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 Spring 2024 Groundwater Conditions
RECOMMENDATION
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Provide an update on the start of the Water Year 2024 with a focus on change in storage and future climate impacts.
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BACKGROUND AND DISCUSSION
A core charge of the Technical Advisory Group (TAG) is to provide guidance on Napa County Groundwater Sustainability Agency responses to groundwater conditions occurring in the Napa Valley Subbasin. This update is to inform TAG members and public of the current groundwater conditions across the Napa Subbasin in response to precipitation events occurring during the first half of Water Year (WY) 2024 (October 1, 2023 through April 30, 2024).
Leading up to WY 2024 (October 1, 2023 through September 30, 2024), WY 2023 experienced total precipitation of 32.91 inches, or 128 percent of average. Temperatures and atmospheric demand were less than the historical average in WY 2023 and groundwater pumping was near the sustainable yield. All of these conditions led to higher groundwater levels and general recovery of groundwater storage throughout the Subbasin.
Precipitation in WY 2024 included near or above average precipitation from December 2023 through March 2024. Overall, as of April 26, 2024, the total precipitation is 22.19 inches, or 89 percent of water year average (24.86 inches). If no other precipitation falls during WY 2024, it will be classified as a Normal (below average), analogous to WY 2016 and WY 2021.
Groundwater storage is estimated based on spring water level data. Groundwater level data are interpolated from wells that are screened primarily or exclusively in the alluvial aquifer. The saturated thickness is calculated by comparing the interpolated groundwater elevation to the depth of the alluvial aquifer to estimated total groundwater in storage. The total groundwater in storage is compared spring-to-spring to estimate the change in groundwater storage. The Spring 2022 to Spring 2023 storage increased by approximately 19,000 acre-feet. Spring 2023 to Spring 2024 storage increased by approximately 3,500 acre-feet with a long-term cumulative storage change of 8,300 acre-feet.
Interpolation of the groundwater elevation was improved in Spring 2024 due to the addition of eight new dedicated monitoring wells. These additional dedicated monitoring wells, which are designed to track groundwater levels in the unconfined part of the aquifer system, improve the quality of the data to track groundwater level responses to natural and human-influenced conditions and address important data gaps identified in the 2022 Groundwater Sustainability Plan.
Groundwater storage changes were not distributed equally across the Subbasin. The St. Helena and Yountville areas experienced the greatest increase in groundwater storage of approximately 1,800 acre-feet (0.33 acre-feet per acre) and 1,000 acre-feet (0.07 acre-feet per acre), respectively. The remaining areas, including Northeast Napa, Calistoga, and Napa saw increases of 130 (0.07 acre-feet per acre), 200 (0.03 acre-feet per acre), and 300 (0.02 acre-feet per acre), respectively.
The small increase in groundwater storage aligns with the knowledge of current conditions throughout the Subbasin, including near average precipitation in the first part of WY 2024 and groundwater pumping near the sustainable yield during the WY 2023 irrigation season. The estimated increase of 3,500 acre-feet is comparable to other Normal (below average) water years.
The increase in groundwater storage provides multiple benefits, including increased baseflow to the Napa River and additional water supply to buffer conditions during dry years. The recently released Fifth National Climate Assessment (https://nca2023.globalchange.gov/) published in 2023, presents current conditions as well as multiple climate scenarios for the United States. Four climate scenarios were assessed, which are based on 1.5°C (2.7°F), 2°C (3.6°F), 3°C (5.4°F), and 4°C (7.2°F) increases in global temperature. All four climate scenarios predict Napa County is likely to experience higher hot temperatures, higher low temperatures, more precipitation, and more extreme precipitation events under all scenarios.
While the degree of change resulting from future climate change is uncertain, national, state, and local data indicate shifting climate patterns and trends. Long-term adaptive management strategies and measures implemented to optimize recharge opportunities and conserve water can help minimize the local impact. Increases in extreme precipitation events provide potential opportunities to increase recharge through best management practices and on-farm strategies to retain precipitation, enhance infiltration, and augment groundwater supplies. Climate variability, including shifts in the timing and duration of precipitation events, can impact groundwater discharge to streams. The relationship between hydrologic variability, streamflow, and potential impacts to groundwater dependent ecosystems are key questions being investigated during implementation of the Interconnected Surface Water and Groundwater Dependent Ecosystems Workplan. Ongoing responses to climate change will require being prepared for potentially hotter years where precipitation events no longer occur in the same pattern as historical events. It is important to continue to embrace “Conservation as a Napa Way of Life” to help build resiliency.
Procedure
Staff introduces.
Questions and answers with the TAG.
Public comments.
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 Agency, Current Conditions - Start of Water Year 2024 (LSCE, May 2024)