydrology and climate. Each of these features is mapped onto a model grid that represents the vertical and horizontal distribution of parameters over the Basin based on the hydrogeological conceptual model. The parameters can also be varied through time over a defined base period to represent seasonal variations in precipitation, streamflow and groundwater pumping. A more detailed discussion of how each of these parameters was developed and entered into the Temescal Basin Model is summarized below.
• Model Setup - representation of the physical groundwater basin • Boundary Conditions – representation of the inflows and outflows from outside of the model • Aquifer Properties – representation of the flow characteristics of the aquifer • Initial Conditions – representation of groundwater conditions prior to the model period The model development was focused on the HCM, as described in GSP Sections 3 and 4, with emphasis on defining boundary conditions and flow paths. Aquifer parameters were assigned on a subregional basis within each model layer to represent reasonable aquifer properties for the aquifer being simulated.

4.2. MODEL SETUP
The model also incorporates spatial distribution of the physical features of the Basin and the temporal distribution of time-varying parameters such as precipitation and recharge. The following describes the basic components required to construct a numerical model.

4.2.1. Model Code Selection
The model setup utilizes the MODFLOW modeling code developed by the United States Geological Survey (USGS). The Temescal Basin Model uses MODFLOW-NWT (Niswonger et. al., 2011), which is a standalone version of MODFLOW-2005 (Harbaugh, 2005) that includes an advanced mathematical solver that provides a more robust solution to complex conditions such as rewetting of dry model cells, unconfined conditions and groundwater-surface water interactions. These features improve the ability of the Model to evaluate complex groundwater-surface water interactions and projects to increase future groundwater levels in the Basin.

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4.2.2. Base Period
The Temescal Basin Model is setup using water years that run from October through to the following September to capture the cause and effect relationship on groundwater levels of wintertime rain and subsequent summertime groundwater pumping. The model simulates the 29-year base period from October 1989 through September 2018 to represent Water Years (WY) 1990 through 2018. This retains the starting date of prior models, which coincides with the beginning of some key data sets and also the beginning of the period of rapid land use conversion from agricultural to urban. The ending year is the most recent year for which all necessary model input data were available. The 29-year simulation period is desirable for model calibration purposes because it includes a wide range of hydrologic and water use conditions, including wet periods, droughts, changes in groundwater pumping and implementation of lake management measures.

To simulate this base period, the model is subdivided into time intervals termed stress periods. For each water year, monthly stress periods were defined to provide the ability of the model to evaluate temporal at a monthly scale. For the base period, a total of 348 stress periods were defined. Timedependent parameters, such as groundwater pumping or precipitation recharge, are assigned to for each stress period.

Conditions during the stress period are constant, but parameters can be varied from stress period to stress period. A stress period can be subdivided into shorter time periods, or timesteps, to allow for more temporal resolution within each stress period to help with model convergence. For the Temescal Basin Model, each stress period was simulated using three (3) timesteps. MODFLOW calculates the groundwater elevations and water balance for each time step. The model results provide the groundwater elevations for the final timestep of each stress period, and the summation of the water balance changes for all timesteps for each stress period.

4.2.3. Model Domain and Grid
MODFLOW requires the application of a rectangular grid that encompasses the entire area, or domain, that will be modeled. The model grid forms the mathematical framework for the model. Each grid cell has to be populated with aquifer properties. Physical features such as streams and wells are mapped onto the model grid. Using this information, the MODFLOW model calculates a groundwater elevation at each model grid cell for each timestep. The density of model grid cells is what defines the resolution of the model in resolving drawdown and other hydrologic effects.

The model domain covers all of the Temescal Basin and a portion of the Chino Area in the Prado area (Figure 7). The Temescal Basin occupies about 75 percent, approximately 37 square miles, of the southern model domain. The extent of the model domain for the Temescal Basin Model is shown on Figure 7. A portion of the Chino Basin was included in the Temescal Basin Model domain to allow for a more natural boundary along the Santa Ana River and Prado wetlands area. The boundary with the Chino Subbasin (DWR Basin No. 8-2.01) to the north is generally marked by the Santa Ana River and a series of low-lying hills in the Norco area. The northern boundary was set at a distance sufficient far so that the assigned boundary condition would not affect groundwater conditions in the Temescal Basin.

The Temescal Basin Model consists of 568 rows, 480 columns and 3 layers. The rows and columns have a uniform spacing of 100 feet. Each 100-foot square represents a model cell. MODFLOW calculates one groundwater level for the center point of each grid cell for each timestep. The total number of grid cells in the Temescal Basin Model is 817,920 cells, of which 425,304 are active cells where MODFLOW calculates a groundwater levels. The active areas, which represent the area within the groundwater

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basin where groundwater elevations are simulated, covers approximately 35,526 acres. Areas outside of the Basin are represented as no-flow cells where MODFLOW does not perform calculations.

4.2.4. Model Layers
The model layers represent the geologic units that compose the Principal and Secondary Aquifers of the Basin based on the geology and HCM presented in GSP Section 3 and summarized in Section 2. Model layers provide vertical resolution for the model to simulate variations in groundwater elevation, aquifer stresses, and water quality with depth. The model layers are based on an evaluation of the following data sets:
• Surficial geology,
• Faulting,
• Lithologic borehole logs.
• Well construction logs, and
• Previously completed local hydrogeologic conceptualizations and cross sections.

This information was collected and translated into a unified GIS compatible database structure for cross section construction and geographic evaluation. This approach allows any hydrostratigraphic structures relevant to groundwater flow in the Basin to be easily translated from GIS for use in other formats.

For the Temescal Basin Model, three model layers were defined to simulate hydrogeologic characteristics of the principal and secondary aquifers within the Temescal Basin. The model layers are numbered from 1 through 3 from top to bottom. In the Temescal Basin, three model layers were defined that represent the following geologic units:
• Model Layer 1 – Channel Aquifer (Principal Aquifer)
• Model Layer 2 – Alluvial Fan Aquifer (Secondary Aquifer), and • Model Layer 3 - Sandstone Aquifer (Secondary Aquifer) Figure 8 shows the general outline of the Channel Aquifer within the Temescal Basin. The top of Model Layer 1 represents the topography that is based on topographic elevation points every 10 meters were extracted from the National Elevation Dataset (http://ned.usgs.gov) throughout the model domain Figure 9.

