TO: Technical Advisory Group for the Napa County Groundwater Sustainability Agency
FROM: Brian D. Bordona, Director of Planning, Building and Environmental Services
REPORT BY: Jamison Crosby, Natural Resources Conservation Manager
SUBJECT: Update on the Napa Valley Subbasin Periodic Evaluation
RECOMMENDATION
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Receive an update on the Napa Valley Subbasin Periodic Evaluation required to be submitted by the Napa County Groundwater Sustainability Agency to the California Department of Water Resources by January 31, 2027.
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Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
BACKGROUND AND DISCUSSION
The Napa Valley Subbasin Periodic Evaluation must be submitted by the Napa County Groundwater Sustainability Agency (NCGSA) to the California Department of Water Resources (DWR) by January 31, 2027.
The Periodic Evaluation is an evaluation and written assessment of an approved Groundwater Sustainability Plan (GSP) to occur at least every five years and when a Plan is amended (the Periodic Evaluation is due no later than five years after initial GSP submittal). The Periodic Evaluation requires the NCGSA to conduct a thorough assessment of how the Plan is performing, whether modifications of the Plan or management objectives are necessary, and a determination that the GSP implementation is on track to meet the Subbasin sustainability goal. Additionally, the Periodic Evaluation is required to:
• Compare current groundwater conditions over the evaluation cycle with relevant sustainable management criteria,
• Evaluate the basin setting in light of significant new information or changes in water use, and an explanation of any significant changes.
• Provide an evaluation of the extent to which progress is being made in implementing projects and management actions (PMAs) and the effect on groundwater conditions resulting from those PMAs.
• Assess the monitoring networks, and
• Discuss any other GSP topics that have changed during GSP implementation.
DWR’s guidance (A Guide to Annual Reports, Periodic Evaluations, and Plan Amendments, 2023) distinguishes the differences between a Periodic Evaluation and a Plan Amendment. A Plan Amendment requires public notice, board adoption, and formal resubmittal of the amended GSP, while a Periodic Evaluation is a GSP implementation review document required at least every five years.
In a January 26, 2023 determination letter, DWR approved the Napa Valley Subbasin GSP (2022) and identified recommended corrective actions, which DWR expects the NCGSA to address by the first Periodic Evaluation:
1. Remove the drought year condition from the Plan’s definition of undesirable results related to the chronic lowering of groundwater levels.
2. Include a metric for cumulative land subsidence in the definition of a minimum threshold for land subsidence.
3. With regard to interconnected surface water:
a. Utilize interconnected surface water guidance issued by DWR to refine minimum thresholds, measurable objectives, and management actions.
b. Continue to fill data gaps, collect additional monitoring data, and implement current strategies to manage depletions of interconnected surface water and define segments of interconnectivity and timing.
c. Collaborate and coordinate with local, state, and federal agencies as well as interested parties to better understand all beneficial uses and users that may be impacted by groundwater pumping-induced interconnected surface water depletions.
The presentation to the TAG focuses on: 1) an overview of the Periodic Evaluation, 2) groundwater conditions over the evaluation cycle, 3) the assessment of the GSP monitoring networks, and 4) refinements to the sustainable management criteria for the reduction in groundwater storage and depletion of interconnected surface water sustainability indicators.
Groundwater Conditions Over the Evaluation Cycle
SGMA defined undesirable results as significant and unreasonable effects for any of the six sustainability indicators. Sustainable management criteria (SMC) are used to quantifiably define undesirable results for the Subbasin, as outlined in Section 9 of the GSP. Each sustainability indicator is evaluated for the Subbasin at representative monitoring sites (RMS), where SMC have been established according to the best available groundwater data and basin characterization. Criteria include minimum thresholds (MT), which define the onset of undesirable results, and measurable objectives (MO), which represent quantifiable groundwater conditions that indicate progress toward achieving the sustainability goal.
The Periodic Evaluation assesses each sustainability indicator against its established SMC to determine whether GSP implementation has been effective and whether groundwater conditions are trending toward the sustainability goal. Conditions from WY 2021 through WY 2025 are evaluated and, where appropriate, compared to conditions since SGMA implementation in 2015, including an assessment of impacts to beneficial uses and users of groundwater in the Subbasin.
Each indicator evaluation also documents SMC revisions and responses to DWR Recommended Corrective Actions. SMC were revised for some indicators to reflect sites newly established as RMS, address a Recommended Corrective Action, or respond to undesirable results occurring during the evaluation cycle.
Assessment of GSP Monitoring Networks
The GSP (Section 5) describes the monitoring networks (including representative monitoring sites and supplemental monitoring sites) associated with the six sustainability indicators. The assessment of the monitoring networks in the Periodic Evaluation includes:
1. A summary of changes to monitoring networks since GSP adoption, including monitoring sites added or removed and any supplemental monitoring methods employed for that sustainability indicator.
