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: Presentation on GSP Annual Report for Water Year 2025
RECOMMENDATION
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Receive an update on the implementation of the Groundwater Sustainability Plan including the Annual Report on conditions in Water Year 2025, implementation of Workplans, increased monitoring, and outreach activities.
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Staff introduces.
Questions and answers with the TAG.
Public comments.
BACKGROUND AND DISCUSSION
Pursuant to California Code of Regulations §356.2, an Annual Report is required to be submitted to the Department of Water Resources (DWR) each year by April 1. This is the fifth Napa Valley Subbasin Groundwater Sustainability Plan (GSP) Annual Report submitted to DWR since the adoption of the GSP. The report contains both a narrative description and data in various formats including DWR provided templates. Annual Reports are submitted to DWR through the state’s Sustainable Groundwater Management Act (SGMA) Portal and are available for public comment at <https://sgma.water.ca.gov>.
The Annual Report (posted here: DWR SGMA Portal) reflects an ongoing commitment by Napa County and the Napa County Groundwater Sustainability Agency (NCGSA) to sustainably manage groundwater resources by implementing an adaptive management approach supported by best available information.
The goal of the GSP is to achieve the sustainability goal by ensuring that there are no Undesirable Results in the Napa Valley Subbasin by 2042. To accomplish the goal, the GSP includes 6 Sustainability Indicators, as follows:
1. Chronic groundwater level decline;
2. Reduction in groundwater storage;
3. Depletion of interconnected surface water;
4. Land subsidence;
5. Degraded water quality; and
6. Seawater intrusion.
These are critical factors used to measure the long-term health of groundwater in the Napa Valley Subbasin. For each Sustainability Indicator, the GSP has established a Minimum Threshold, which defines when the Sustainability Indicators are declining to a point where the GSA should evaluate the conditions and determine the necessary responses needed to maintain or achieve sustainability, including implementing Projects and Management Actions (PMAs) or other response actions to avoid Undesirable Results. Each Sustainability Indicator also has a defined Undesirable Result, which indicates conditions that need to be avoided to protect the long-term health of the Subbasin groundwater and interconnected surface water and to achieve the sustainability goal.
Water Year 2025 Conditions
Water Year (WY) 2025 (defined as October 1, 2024 through September 30, 2025) saw normal (below average) precipitation throughout Napa County and the Napa Valley Subbasin. WYs 2020 and 2021 registered as the driest consecutive years since at least the 1890s, as measured by the precipitation gauge at the Napa State Hospital. WY 2022 registered 21.24 inches of precipitation and was classified as a normal (below average) year with most precipitation falling before Spring 2022, and WY 2023 registered 128 percent of average precipitation and was classified as a wet year. WY 2024 experienced a total of 22.94 inches, defining it as normal (below average). WY 2025 precipitation was slightly less than WY 2024 and recorded a total of 21.59 inches, again normal (below average).
Most rainfall occurred in November and December 2024, while no precipitation was measured from June through August 2025. This pattern reflects the region’s Mediterranean climate, characterized by dry summers and wet winters. In WY 2025, precipitation varied across the region, with the highest amounts occurring to the north and west and decreasing toward the south and east. The Napa State Hospital rain gage is in a comparatively drier part of the watershed and Subbasin. PRISM data (provided by the PRISM Group of the Northwest Alliance for Computational Science and Engineering and housed at Oregon State University) are used for water year hydrologic data updates to the Napa Valley Integrated Hydrologic Model (NVIHM).
Temperature patterns also affect groundwater conditions because of the influence of temperature on the consumptive use of water through evaporation and transpiration, and associated demands of environmental, agricultural, and urban uses. In WY 2025, there were 30 days above 90 degrees, which is similar to the historical average, but 19 days fewer than observed in WY 2024. In WY 2025, there were 13 days above 95 degrees, which is the same as the historical average. WY 2025 temperatures align with historical averages; however, temperatures are trending upwards.
Total water use in the Napa Valley Subbasin in WY 2025 is estimated to have been approximately 41,493 acre-feet (similar to WY 2024 at 41,560 acre-feet), including uses by agriculture, cities, small public water systems, individual well users, groundwater dependent ecosystems (GDEs), and other native vegetation. This is within the range of total annual water use documented since 1988, which has varied between approximately 38,000 and 47,000 acre-feet per year.