The model layers represent the aquifers within the Temescal Basin. Figures 10 through 12 show the areal extent and bottom elevation of each of the model layers over the entire model domain. Figure 13 shows two cross section help to illustrate the shapes and relative thicknesses of three model layers in the Temescal Basin. These cross sections follow along the model grid with the upper cross section on Figure 13 located along model-grid row 262 and the lower cross section on Figure 13 located along model-grid column 322 (Figure 8). These The following discussion provides a summary of the geologic units represented by each model layer in accordance with the HCM.

Model Layer 1 represents the Channel Aquifer in the Temescal Basin, which is the primary water supply unit in the Basin (Todd and AKM 2008) where the larger wells are completed. The alluvial deposits are a mix of interlayered gravels, sands, silts, and clays resulting from alluvial fan and fluvial processes. Model Layer 1 ranges up to 200 feet thick. Alluvial aquifer materials are present in other parts of this hydrologic area, but their extent and production capacity are uncertain. In these areas, Model Layer 1 represents a relatively thin layer that is rarely saturated. The extension of Model Layer 1 is a requirement of MODFLOW to provide for continuity across the model domain. In these areas, Model Layer 1 has a minimum thickness of ten feet, and is conceptualized as the soil and shallow unconsolidated sediments that overlie the older alluvial fan and consolidated sedimentary geologic units.

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Model Layer 2 represent the Alluvial Fan Aquifer (Secondary Aquifer) that is composed of heterogeneous sand and fine-grained sediments. Although these deposits are relatively thick, the entire unit is heterogeneous and cannot be considered one single aquifer. Rather, sand lenses within the deposits collectively form the Alluvial Fan Aquifers. Lithologic data from wells are insufficient to map out the extent of the aquifers or characterize the deposits. Limited data indicate relatively fine-grained textures throughout much of the area, especially with depth (Todd and AKM 2008).

Model Layer 3 represents the Sandstone Aquifer (Secondary Aquifer) that consists of sedimentary rocks of Tertiary age containing sandstone layers that are penetrated by several Corona wells (Todd and AKM 2008). Due to the limited production, depth, and relatively low permeability in most areas, the Sandstone Aquifer is not considered a primary source of water supply. The bottom of Model Layer 3, which the lowest model layer in the Model, is a no-flow boundary condition, representing the older bedrock formations that are assumed to be relatively impermeable.

In the Chino Basin, Model Layers 1, 2 and 3 represents the aquifer layer defined in the Chino Basin Model (WEI, 2015, 2020). The definition of the model layers in the Chino Basin were set up to conform as well as possible to the model layers used in the Chino Basin Model (WEI, 2015, 2020). These Chino Basin layers were correlated to the three model layers defined in the Temescal Basin.

4.2.5. Faults
The Elsinore and Chino fault zones bound the Basin on the west and parallel the base of the Santa Ana Mountains. The faults within the Basin were simulated using the Horizontal Flow Boundary (HFB) Package in MODFLOW that allows a conductance parameter to be placed between adjacent model cells to limit groundwater flow. Flow across the faults was based on assigned conductance values that ranged from 0.01 ft2
/d in the alluvial sediments in the Prado area to 0.000002 ft2/d in the sandstone aquifer in the Temescal Basin. The fault locations within the Temescal Basin model are shown on Figure 14. For the model, all faults extended across model Layers 1 through 3. The fault hydraulic conductivities were based on an initial estimate that was refined during model calibration.

The HFB Package was also used to assign a low conductance (0.0000003 ft2 /d) to represent the
engineered clay core of the Prado Dam that is designed to limit underflow underneath the dam through the unconsolidated sediments in Model Layer 1.

4.2.6. Aquifer Conditions
Groundwater conditions for each model layer can be defined as unconfined, fully-confined, or convertible between confined and unconfined based on the relation of the simulated groundwater level to the top of the model layer. Unconfined conditions exist when groundwater levels are below the top of the physical aquifer layer whereas confined conditions exist when groundwater levels are above the top of the physical aquifer layer. For the Temescal Basin Model, Model Layer 1 is defined as unconfined throughout the model domain. Model Layers 2 and 3 are defined as convertible between confined and unconfined conditions.

Because of the historical changes in groundwater levels, areas within the Basin can be temporarily unsaturated. Prior MODFLOW versions set a dewatered cell to a no-flow condition for the rest of the simulation if the cell is dewatered. An important advantage of using MODFLOW-NWT compared to previous MODFLOW versions is that unsaturated groundwater heads will be calculated for dry cells, whereas standard MODFLOW excludes these calculations (Niswonger et. al., 2011). This resaturation capability of MODFLOW-NWT was utilized for the Temescal Basin Model.

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In MODFLOW-NWT, head is simulated continuously from saturated to unsaturated conditions.

MODFLOW-NWT will calculate a head in an unsaturated cell while not allowing water to flow out of that cell, which provides a continuous solution for groundwater flow. Inflow to an unsaturated cell, either from adjacent cells, overlying cells, or an external source simulated by one of the stress packages, automatically flows downward to an underlying saturated cell if there are deeper layers. An unsaturated cell has a head below the cell bottom and is considered to have no water in storage, so changes in storage also are zero for these cells. The model accounts for this situation by setting the storage coefficient for an unsaturated cell to zero. This allows for the continuous solution of head not to affect the overall water balance results (Niswonger et. al., 2011).

Because groundwater heads are calculated for unsaturated cells using this approach, it is necessary for the model user to interpret the head in a cell relative to the cell bottom. If the head in a cell is at or below the cell-bottom altitude, then the water table is not contained within this cell (Niswonger et. al., 2011).

4.3. BOUNDARY CONDITIONS
Model boundary conditions represent the hydrologic budget by simulating where groundwater enters and exits the basin. Boundary condition data must be entered for each stress period at each model grid cell where a boundary condition is defined in the model. MODFLOW NWT provides a number of boundary condition options to numerically represent the different physical processes included in the hydrologic budget. The physical distribution and volumes of groundwater inflow and outflow for each budget component need to be accounted for geographically within the model domain. A discussion of each boundary condition of the groundwater budget is provided below.

4.3.1. Surface Recharge
The rainfall-runoff-recharge model outlined in Section 3 describes the methodology to define both the spatial distribution and monthly volume of surface recharge to groundwater within the Temescal Basin model. The rainfall-runoff-recharge model calculates the monthly contributions from precipitation and return flows to surficial groundwater recharge. The surface recharge is spatially distributed across the model domain using zones that are defined by a combination of geology and land use. This calculated surface recharge is applied using the MODFLOW recharge package.

4.3.2. Streams
The groundwater model dynamically simulates groundwater recharge from stream percolation and groundwater discharge into streams. Percolation from streams is a function of stream flow and—where the water table is equal to or higher than the stream bed elevation—the difference in water level between the creek and water table.