2. An assessment of monitoring network function with an analysis of data collected to date, identification of data gaps, and the actions necessary to improve the monitoring network.
3. For data gaps identified by the NCGSA, the Periodic Evaluation describes a program for the acquisition of additional data sources, including an estimate of the timing of that acquisition, and for incorporation of newly obtained information into GSP implementation including the development of SMC as appropriate. The assessment considers whether a sufficient number (density) of monitoring sites is included as relevant to each sustainability indicator, the monitoring frequency, and/or whether previously monitored sites have subsequently been updated/replaced with more appropriate data sources and/or improved monitoring facilities that better achieve minimum standards for the monitoring network(s) and achieve the Subbasin sustainability goal.
4. The Periodic Evaluation describes priorities for potential new data collection facilities (e.g., recruit volunteered wells for monitoring in data gap areas), collection of new data, and analysis of new data based on the needs of the Subbasin.
Refinement of Sustainable Management Criteria
During the first GSP evaluation cycle, the GSP team recognized the need to refine the SMC for the reduction in groundwater storage and depletion of interconnected surface water sustainability indicators.
Refinement of Reduction in Groundwater Storage Sustainable Management Criteria
The SMC developed to assess the change in Subbasin groundwater storage in response to variable water year types and natural and human-influenced stresses on the groundwater system and the effectiveness of management actions to achieve long-term sustainability necessarily represent a multi-year period. The original SMC incorporated short-term considerations for the MT definition and long-term sustainability and time elements in the definitions for the MO and undesirable results definitions, including:
Measurable Objective: Net annual groundwater extraction by pumping is less than or equal to the sustainable yield for the Subbasin.
Minimum Threshold: Net groundwater extraction by pumping exceeding the sustainable yield for the Subbasin, where net groundwater extraction is the volume extracted less any volume of augmented recharge achieved by projects implemented in the Subbasin.
Undesirable Results: The seven-year average annual net groundwater extraction in the Subbasin exceeds the sustainable yield.
Based on the criteria in the GSP, it was estimated that the sustainable yield of the Subbasin is 15,000 AFY. The GSP also indicates that “assuming potential uncertainty of roughly 15 percent associated with the water budget estimates and given the variability in historical and projected estimates, the sustainable yield could range from 13,000 to 17,000 AFY.”
Historically, groundwater conditions responded with a decrease in groundwater volume during dry years and replenishment of the system in wetter years; overall groundwater conditions remained stable without persistent groundwater storage declines. This historical pattern was typical and evidenced in Napa Valley Integrated Hydrologic Model (NVIHM) historical water budget results. When the MO, MT, and undesirable results definitions were defined for the GSP, the NVIHM historical period covered 1988-2019. This model period preceded the unprecedented sequential very dry water year conditions in 2020 and 2021, which were followed by a different but equally unprecedented dry period in winter and spring of 2022. In response to this unusually hotter, drier period, the cumulative change in groundwater storage was about -20,880 AF in 2021, which was the largest cumulative decrease in groundwater storage during the 1988-2021 period. Groundwater levels recovered in response to a wetter year in Water Year (WY) 2023 and more typical normal (below average) precipitation in WYs 2024 and 2025, and the cumulative change in storage increased to 5,170 AF. Groundwater conditions in the narrow alluvial river valley Subbasin setting respond quickly to hydrologic and human influences. Unusual hotter, drier conditions occurred in March 2026, groundwater levels decreased somewhat, and the Spring 2025 to Spring 2026 annual change in groundwater storage was - 4,810 AF with the cumulative storage change of 360 AF.
Climate and weather pattern effects significantly influence the volume of groundwater extraction, and corresponding groundwater storage volume changes (increases and decreases) have occurred over short time periods. While it remains important to assess long-term Subbasin sustainability over a longer period, the current definition of undesirable results for groundwater storage reduction, which involves an average net extraction value over seven years, does not adequately reflect recent hydrologic factors and management actions that contribute to improved groundwater conditions. Also, the Subbasin includes one designated management area, the NENMA, that requires additional management considerations due to its unique hydrogeologic setting. In the NENMA, groundwater level declines have been observed in monitoring wells constructed in the older, more consolidated formations underlying the alluvium. As part of Napa County’s Water Availability Analysis update, the NVIHM model scenario, spanning a period from WY 1997 through WY 2023, was used to evaluate the sustainable yield within the NENMA. The analysis resulted in an estimated sustainable yield of 550 acre-feet per year for the NENMA. Based on the Napa Valley Subbasin estimate of sustainable yield for the entire Subbasin, the non-NENMA area has a sustainable yield of about 14,450 AF.
To better reflect the temporal sensitivity of the Subbasin to natural and human influences, and to account for management differences for the non-NENMA area compared to the NENMA, slight changes in the MT and undesirable results’ definitions are proposed for the groundwater storage reduction sustainability indicator.
