TO: Napa County Groundwater Sustainability Agency (NCGSA)
FROM: Brian D. Bordona - Director of Planning, Building and Environmental Services
REPORT BY: Jamison Crosby - Natural Resources Conservation Manager
SUBJECT: Napa Valley Subbasin Annual Report for Water Year 2024 and GSP Implementation Progress
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 2024, implementation of Workplans, increased monitoring, and outreach activities. (No Fiscal Impact; Mandatory)
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BACKGROUND
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 fourth 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.
This Water Year 2024 Annual Report <https://www.countyofnapa.org/DocumentCenter/View/37446/2024-Napa-County-Groundwater-Monitoring-Annual-Report?bidId=> 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 Project and Managements Actions (PMA) 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 to achieve the sustainability goal.
Water Year 2024 Conditions
Water Year (WY) 2024 (defined as October 1, 2023 through September 30, 2024) 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. All data from the Napa State Hospital weather station is reviewed for completeness and is appropriately compensated when necessary. 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). Precipitation fell in an ideal pattern with small amounts beginning in October 2023 and periodic precipitation events through May 2024. Summer conditions were some of the hottest on record leading to significantly higher evapotranspiration than previous years. Overall, WY 2024 can be classified as having a cool wet spring with an extremely hot dry summer.
Total water use in the Napa Valley Subbasin in WY 2024 is estimated to have been approximately 41,560 acre-feet (approximately 2,880 acre-feet more than WY 2023), 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 was greater in WY 2024 compared to WY 2023. Groundwater extraction by wells totaled approximately 16,210 acre-feet in WY 2024, representing 39 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 2024 estimated extraction is higher than the sustainable yield of 15,000 acre-feet for the Subbasin and 910 acre-feet greater than WY 2023 extraction. While the cool wet spring created ideal conditions at the beginning of the water year, the record-breaking heat in Summer 2024 slightly increased groundwater extraction by all sections, including agricultural, municipal, small public water systems, and self-supplied users. Direct uptake of groundwater by GDEs, native vegetation, and managed wetlands accounted for another 21 percent (approximately 4,430 acre-feet, 4,150 acre-feet, and 350 acre-feet, respectively) of total water use. The percentage distribution of water use falls within the observed range for the Subbasin.
In WY 2024 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. Two wells located in the Northeast Napa Management Area exceeded the Minimum Threshold. 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 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 measured Minimum Threshold exceedances 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 3,240 acre-feet, approximately 20 acre-feet above the Minimum Threshold or less than one percent above the Minimum Threshold. The stream depletion at the Napa River at Pope Street was modeled as 1,490 acre-feet, approximately 100 acre-feet above the Minimum Threshold or seven percent above the Minimum Threshold. Investigations into the cause of exceedance showed a complex interaction of many factors including the high streamflow and groundwater elevations produced by the cool wet spring and the substantially higher direct groundwater uptake of vines and native vegetation. The extremely hot and dry summer conditions led to above average pumping as well as direct uptake of vines and native vegetation, further contributing to streamflow depletion. The factors contributing to streamflow depletion in WY 2024 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 2024. 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 2024, 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 projects and management actions 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, i.e. the change in the amount of water stored in the aquifer, is reported using two separate analyses. The first analysis is a spring-to-spring measurement based on measured groundwater elevations. These measurements are typically conducted in March. The second analysis provides a fall-to-fall estimated based on the Napa Valley Integrated Hydrologic Model (NVIHM) water budget. The NVIHM water budget calculation takes into account factors such as subsurface inflow and stream depletion related to pumping influence. The spring-to-spring change in groundwater storage analysis typically lessens 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 2024, groundwater storage remained steady across most of the basin with a -640 acre-feet decline based on spring-to-spring groundwater level measurements and +2,390 acre-feet based on fall-to-fall model-simulated storage change.
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). This analysis is critical to ensure careful consideration of groundwater conditions. Minimum Threshold, Measurable Objectives, and Undesirable Result are summarized in Table 1.
GSP Implementation Progress
Implementation of GSP elements, including specific 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 the Periodic Evaluation in January 2027. The Annual Report Section 7 includes a description of all PMA progress.
In WY 2024, the following key GSP implementation activities underway and completed include:
• Completed three GSP implementation plans/workplans described in the GSP (GSP Sections 5, 9, 11 and 12) to address: 1) Interconnected Surface Water (ISW) and Groundwater Dependent Ecosystems (GDEs) Workplan: Napa Valley Subbasin (March 2024); 2) Napa County Water Conservation Workplan: A Guide for Vineyards, Wineries and Other Users (March 2024); and 3) Groundwater Pumping Reduction Workplan: Napa Valley Subbasin (March 2024);
• Installed groundwater monitoring facilities and associated instrumentation at eight monitoring sites for the purpose of enhancing the understanding of interconnected surface water and groundwater;
• Conducted ongoing groundwater monitoring and initiating expanded monitoring efforts as described in GSP Sections 5, 9, and 12 including evaluating ways to address data gaps;
• Implemented the Water Conservation and Groundwater Pumping Reduction Workplans (Management Actions 1 and 2), including a Water Certification Partnership Technical Memorandum (April 2025), a Vineyard Replacement Strategy Technical Memorandum (in progress), and significant outreach efforts;
• Efforts underway to update the Water Availability Analysis guidance document (in progress) and new well permitting standards (Management Action 3)
• Implementation of the ISW and GDEs Workplan, including aquatic and terrestrial surveys (additional types of monitoring) at four intensive survey sites (monitoring in 2025 to occur at these and two other sites where access agreements were more recently completed);
• Published an interactive webmap that presents information related to the Napa Valley Subbasin GSP including monitoring activities, historical data, and sustainable management criteria information; and
• Updated the Communication and Engagement Plan (February 2024) and continued ongoing public outreach and community engagement.
FISCAL & STRATEGIC PLAN IMPACT
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Is there a Fiscal Impact? |
No |
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Is it Mandatory or Discretionary? |
Mandatory |
ENVIRONMENTAL IMPACT
ENVIRONMENTAL DETERMINATION: The proposed action is not a project as defined by 14 California Code of Regulations 15378 (State CEQ Guidelines) and therefore CEQA is not applicable.