The MODFLOW stream flow routing (SFR2) package is used to simulate these processes. Each stream in the basin is simulated as a sequence of reaches, each of which is a model grid cell along the alignment of the channel. Flow is specified at the upstream end of each stream segment and routed down the reaches, with flow to or from the aquifer calculated on the basis of wetted channel area, channel bed hydraulic conductivity and the difference in elevation between the stream surface and the simulated groundwater level at that reach. By this means conservation of mass is applied concurrently to the stream and the aquifer. Streams can dry up completely as they cross the basin; and conversely, groundwater discharge can create stream flow in a segment that is dry farther upstream. The stream

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flow routing module allows for a network of channel segments, with multiple inflows or diversions at the start of each segment.

The Temescal Basin model includes a network of 39 stream segments containing a total of 2,688 stream reaches (Figure 15). Seventeen segments are used to simulate eight streams that drain watersheds in the Santa Ana Mountains along the west side of the Basin. Streams that flow across the Temescal Basin to Temescal Wash are divided into multiple segments to represent the natural, concrete-lined and unlined engineered streambed conditions present on these streams. Temescal Wash is composed of seven segments that represent varying lined and unlined conditions along Temescal Wash (Figure 15).

An additional five segments represent the short sections of four streams that drain watersheds from upland areas east of Temescal Wash.

In general, the upland stream reaches are more than 20 feet above the water table and are not hydraulically coupled to groundwater. Percolation from those reaches is independent of groundwater levels and not affected by pumping. Reaches where groundwater appears to be hydraulically coupled to surface water primarily include most of the length of Temescal Wash, Santa Ana River, and the lower ends of some larger tributaries in the Prado wetland area.

In the Chino Basin, the Santa Ana River is represented by five segments (Figure 15). The areas upstream of the Prado wetland area were represented by a single long segment, and the other four segments defined areas within the Prado wetland area and downstream of the Prado Dam. Five other segments simulated streams in the Chino Basin. Stream bed permeability was estimated by model calibration. For unlined streams, calibrated values for the stream bed permeability ranged from 0.1 to 25.0 feet per day (ft/d); whereas for concrete-lined streams, the stream bed permeability ranged from 0.00003 to

0.03 ft/d.

To develop estimates of surface and subsurface inflows from these tributary areas to the groundwater basin, a rainfall-runoff-recharge model (see Section 3) is used to simulate the entire watershed tributary to the Basin. This model simulates all near-surface hydrologic processes, including rainfall, runoff, infiltration, evapotranspiration, effects of impervious areas and irrigation, soil moisture storage and percolation to stream base flow and deep groundwater recharge. The calculated runoff is included in the SFR2 Package. Inflows for Temescal Wash are coordinated with output from the Bedford-Coldwater numerical model and USGS gauge data as discussed as part of the rainfall-runoff model documentation in Section 3.

The lower Temescal Wash and Butterfield Drain in the Prado area along the northern boundary of the Temescal Basin were simulated using the MODFLOW Drain Package. This is because the Prado Area is primarily a groundwater discharge area (Figure 15). During model calibration, these two stream segments experienced numerical instability due to the interaction of two head-dependent boundaries of groundwater-surface water interactions using SFR2 and the high evapotranspiration applied to the Prado Area. During model calibration, the MODFLOW output showed that these areas were principally areas of groundwater discharge to streams or ET, so converting these areas to drains was appropriate to improve the overall model performance.

Similarly, areas in the Norco area include large storm drain channels for drainage and stormwater management (Figure 15). Occasional shallow groundwater conditions during high rainfall periods led to numerical instability that was relieved by converting these drainage channels to the MODFLOW Drain Package.

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4.3.3. Mountain Front Recharge
Groundwater inflow into the basin from adjacent uplands—also called mountain front recharge (MFR)— were calculated by the rainfall-runoff-recharge model (see Section 3). MFR represents subsurface inflow of groundwater from the low-permeability rocks adjacent from the surrounding watershed to the groundwater Basin. The MODFLOW well package was applied along the basin margin in Model Layer 3 which represents the weathered bedrock. The distribution of the cells assigned to represent MFR are shown on Figure 16.

The rainfall-runoff-recharge model (see Section 3) was used to calculate a monthly subsurface inflow from each watershed based on precipitation recharge in the upstream watershed, with delays and attenuation due to long travel times through bedrock fractures. A set of cells in the well package were assigned to each watershed and the monthly inflow was distributed evenly to those cells assigned to that watershed. Therefore, the distribution of inflow incorporates the size and rainfall for each of the defined watersheds that contribute flow to the Basin.

4.3.4. Evapotranspiration
Evapotranspiration (ET) represents groundwater outflow from evaporation to the atmosphere and uptake by plants from the saturated zone. This is distinct from ET associated with soil moisture before it reaches the groundwater aquifer that is sustained by the total available precipitation not accounted for by runoff or recharge (see Section 3).

The MODFLOW evapotranspiration (EVT) package is used simulate ET directly from the groundwater aquifer. ET is defined over the entire model domain; however, ET only occurs in areas of shallow groundwater. In the Basin, this is generally limited to riparian areas adjacent to streams. ET includes uptake from both phreatophytes (plants that require groundwater) and mesophytes (plants that can utilize groundwater) either directly from the saturated zone or from the overlying capillary fringe (Meinzer, 1927; Robinson, 1958; and Lewis and Burgy, 1964). ET from the capillary fringe is replenished with groundwater from the underlying aquifer, so it is also considered a loss of groundwater (Lubczynski, 2011).

In the MODFLOW EVT package, the ET rate decreases with increasing depth to the water table because fewer shrub and tree roots are able to reach the water table and the energetics of withdrawing the water become less favorable. In the groundwater model, the consumptive use of groundwater due to ET decreases from the maximum rate when the water table is at the land surface and diminishes linearly down to zero when the water table reaches the extinction depth for that location.

In the Temescal Basin Model, three ET zones were defined as shown on Figure 17. The first zone represents locations where aerial photographs indicate the presence of dense, lush riparian vegetation indicates areas of shallow groundwater where the plants (phreatophytes) can regularly uptake water directly from the water table to meet evaporative demand. These primarily occur in the Prado wetland area, along the Temescal Wash, Santa Ana River and in some of the upland canyons along the basin margin. The extinction depth for these locations was set at 20 feet below the ground surface. Over most of the remaining model domain, the extinction depth was set at the ground surface. The third area represents areas the Norco area where the extinction depth was set at 1.0 feet below the ground surface to better control periods of high groundwater. ET rates applied in the Temescal Basin Model use the ET data from the rainfall-runoff-recharge model (see Section 3).