Refinement of Interconnected Surface Water Sustainable Management Criteria
The SMC developed to assess interconnected surface water in response to variable water year types and natural and human-influenced stresses on the groundwater system and the effectiveness of management actions to achieve sustainability include two types of criteria - one based on groundwater levels and the other based on the volume of streamflow depletion during the June through October period. This presentation focuses on revisions to the interim SMC for the volume of streamflow depletion. The original interconnected surface water criteria for the volume of streamflow depletion were defined as follows:
Measurable Objective: A reduced volume of streamflow depletion corresponding to a 10 percent reduction in average annual historical (2005 to 2014) pumping for all non-de minimis groundwater users. The reduced volume of streamflow depletion during summer/early fall (June to October) is 1,300 AF for the Napa River at Pope Street and 2,300 AF for the Napa River at Oak Knoll Avenue.
Minimum Threshold: Summer/early fall (June to October) streamflow depletion volumes exceeding the second highest seasonal volume of streamflow depletion that occurred from 2005 to 2014 at two RMS on the Napa River at Pope Street and Oak Knoll Avenue.
Undesirable Result: Exceedance of the MT for the volume of streamflow depletion occurring for three consecutive years at either of the Napa River at Pope Street or Napa River at Oak Knoll Avenue locations.
The GSP regulations (California Code of Regulations §354.28 (c)(6)) for depletion of interconnected surface water require the MT to be the:
“rate or volume of depletions caused by groundwater use that has adverse impacts on beneficial uses of surface water and may lead to undesirable results.”
There is an important complement to the GSP regulations that is not directly expressed. To clearly understand the beneficial uses of surface water, and how and when the users and uses may be affected by streamflow depletion caused by groundwater use, requires the characterization of:
• streamflow depletion resulting from groundwater use (simulated);
• streamflow occurring with groundwater use (simulated);
• streamflow that would have occurred without depletion due to groundwater use (simulated);
• relationship between the streamflow depletion and the streamflow that would have occurred without depletion due to groundwater use (percent streamflow depletion from simulated estimates);
• observed streamflow (measured); and
• ecologic streamflow needs of various aquatic and terrestrial GDEs for critical life stages.
As part of the Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystems (GDEs) Workplan: Napa Valley Subbasin (2024), the California Environmental Flows Framework (CEFF) approach includes evaluation of actual surface water flows and the effect of depletion on the functional flows needed to support healthy GDEs.
The 2022 GSP interim SMC for the volume of streamflow depletion during a typical dry period (June through October) provided a useful metric while the ISW and GDEs Workplan was under development and as initial aquatic and terrestrial GDE data collection occurred during Workplan implementation in 2024 to 2026. However, it became apparent that the summer/early fall “dry” period (i.e., June through October) included in the SMC definition encompasses a longer period than appropriate for consideration of the overall health of GDEs at critical life stages, particularly when related to instream flow conditions rather than depths to groundwater. Additionally, streamflow depletion volumes or rates are not particularly useful unless the volumes or rates are contextualized with:
1. information that demonstrates the relative effect of the depletion (volume of stream depletion relative to streamflow without pumping effects and shown as a percent depletion); and
2. the actual streamflow during the critical period(s) when instream flow conditions are necessary to support GDE health.
In coordination with the implementation of the ISW and GDEs Workplan, monitoring conducted by Stillwater Sciences, Napa County Resource Conservation District, Napa County, and LSCE staff, installation of new monitoring facilities, and collection and analysis of new aquatic and terrestrial data, the interconnected surface water SMC are being refined. The Periodic Evaluation provides a detailed description of the rationale for the refined interim SMC, ongoing analyses of stream gage data, integrated hydrologic modeling to assess the percentage of streamflow depletion during May through July, ongoing stream reach connectivity surveys, and GDEs’ surveys. Recommendations for new field work and data collection are provided in the report, Napa Valley Subbasin California Environmental Flows Framework Sections A and B (Stillwater Sciences; an appendix in the Periodic Evaluation). These recommendations are for purposes of further refining the interconnected surface water SMC during the next evaluation cycle.
PUBLIC REVIEW
The draft Periodic Evaluation will be posted by October for a 30-day public review and comment period.
ENVIRONMENTAL IMPACT
ENVIRONMENTAL DETERMINATION: The proposed action is not a project as defined by 14 California Code of Regulations 15378 (State CEQA Guidelines) and therefore CEQA is not applicable.
SUPPORTING DOCUMENTS
A. Overview of Napa Valley Subbasin Periodic Evaluation, Assessment of Monitoring Networks, and Refinement of Sustainable Management Criteria. Presentation by Luhdorff & Scalmanini, Consulting Engineers, September 10, 2026.