The amount of groundwater pumping in WY 2025 was similar to WY 2024. Groundwater extraction by wells totaled approximately 16,580 acre-feet in WY 2025, representing 40 percent of total water use. The highest level of pumping was in 2021 (22,990 acre-feet) and the second highest year of groundwater pumping was in 2020, when 19,570 acre-feet of groundwater was used. WY 2025 estimated extraction is higher than the sustainable yield of 15,000 acre-feet for the Subbasin. Direct uptake of groundwater by GDEs, native vegetation, and managed wetlands accounted for another 22 percent (approximately 9,030 acre-feet) of total water use. The percentage distribution of water use falls within the observed range for the Subbasin.
In WY 2025, the Minimum Thresholds for the following Sustainability Indicators have been exceeded:
1. Chronic groundwater level decline;
2. Reduction in groundwater storage;
3. Depletion of interconnected surface water.
There was substantial groundwater level recovery in the majority of the Subbasin’s representative monitoring site wells in WY 2023. Groundwater elevations in WY 2024 slightly declined after the recovery in WY 2023. Groundwater elevations in WY 2025 remained similar to WY 2024. Two wells located in the Northeast Napa Management Area exceeded the Minimum Threshold for chronic groundwater level decline. The Northeast Napa Management Area is located east of the Napa River and west of the Milliken-Sarco-Tulucay area in the southeast part of the Subbasin. Water level recovery in the Northeast Napa Management Area generally does not respond as quickly as the rest of the Subbasin. Factors contributing to the ongoing groundwater decline in the deeper portion of the aquifer system in this area include:
• the thin alluvium,
• the presence of low permeability volcanic rocks from which groundwater is produced in this area,
• local faults,
• delayed recharge and groundwater level recovery due to the low permeability volcanic rocks, and
• more pumping than can be supported by recharge to this area.
All representative monitoring wells co-located at five stream monitoring sites to assess groundwater levels relative to interconnected surface water did not exceed Minimum Thresholds. However, modeled results for the rate of depletion of interconnected surface water exceeded interim Minimum Thresholds at the Napa River at Oak Knoll and the Napa River at Pope Street gaging stations. The streamflow depletion is measured as a volume of depletion over the period from June through October. The stream depletion at the Napa River at Oak Knoll station was modeled as 3,650 acre-feet, approximately 430 acre-feet above the interim Minimum Threshold. The stream depletion at the Napa River at Pope Street was modeled as 1,500 acre-feet, approximately 110 acre-feet above the interim Minimum Threshold. This is the third interim Minimum Threshold exceedance at the Napa River at Oak Knoll gage, which by definition constitutes an interim Undesirable Result. Investigations into the cause of exceedances showed a complex interaction of many factors, including the higher streamflows in June and July at both stations, similar groundwater conditions observed in both WY 2024 and WY 2025, and the substantially higher direct groundwater uptake of vines and native vegetation. Implementation of the Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystems (GDEs) Workplan (March 2024) along with additional groundwater levels measured at 28 new dedicated monitoring wells and wet-dry river reach mapping in the vicinity of the six intensive survey sites lend insights to the sustainable management criteria for interconnected surface water. The GSP regulations for depletion of interconnected surface water (California Code of Regulations §354.28 (c)(6)) require the minimum threshold 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.” An important complement to this definition, which is not expressly in the GSP regulations but is being implemented as part of the Napa Valley Subbasin ISW and GDEs Workplan, are the actual surface water flows and the effect of depletion on the functional flows needed to support the health of GDEs. The factors contributing to streamflow depletion in WY 2025 will be further discussed during the presentation.
Because the 7-year average of annual groundwater extraction has exceeded the estimated sustainable yield of 15,000 acre-feet/year for the Napa Valley Subbasin, an Undesirable Result occurred for the Sustainability Indicator for reduction in groundwater storage for WY 2025. Groundwater extractions in 6 of the past 7 years have exceeded the sustainable yield, and as expected the 7-year average exceeded the sustainable yield.
Although groundwater levels in the main part of the Subbasin were mostly above the minimum threshold in WY 2025, achieving the sustainability goal at least by 2042 involves protecting and enhancing conditions and achieving measurable objectives, especially for interconnected surface water. Factors to be considered during GSP implementation include PMAs that augment recharge, reduce pumping, and help mitigate climate-related effects on reduction in groundwater storage such as occurred during WYs 2020 through 2022.