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4.3.5. Groundwater Pumping
Groundwater pumpage is the largest groundwater outflow from the Basin. Corona is the primary producer of groundwater in the Basin. Corona has 18 wells that extract water from the Basin for the purpose of potable water supply (Michael Baker 2021). Norco has four active wells but they are located in the unadjudicated portion of the Chino Subbasin not the Basin. Thirty-eight wells within the Basin produced groundwater in one or more years during 1990-2018, and the reported annual pumping amounts were obtained from WMWD.

A number of private wells were historically installed in the Basin. There are no records of which of these wells are currently active. However, the GSA agencies searched for existing active wells within the Basin.

This search included reviewing water use records and contacting owners of large private properties (domestic, commercial, and industrial), inquiring about private wells in discussions with knowledgeable local residents and community leaders, and polling interested parties during public meetings. This effort indicated that the only private pumpers in the Basin are All American Asphalt, Dart Corporation, and 3M. No active private domestic wells were identified in this search. Figure 18 shows the locations of wells with measured pumping rates in the Basin by Corona, Norco and private pumpers.

Citrus orchards irrigated with groundwater were common in the Basin in the early 1990s, but except for one small grove those have all been replaced by urban development. Agricultural irrigation pumping of the orchards was estimated by the rainfall-runoff-recharge model, with pumping assigned to a hypothetical irrigation well at the center of each irrigated recharge zone. This pumping was phased out over time as urban development occurred. Urban irrigation is supplied by the municipal water system, which uses imported water and local wells. Locations of agricultural pumping are distributed based on the estimated agriculture pumping requirements calculated using the rainfall-runoff model (Figure 19).

Municipal well extractions are measured and these data are entered directly into the model. Annual production by municipal wells is shown in Figure 20. All pumping wells are included as analytical elements that are simulated by the MODFLOW well package in the model. Table 1 summarizes the average annual groundwater pumping for each well over the simulation period along with the assigned model layer.

4.3.6. Recycled Water Recharge Ponds
Wastewater is treated at three Corona-owned and operated Water Reclamation Facilities (WRF-1, WRF-2 and WRF-3). The average annual production of treated wastewater (effluent) from these sources is approximately 11.35 mgd, or 12,700 acre-feet per year (AFY). Supply is anticipated to increase incrementally due to population growth by an additional 0.88 mgd through 2040 (about 7.8 percent).

WRF effluent is allocated to three end uses: 1) discharge to Temescal Wash or the Santa Ana River (SWRCB 2021), 2) reuse via the reclaimed water distribution system, and 3) discharge to offsite percolation ponds. WRF-1 and WRF-2 both contribute effluent to all of these end uses while WRF-3 only contributes effluent to the reclaimed water system.

The MODFLOW Well Package was used to simulate recharge at the WRF recharge pond as recharge wells. The volume of flow for each recharge pond was distributed evenly over the area of the ponds. The three offsite percolation ponds overlie the Basin and allow for recharge. One of the ponds is located along Lincoln Avenue and the other two at the end of Rincon Street near Cota Street, as shown in Figure 21. Average annual WRF discharge to the recharge ponds from 2016 through 2020 ranged from 1,364 to 5,273 AFY at WRF-1, and 734 to 1,462 AFY at WRF-2.

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4.3.7. Subsurface Flow with Adjacent Groundwater Basins To simulate potential subsurface groundwater and outflow with adjacent groundwater basins, either a specified head or general head boundary was defined using MODFLOW. Constant head boundaries allow sufficient inflow or outflow at that model cell to achieve the specified head. Head boundaries were defined at the locations shown on Figure 22 at the following areas:
• Arlington Gap - The Basin margin with the Upper Santa Ana Valley – Riverside-Arlington Basin at the location known as the Arlington Gap.
• Santa Ana River Flow Boundary - The Basin margin with the Coastal Plain of Orange County Groundwater Basin near the outflow of the Santa Ana River from the Temescal Basin. • Bedford-Coldwater Flow Boundary - The basin margin along the far southeastern corner of the Basin to coordinate with a similar boundary condition applied in the groundwater model for the Bedford-Coldwater Basin.
• Chino Basin Interior Flow Boundary - The northern model domain boundary located within the Chino Basin.

At the Arlington Gap, a MODFLOW General Head Boundary (GHB) package was applied along the basin margin. The distribution of the GHB cells is shown on Figure 22. The MODFLOW general head boundary (GHB) package allows for a more flexible simulation of the groundwater elevation at the cell by calculating the inflow or outflow at that model cell based on a conductance and a specified head at a user-defined distance on the external side of the boundary. The GHB boundary was defined based on an earlier groundwater flux calculation for the Arlington Gap (Todd and AKM 2008). The GHB parameters were varied during calibration to better match measured groundwater levels in the area.

The boundary with the Coastal Plan of Orange County Basin is a narrow canyon where the Santa Ana River exits the Temescal Basin. A MODFLOW constant head boundary was applied at this location. The specified head at this boundary was set at a comparable level with the stage of the Santa Ana River to simulate subsurface flow towards the Coastal Plan of Orange County Basin.

The Bedford-Coldwater Basin boundary is generally considered have little to no groundwater flow except where the alluvium along Temescal Wash thins as the wash leaves the subbasin and traverses northward through bedrock (a reach referred to as Temescal Canyon) before entering Temescal Basin. A MODFLOW constant head was applied at this location at a comparable level with the stage as was applied in the Bedford-Coldwater Basin to allow for groundwater flow through the channel deposits of Temescal Wash.

The Chino Basin interior flow boundary is located along the northern model domain. A portion of the Chino Basin was included in the Temescal Basin Model domain to allow for a more natural boundary along the Santa Ana River and Prado wetlands area. As a result, a boundary condition was defined within the Chino Basin. This northern boundary was set at a distance sufficiently far from the Santa Ana River that the assigned boundary condition would not affect simulated river-aquifer interactions or simulated groundwater conditions in the Temescal Basin. A MODFLOW constant head was applied to represent the general groundwater elevation pattern in the Chino Basin based on available groundwater elevations with some minor adjustments during model calibration. The objective of this boundary condition is to provide a realistic representation of groundwater conditions within the Chino Basin with respect to understanding the water balance for the Temescal Basin.

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4.4. AQUIFER PROPERTIES
Aquifer properties represent the physical and hydrogeologic characteristics of the aquifers within the Basin that control groundwater flow. Aquifer properties must be assigned to each active grid cell in the model. The conceptual model provides the framework necessary to define aquifer properties.

