TO: Technical Advisory Group for the 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: Technical Advisory Group Meeting: March 13, 2025
RECOMMENDATION
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Receive a presentation on the Water Year 2024 Annual Report model results and an update on groundwater conditions in Napa County with a focus on the Napa Valley Subbasin and an update on Groundwater Sustainability Plan implementation.
Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
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EXECUTIVE SUMMARY
As in the past ten consecutive annual reports, this Water Year 2024 Annual Report includes an update on groundwater conditions in the Napa Valley Subbasin and elsewhere in Napa County. This is the fourth Annual Report prepared to support implementation of the Napa Valley Subbasin Groundwater Sustainability Plan (GSP), adopted by the Napa County Groundwater Sustainability Agency (NCGSA) in January 2022 and approved by the California Department of Water Resources (DWR) on January 26, 2023. This Report reflects an ongoing commitment by the County and NCGSA to sustainably manage groundwater resources by implementing an adaptive management approach supported by best available information.
Water Year 2024 (defined as October 1, 2023 through September 30, 2024) was characterized by a cool wet spring with an extremely hot, dry summer. The Napa Valley Subbasin was characterized by severe drought conditions in Water Years 2020 and 2021 with recovery occurring in normal (below average) and wet Water Years in 2022 and 2023, respectively. Water Year 2024 had consistent precipitation throughout the winter and spring, starting in October 2023 and continuing through May 2024 with a total of 22.94 inches of rain measured at the Napa State Hospital. The average precipitation is 24.87 inches per year, and Water Year 2024 was classified as a normal (below average) year. The summer irrigation season had multiple heat waves with substantially higher temperatures than average. Based on local weather stations, Calistoga, St. Helena, and Oakville experienced 35, 35, and 12 days above 100 degrees Fahrenheit, respectively.
As documented in the Water Year (WY) 2024 Annual Report, 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.
After the substantial recovery of the aquifer system in Water Years 2022 and 2023, groundwater elevations remained steady in representative monitoring wells in the main part of the Subbasin. Two wells located in the Northeast Napa Management Area exceeded the Minimum Threshold. All wells used to monitor the level of interconnected surface water remained above their Minimum Thresholds. Minimum Threshold exceedances for depletion of interconnected surface water were measured in the rate of stream depletion at the Napa River at Oak Knoll and the Napa River at Pope Street gaging stations. Subsidence measurements were conducted through InSAR measurements to evaluate land subsidence and found to be very small (less than the accuracy of the measurement) and were less than the Minimum Threshold. Groundwater levels measured at representative wells to assess potential causal factors for land subsidence were above the minimum historical groundwater level elevation.
Groundwater pumping in the Subbasin increased compared with WY 2023. 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 reduction in groundwater storage in WY 2024. In WY 2024, groundwater storage change was negligible across most of the basin at +250 acre-feet based on spring-to-spring groundwater level measurements and +2,370 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), including the extent to which the drought has contributed to these conditions.
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.
BACKGROUND AND DISCUSSION
GROUNDWATER SUSTAINABILITY PLAN (GSP)
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 following adoption of a GSP. This fourth Napa Valley Subbasin GSP Annual Report is due April 1, 2025 and covers the period from October 1, 2023 through September 30, 2024. DWR has provided forms and instructions for submitting the materials electronically through the DWR online reporting system. The GSP Annual 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 2024 Annual Report reflects an ongoing commitment by the County and 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 sustainability 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 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 to achieve the sustainability goal.
WATER YEAR (WY) 2024 CONDITIONS
WY 2024 (defined as October 1, 2023 through September 30, 2024) saw near 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 of the precipitation occurring 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 going 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 41,550 acre-feet (approximately 3,060 acre-feet more than WY 2023), including uses by agriculture, cities, small public water systems, individual well users, groundwater dependent ecosystems, 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 16,200 acre-feet in WY 2024, representing 39 percent of total water use. The highest level of pumping was in 2021 (22,550 acre-feet) and the second highest year of groundwater pumping was in 2020, when 19,310 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 1,100 acre-feet greater than WY 2023 extraction. While the cool wet spring created ideal conditions to start the year, the record-breaking heat in Summer 2024 slightly increased all sectors of groundwater extraction, including agricultural, municipal, small public water systems, and self-supplied users. Direct uptake of groundwater by groundwater dependent ecosystems and native vegetation accounted for another 21 percent (approximately 4,440 acre-feet and 4,150 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 with WY 2024 sustaining the recovery across most of the Subbasin. 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 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 3,250 acre-feet, approximately 40 acre-feet above the Minimum Threshold or 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 extreme 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. In WY 2024, groundwater storage remained steady across most of the basin by +250 acre-feet based on spring-to-spring groundwater level measurements and +2,370 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.
SUPPORTING DOCUMENTS
A. Napa County Groundwater Sustainability, Annual Report - Water Year 2024 Presentation (LSCE, March 2025)
B. Table 1, Sustainable Management Criteria Summary