Groundwater storage change refers to the difference in the amount of water stored in the aquifer over time and reflects gains from recharge and losses from pumping and natural discharge. The change in the amount of water stored in the aquifer is reported using two separate methods. The first method analyzes seasonal high groundwater levels at wells throughout the Subbasin in spring within the principal aquifer, where the year-to-year (spring-to-spring) change is used to calculate the change in groundwater storage as required by SGMA regulations §356.2(b)(5). These measurements are typically conducted in March. The second method utilizes the NVIHM to calculate the change in storage for the period from October 1, 2024 to September 30, 2025. The second method takes into account factors such as subsurface inflow and stream depletion related to pumping influence. Whereas, the first method typically mutes the effects of these factors on the condition of the groundwater basin following the winter to early spring period when groundwater basins typically exhibit groundwater level recovery. In WY 2025, the change in groundwater storage of 580 acre-feet remained steady across most of the basin; this represents a slight increase in groundwater storage relative to WY 2024 based on spring-to-spring groundwater level measurements as required by SGMA regulations §356.2(b)(5).
As described in the GSP, once Minimum Thresholds and/or Undesirable Results have been exceeded, the GSA should assess the causal factors resulting in the exceedance(s) (Table 1). This analysis is critical to ensure careful consideration of groundwater conditions. The factors contributing to the exceedances have highlighted other important considerations such as the need for an understanding of the relationships between surface water flows and the effects of pumping on streamflow depletion and the functional flows needed to support healthy GDEs. Additional analyses pertaining to ecologic and streamflow relationships will be included in the upcoming Periodic Evaluation due to DWR in January 2027.
GSP Implementation Progress
Implementation of GSP elements, including Projects and Management Actions, is intended to avoid Undesirable Results and achieve the Subbasin sustainability goal by 2042, as required by SGMA. The GSP adopted in January 2022 describes PMAs (Napa Subbasin GSP Section 11) along with actions developed to support sustainable groundwater management. The purpose of the Annual Report is to provide updates to each PMA that is being implemented. A full assessment of how each PMA is performing will be conducted during preparation of the Periodic Evaluation, which is due in January 2027. The Annual Report Section 7 includes a description of all PMA progress.
GSP implementation activities completed prior to and/or during WY 2025 through Spring 2026 include efforts related to the following GSP PMAs and other supplemental actions (see Annual Report Section 7 for additional details):
• GSP Project #1 (P1): Managed Aquifer Recharge
o Evaluate recharge feasibility scenarios, assess economic feasibility relative to potential benefit, and prepare technical memorandum; explore the opportunity for NCGSA to apply for a 5-year temporary recharge permit.
• GSP Project #2 (P2): Expansion of Recycled Water Use
o Coordinate with the City of St. Helena regarding the City’s opportunities for potential in-lieu use of recycled water and/or direct recharge of recycled water.
• GSP Management Action #1 (MA1): Water Conservation (Vineyards, Wineries, and Other Water Users)
o Continue implementation of water conservation activities related to the Napa County Water Conservation Workplan - A Guide for Vineyards, Wineries, and Other Water Users (March 2024).
• GSP Management Action #2 (MA2): Groundwater Pumping Reductions
o Continue implementation of activities related to the Groundwater Pumping Reduction Workplan: Napa Valley Subbasin (March 2024); implement Napa Water Certification Partnership pilot concept; prepare Subbasin Vineyard Replant Extension concept, which is a voluntary initiative currently under development that, if implemented, could achieve groundwater pumping reduction by incentivizing landowners to delay vineyard replanting cycles.
• GSP Management Action #3 (MA3)
o Release the Public Draft Water Availability Analysis Guidance Document and accompanying Technical Report for public review and comment (posted March 2026); periodic updates to countywide well inventory.
• GSP Section 6 to Address Data Gaps
o Continue activities described in the ISW and GDEs Workplan, including monitoring of physical and biological characteristics at six intensive survey sites; installation of surface water monitoring facilities at five sites under the DWR California Stream gage Improvement Project (CalSIP) (in progress); installed two monitoring wells in Calistoga area for groundwater and interconnected surface water monitoring.
• Integrated Hydrologic Modeling
o Refine NVIHM, including incorporation of soil moisture storage and utilization within the water supply and demand model framework; refine crop coefficients and evapotranspiration (ET) estimates based on locally measured and remotely sensed ET estimates; refine the upper watershed modeling platform; refine the distribution of groundwater pumping within water balance regions based on the County’s updated well inventory; other updates have included utilization of stream channel geomorphology to better reflect real-world conditions; continue coordination with State Water Resources Control Board on model development for Napa River Watershed and the State’s evaluation of streamflow diversions.
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. Napa County Groundwater Sustainability Annual Report, Water Year 2025 Presentation (LSCE, 2026)
B. Napa County Groundwater Sustainability Annual Report, Water Year 2025 (Executive Summary)
C. Table 1 - Sustainable Management Criteria Summary