4.4.1. Aquifer Characteristics
The groundwater model represents the basin fill materials in terms of their ability to store and transmit groundwater. Horizontal and vertical hydraulic conductivity define the permeability of the aquifer, which is its ability to transmit groundwater flow. The ability to store water consists of two components.

At the water table, storage of water associated with filling or draining the empty (air-filled) interstices between mineral grains is represented by the specific yield of the aquifer. In deep aquifers, there is a much smaller ability to store and release groundwater that derives from the compressibility of the water and aquifer materials (specific storativity). Thus, the initial response to pumping from a deep aquifer is a large drop in water level (head) within that aquifer. With sufficient time, however, the decrease in head creates downward movement of groundwater that eventually accesses the storage capacity at the water table. In other words, the storage response of the aquifer depends partly on the duration of pumping and observation. For groundwater management purposes, storage responses over periods of months to decades are usually the most relevant.

Aquifer characteristics can be estimated in two ways. The first is by means of an aquifer test in which one well is pumped while water levels are measured at a nearby well. This approach typically measures horizontal hydraulic conductivity over distances of tens to hundreds of feet and storage responses over periods of 1 to 3 days. The second approach is to calibrate a groundwater flow model such that the aquifer characteristics reproduce measured historical water levels throughout the basin given estimates of historical recharge and pumping. The latter approach produces estimates of aquifer characteristics averaged over spatial scales of thousands to tens of thousands of feet and time scales of months to decades. The estimates account for preferential flow through localized sand and gravel lenses in the basin fill materials and for delayed water-table responses to deep pumping. Also, model calibration provides estimates of vertical hydraulic conductivity across the layers of alluvial deposits, which is rarely measured by aquifer tests. The temporal and spatial scales represented by the model calibration approach are better for addressing most long-term groundwater management questions.

4.4.2. Zone Approach
Because of the limited data for aquifer properties for the Basin, a zoned distribution pattern was used that applied aquifer properties over subregional areas with similar geologic conditions. Although the units are heterogeneous, the approach was to get a representative average value for each aquifer property for limited number of zones around the basin. This was to avoid the patchwork quilt type of aquifer property distribution that does not show any relation to the underlying geologic conditions that define the aquifer property. Figure 23 shows the distribution of aquifer characteristics after calibration of hydraulic conductivity and specific storage, respectively. The initial estimates of hydraulic conductivity and specific yield were from available local data, which incorporated major geologic features such as relatively permeable sediments in the upper parts of alluvial fans.

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4.4.3. Hydraulic Conductivity
Hydraulic conductivity represents the ability of the water to flow through the aquifer, and is defined horizontally within a model layer to represent groundwater flow through the aquifer and vertically between adjacent model layers to represent groundwater exchange between aquifers. The determination of the horizontal hydraulic conductivity is based on an assessment of lithologic description, available aquifer test data and model calibration. Since each model layer represents a thick interval composed of varying lithologies, the horizontal hydraulic conductivity represents an average value over the entire vertical thickness that includes the finer-grained layers in addition to any specific sand and gravel zone. For the Temescal Basin model, horizontal hydraulic conductivity is defined using regionalized blocks based on the geologic character of the unit and refined during calibration. The hydraulic conductivity used in the Temescal Basin model varies within a reasonable value range for the aquifer characteristics for each aquifer to achieve the model calibration. The final simulated horizontal hydraulic conductivities are listed in Table 2.

4.4.4. Vertical Conductance
In general, groundwater flow within an aquifer is dominantly horizontal whereas flow between adjacent aquifers is essentially vertical. The application of vertical hydraulic conductivity recognizes the inherent anisotropy present in natural geologic formations. Vertical groundwater flow is equivalent to Ohm’s Law for serial electrical flow through different resistivity layers. Based on this analogy, vertical groundwater flow, similar to serial electrical flow, is limited by the lowest conductivity (or highest resistivity) layer encountered. Therefore, vertical groundwater flow is defined by the lowest-permeability, areally extensive layer that controls the exchange of groundwater between aquifer or model layers. MODFLOW requires the input of a vertical hydraulic conductivity (Kz) for each layer. The vertical hydraulic conductivity values used in the model to calculate the VCONT are summarized in Table 2.

4.4.5. Specific Yield and Specific Storage
Aquifer storage defines the ability of the aquifer to take in or release water. Under unconfined conditions, water released from or put into aquifer storage represents the physical draining of groundwater from interstitial pore space within the aquifer. Unconfined storage is defined by specific yield. Under confined conditions, water released from or put into aquifer storage is derived from the compressibility of water as a result of changes in the aquifer pressure within the interstitial pore space.

MODFLOW NWT requires the use of specific storage, which is in the units of feet-1 . Reasonable ranges
for the specific yield and specific storage were varied within a reasonable range during the model calibration and the values are listed in Table 2, respectively.

4.5. INITIAL CONDITION
The model also requires that groundwater levels be specified at the start of the simulation. They were estimated based on contouring of available water level data. As the initial heads may not be representative of stable initial conditions, the first stress period representing pre-1990 conditions was run as steady-state condition to facilitate the calculation of a stable hydrologic system. In addition, initial conditions for the earlier groundwater model (Todd and AKM 2008), and simulated groundwater conditions for September 1989 from the Chino Model (WEI, 2015, 2020) were included to help guide the contouring. Figure 24 provides the starting head used to provide a reasonable representation of the September 1989 groundwater conditions for Layers 1, 2 and 3.

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5. HISTORICAL MODEL RESULTS
The Temescal Basin model was calibrated to reduce uncertainty by matching model results to observed data. An extensive calibration process was designed to better constrain the range of aquifer properties and boundary conditions for the model, thereby reducing uncertainty in the results.

5.1. CALIBRATION METHODOLOGY
For the Temescal Basin model, the calibration simulation uses a 29-year period that covers water year (WY) 1990 to WY2018. This aspect of the calibration is important to demonstrate that the model has the capability to simulate historical changes in groundwater elevations, and is therefore capable of forecasting future changes in groundwater elevations. This capability is necessary for the model to serve as a useful groundwater management tool.

5.1.1. Approach
The transient calibration is a process that compares the simulated groundwater levels from the model to observed groundwater level measurements. During calibration, boundary condition parameters and aquifer properties are varied within the reasonable range defined by the hydrogeological conceptual model. Different combinations are tested to determine the set of parameters and properties that produce an acceptable correlation between simulated and measured groundwater elevations. Other data sets, such as key water budget components, surface water conditions, or hydrogeological conceptual model, were also used to further constrain the calibration.

There are multiple combinations of aquifer properties and boundary conditions that can be used to match a single set of groundwater elevation data. Calibrating to multiple data sets under differing stresses (i.e. recharge and discharge rates) reduces this “non-uniqueness”, thereby reducing the uncertainty. Performing a comprehensive calibration over a 29-year base period infers the calibration has been performed over wet, dry, and normal years with varying degrees of pumping. To that end, the Temescal Basin model was primarily calibrated using groundwater levels. The measures of calibration are primarily from a statistical analysis along with a visual assessment groundwater level trends from hydrographs. The groundwater elevation maps and water budget data considered during the model calibration are assessed in context with the model results, so are discussed in the next section.

5.1.2. Calibration Methodology
Joint calibration of the rainfall-runoff-recharge model, the surface water budget models and the groundwater flow model applied heuristic methods (i.e. trial-and-error adjustments) to selected variables, as informed by the timing and location of model residuals. In accordance with the principle of parsimony in modeling (DWR 2016a), calibration began with a small number of broad zones for hydraulic conductivity and storage. Zones were subdivided during calibration if a pattern of residuals at multiple wells warranted it. Although storage and hydraulic conductivity are not necessarily correlated, in practice they often are to some degree. Thus, for simplicity, similar zonation patterns were used for both variables.

In practice, most of the calibration effort focused on adjustments to horizontal and vertical hydraulic conductivity, the locations and conductances of faults, stream bed vertical hydraulic conductivity, and several tributary watershed parameters: root depths of natural vegetation, rainfall-runoff thresholds and slopes, and the leakage and recession rates for shallow groundwater. Variables that were not

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adjusted during calibration include land use, crop root depths, pumping locations, and groundwater pumping.

The model calibration process was also evaluated using a statistical comparison of differences (or residuals) between measured and simulated groundwater elevations. An initial sensitivity analysis was performed using an automated parameter estimation process to provide an initial estimate of hydraulic parameter values and zonation. During the final model calibration, adjustments were made to model inputs and parameters in areas based on the degree and pattern of discrepancies between measured and simulated water levels. Water levels for some wells were easy to reproduce with the model, while others were more difficult. Additionally, a visual inspection of superimposed measured and simulated water-level hydrographs was used to verify consistency with long-term trends. This process of manually calibrating a groundwater model also produced considerable insight into the groundwater flow system and the factors that influence it.

5.2. SENSITIVITY ANALYSIS
The sensitivity analysis was used to determine which model parameters should be calibrated. The model parameters include the hydraulic properties of the aquifer, boundary conditions, as well as any other aspect of the model that can be parameterized. The objective of the sensitivity analysis was to identify those model parameters with a high sensitivity with respect to simulation of groundwater elevations.

For the sensitivity analysis, PEST (Doherty, 2004) was selected due to its robust capabilities to automate the parameter estimation and further evaluate the model.

Parameter sensitivity measures the impact of a small parameter change on the calculated system response. If a small model parameter changes results in a large change in the simulated water levels of the model domain, the parameter is regarded as highly sensitive. For the initial sensitivity analysis, all model parameters were included. The purpose was to exclude insensitive parameters from the final adjusted parameter set. During these processes, the covariance matrix from sensitivity analysis was used as the basis for eliminating insensitive parameters from the final test. A total of 21 hydraulic parameters were included in the initial sensitivity analysis. The results indicated that results are relatively sensitive to the hydraulic conductivities and specific yield of layer 1, and also to the hydraulic conductivity of Layer 2 in some areas.

5.3. STATISTICAL CALIBRATION
Model calibration was based on observed groundwater elevations from 3,166 measurements in 29 wells over the 29-year base period from October 1989 through September 2018 (WY1990-2018). The locations of these wells are shown on Figure 25.

The statistical calibration consists of a rigorous analysis comparing the difference, or residual, between measured and simulated groundwater elevations. An initial assessment of the model calibration is a comparison of observed versus simulated groundwater elevations of the entire calibration data set using a scatter plot (Figure 26). As indicated on Figure 26, the scatter along the correlation line is minor in comparison to the range of the data. The correlation coefficient for the data on this graph is 0.934, which indicates a strong correlation between simulated and observed groundwater elevations.

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A more detailed tabulation of the statistical analysis for the model calibration is presented in Table 3. A summary of the key statistical measures shown on Table 3 are provided below:
• The residual mean is computed by dividing the sum of the residuals by the number of residual data values. If the mean is significantly higher or lower than zero, it indicates overall model bias toward high or low water levels. The residual mean is -1.7 feet, which is close to zero. • The absolute residual mean is the arithmetic average for the absolute value of the residual so it provides a measure of the overall error in the model. The absolute residual mean is 11.1 feet. • The residual standard deviation evaluates the scatter of the data. A lower standard deviation indicates a closer fit between the simulated and observed data. The standard deviation for the calibrated model is 8.7 feet.
• The Root Mean Square (RMS) Error is the square root of the arithmetic mean of the squares of the residuals is provides another measure of the overall error in the model. The RMS Error for the calibrated model is 11.3 feet.
• The scaled absolute residual the ratio of the absolute residual mean is divided by the range of observed groundwater elevations. This ratio helps to put the variation of the residuals into perspective with respect to the scale of the groundwater basin. This ratio for the Temescal Basin Model is 0.024, which puts the statistical variability at less than 2.5 percent of the range. A ratio below 0.10 is generally considered a well calibrated (ESI 2020).

The statistical comparison is also consistent when evaluated by aquifer as shown on Table 3, which summarizes the statistical parameters for calibration wells screened primarily in the Channel Aquifer and the Secondary Aquifers (combined Alluvial Fan Aquifer and Sandstone Aquifer). The variability is primarily attributed to the greater number of groundwater levels from active pumping that increases the variability of the observed data over the calibration period. The statistical results are of high quality and are one indication that each aquifer is well calibrated. Table 4 provides a summary statistics for each of the 29 wells used in the calibration process. The statistical parameters are considered reasonable, indicating that the model is well calibrated.

It should be noted that some degree of difference (or residual) between the observed and simulated groundwater elevations is expected. Residuals may be due in part to localized effects or data quality issues. Therefore, a limited outlier analysis was applied to remove groundwater elevations that did not reflect groundwater conditions in the aquifer. For example, data quality issues, which typically look like isolated spikes along an otherwise consistent long-term trend, were removed. Elevated residuals can result from using groundwater elevations from pumping wells as calibration targets due to excessive drawdown due to a low well efficiency of the pumping well. Pumping well groundwater levels used for the calibration data set except those where the drawdown highly deviated from the long-term trend that were interpreted to be represent well efficiency issues within the pumping well.

5.4. GROUNDWATER LEVEL TRENDS
Hydrographs provide a detailed time history of groundwater elevations for specific wells. This time history data includes the impact of varying climatic and pumping stresses on the groundwater basin.

Comparing hydrographs of model results versus observed data provides another measure of model accuracy. For calibration purposes, the hydrographs were inspected to evaluate how well the model results matched the overall magnitude and trend of the observed groundwater elevation data over time.

For the transient model, it was considered more important to honor the overall trend of the data. A

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hydrograph was considered a good match if the model simulated the trend, even if the simulated groundwater elevations were consistently offset from the measured ones.

Groundwater elevation data for 28 hydrographs from different parts of the basin are included on Figures 27 through 33. Locations of the wells used for the hydrographs are shown on Figure 25. To facilitate a comparison of the relative groundwater trends observed in these wells, a consistent vertical scale of 200 feet is used on Figures 27 through 33. The vertical scale on the hydrographs ranges from 450 to 650 feet, except for Corona 27 well, which is located in the upland areas and has groundwater elevations outside of that range.

The majority of the hydrographs are from wells completed in the Channel Aquifer and adjacent Secondary Aquifers located in the northern portion of the Temescal Basin. The hydrographs from wells in this area show several trends that can be summarized as follows:
• From 1990 through 2000, groundwater elevations typically showed a stable to increasing trend with cumulative groundwater changes ranging from near zero to increases of over 20 feet. • From 2001 through 2009, groundwater elevations showed a general declining trend of 20 to 40 feet of cumulative decline over this period.
• From 2010 through 2018, groundwater levels showed a variable, but overall stable trend, with groundwater levels fluctuating by plus or minus 10 to 20 feet.

During these periods, the average groundwater recharge was roughly similar, with a mixture of wet, normal and dry water year types. From 1990 to 2000, the average annual groundwater pumping in the Channel Aquifer area was at its lowest levels for the simulation period. Groundwater pumping in the Channel Aquifer area peaked during 2001 through 2009, then declined from 2010 to 2018. Based on this, the primary factor affecting the groundwater levels in the Channel Aquifer area is the amount of groundwater pumping.

In summary, trends in simulated groundwater elevations are similar to trends in the measured groundwater level data, indicating good model calibration. As noted above, most of the differences are due to using groundwater level data from active production wells. Groundwater elevations near active production wells can be chronically lower than in nearby surrounding areas due to residual pumping drawdown. MODFLOW calculates the average groundwater elevation over the entire area of each model cell rather than the elevation at the well location itself. It does not simulate localized pumping drawdown around the well.

5.5. EVALUATION OF GROUNDWATER FLOW
The Temescal Basin Model simulates monthly groundwater elevations for 348 months from October 1989 through September 2018. In general, the overall groundwater flow directions remained generally consistent over this period with some variations observed near the major groundwater pumping centers. To evaluate the range of groundwater elevations, we have selected a few key time periods.

These include:
• Figure 34 – September 2018 for Model Layer 1 – End of Historical Simulation Period • Figure 35 – September 2018 for Model Layer 3 – End of Historical Simulation Period • Figure 36 – January 1997 for Model Layer 1 – Period of consistently high groundwater levels • Figure 37 – January 1997 or Model Layer 3 – Period of consistently high groundwater levels • Figure 38 – August 2014 for Model Layer 1 – Period of consistently low groundwater levels • Figure 39 – August 2014 or Model Layer 3 – Period of consistently low groundwater levels

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The high and low conditions represent a combination of climatic conditions and groundwater pumping demands. For the purposes of evaluating groundwater flow directions, we have selected Layers 1 and 3 as representative of the three layers. In general, the groundwater map for Layer 1 is representative of groundwater conditions in the Channel Aquifer, the upper part of the Alluvial Aquifer in the Temescal Basin, and the shallow aquifer in the Chino Basin. The groundwater map for Layer 3 is representative of groundwater conditions in the Secondary Aquifers.

Figure 34 shows the groundwater level contours and flow directions for Layer 1 at the end of the historical simulation period representing September 2018 conditions Groundwater flow in the Channel Aquifer was from east to west, generally following Temescal Wash. In the Norco area, groundwater flow in Layer 1 was localized to internal drainage and downward percolation. In the Chino Basin, groundwater flow was from northeast to southwest generally following the Santa Ana River. For much of the southern Temescal Basin and areas in the Norco area, Layer 1 is unsaturated, shown on the figures by the purple areas.

In the Secondary Aquifers, groundwater flow was generally from the basin margins towards the Santa Ana River and the Prado wetlands area in September 2018 (Figure 35). Underneath the Channel Aquifer, groundwater flow in the Secondary Aquifers flowed from southeast to northwest generally parallel to Temescal Wash. In the Norco area, groundwater flow was generally southwest along the long axis of the valley where it then converged with groundwater flowing northwest toward the Santa Ana River. In the Chino Basin, groundwater flow is from northeast to southwest along the Santa Ana River.

In the southern Temescal Basin, the groundwater gradient in September 2018 was steeper due to the geology of the area. The fault zone along the western margin is configured to be a groundwater barrier limiting flow across the fault. This is based on the HCM and groundwater levels from the Corona 27 well.

Groundwater flow in the faulted area flowed to the north where it reached the Santa Ana River downstream of Prado Dam. Below Prado Dam, groundwater flow is towards the Coastal Plain of Orange County Groundwater Basin through either discharge to the Santa Ana River or subsurface flow.

Figure 36 shows simulated groundwater elevation contours for January 1997 in Layer 1. During this period, widespread high groundwater levels were observed reflecting a period of high precipitation and below average groundwater pumping rates. In spite of the contrast in hydrologic conditions, the general groundwater flow directions were generally consistent with those in September 2018 (Figure 34).

Groundwater elevations in January 1997 in Layer 3 were also generally consistent with September 2018, with groundwater flowing from the basin margins towards the Santa Ana River and the Prado wetlands (Figure 37). The most significant difference was lower groundwater elevations with a localized groundwater depression in the southern Temescal Basin as a result of estimated agricultural groundwater pumping at this time.

Figure 38 shows the groundwater elevations in layer 1 for August 2014. During this period, widespread low groundwater levels were observed reflecting several preceding dry years. In general, the groundwater flow directions were similar to those in 1997 and 2018 (Figures 34 and 36). The main differences are lower groundwater levels due to groundwater pumping and limited recharge in the Channel Aquifer.

In the Secondary Aquifers (Layer 3), groundwater elevations were also similar to September 2018 and January 1997, with groundwater flowing from the basin margins towards the Santa Ana River and the Prado wetlands area (Figure 39). By 2014, agricultural pumping in the Temescal Basin had disappeared due to urbanization. As a result, groundwater pumping in the southern part of Temescal Basin decreased to near zero, and flow was consistently towards the north.

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The simulated groundwater flow patterns were consistent with the hydrogeological conceptual model.

These maps are included to demonstrate that the model provides reasonable simulation of groundwater elevation and flow direction even during the more extreme climatic periods during the base period. This further demonstrates that the model is well calibrated and can accurately simulate wet and dry weather periods.

5.6. MODEL-BASED HYDROLOGIC BUDGET
GSP regulations (§354.18(c)(2)(B)) indicate a need to identify an average hydrologic study period that cover as least 10 years that includes a range of hydrologic conditions (e.g. wet, normal, dry and critically dry) for purposes of the groundwater analyses in the basin-wide water budgets. In order to select a consistent study period, the Temescal GSA is using a 29-year base period covering the simulation period from WY1990 through WY2018. Water years used for the Temescal Basin Model run from October through to the following September to capture the cause and effect relationship on groundwater levels of wintertime rain and subsequent summertime groundwater pumping. Additional analysis of the historical water budget is provided in GSP Section 5 (“Water Budget”) and tables summarizing the water budget results are presented in Appendix K.

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6. SIMULATION OF FUTURE CONDITIONS
GSP regulations §354.18(c)(3) require simulation of several future scenarios to determine their effects on water balances, yield and sustainability indicators. The following scenarios to simulate future conditions include:
• Baseline Scenario - This represents a continuation of existing land and water use patterns, imported water availability, and climate.
• Growth Plus Climate Change Scenario - This scenario implements anticipated changes in land use and associated water use, such as urban expansion, and anticipated effects of future climate change on local hydrology (rainfall recharge and stream percolation) and on the availability of imported water supplies.

The historical period used for model calibration consisted of only 29 years (water years 1990 through 2018). The Sustainable Groundwater Management Act requires that future simulations cover a 50-year period. To obtain 50 years of hydrology, rainfall, reference ET and streamflow were assumed to repeat the 1993 to 2017 sequence twice. Rainfall during that period equaled 99 percent of the long-term average. Surface and subsurface inflows from tributary watersheds simulated using the rainfall-runoffrecharge model were also replicated to obtain 50 years of data. The initial conditions for the future baseline simulation equaled the ending water levels of the calibration simulation, or September 2018.

Thus, the future simulation period nominally covers water years 2019 to 2068.

Both of the future simulations assumed that the level of development and related water demand are constant throughout the simulation. That is, development in the growth plus climate change simulation is not phased in over time but rather corresponds to 2068 development throughout the simulation. This is the best way to demonstrate whether 2068 land use is sustainable because it allows for assessment of the effects of variations in climatic conditions (wet and dry cycles) on groundwater conditions, avoids subjective decisions about the concurrent timing of droughts and development, and provides time for the full effect of future conditions on groundwater to become apparent.

Additional details regarding assumptions and inputs for the future scenarios are presented in GSP Section 5.5.3 “Simulation of Future Conditions”. Water budget results for the two future scenarios are described in Section 5 of the main GSP text. Both scenarios showed essentially no net change in groundwater storage from 2018 to 2068 (see GSP Figure 5-10). Contours of simulated groundwater elevations in model layers 1 and 3 were also very similar to those in 2018, consistent with the water budget results and with the SMCs for groundwater elevations and storage, which preclude future declines to levels below minimum historical levels.

The future Baseline Scenario and Growth Plus Climate Change Scenario can serve as reference conditions against which to compare alternative management scenarios. Additional data and assumptions used in the future baseline simulation are described in GSP Section 5.5.3 (“Simulation of Future Conditions”). Inputs and results of other scenarios related to specific management actions recommended in the GSP are also described in Section 8 (“Management Actions”) and water budget results are presented in Appendix K.

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7. SGMA REQUIREMENTS
As noted in the SGMA Modeling Best Management Practices (BMP) guidelines (DWR 2016a), the description of the model application should include detailed information on the model conceptualization, assumptions, data inputs, boundary conditions, calibration, sensitivity and uncertainty analysis, and there applicable modeling elements such as model limitations. A DWR requirement for using model results in future water budget reporting for Annual Reports is to report the model accuracy. The following information addresses these reporting requirements.

7.1. MODEL DATA GAPS
When evaluating model results, it is important to consider the strengths and limitations of the numerical model. The horizontal and vertical resolution used to construct the model dictates the range of scales that the model can evaluate. The Temescal Basin Model is designed as a regional or basin-wide model to evaluate long-term, regional trends and the overall groundwater inflow and outflow to the basin. Within that scale, conditions are averaged. However, this model may not contain the site-specific details necessary to evaluate some localized conditions due to geologic complexity or unique localized effects.

For these areas, a more localized model may be required if such a detailed analysis is necessary. The regional model can provide a broader regional context to support the development of these localized models.

The groundwater flow model is an appropriate tool for evaluating groundwater conditions at the basin and subarea scale over periods of months to decades. Given its reasonable calibration under a wide range of historical hydrologic and water management conditions, it should produce reliable results under a similar range of future conditions. However, some aspects of the model and some types of applications may be less reliable. Limitations in model accuracy and in types of applications include the following:
• As with any regional model, the model cannot simulate details of water levels and flow at spatial scales smaller than one model cell. It cannot, for example, simulate drawdown within a pumping well. It can only simulate the average effect of that pumping on the average water level of the cell in which the well is located.
• The monthly stress periods of the model preclude simulation of brief hydrologic stresses. For example, the model cannot simulate the effects of daily pumping cycles on water levels, or the amount of recharge associated with peak stream flow events. • Surface and subsurface inflows from tributary watersheds around the perimeter of the basin remain uncertain. The rainfall-runoff-recharge model simulates watershed hydrology explicitly but flows from the watersheds to the groundwater basin are small compared to rainfall and ET.

Accurate data for those variables within the watershed areas are not available, and a small error in rainfall or ET can result in a large error in simulated watershed outflow. • Model calibration is better in some parts of the basin than others. Any future model calibration would benefit from additional groundwater elevation data in areas outside of the Channel Aquifer.