TO: Technical Advisory Group for the Napa County Groundwater Sustainability Agency
FROM: Brian Bordona - Director of Planning, Building and Environmental Services
REPORT BY: Jamison Crosby, Natural Resources Conservation Manager
SUBJECT: Water Year 2023 Annual Report
RECOMMENDATION
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The Technical Advisory Group (TAG) members will receive a presentation from Luhdorff and Scalmanini, Consulting Engineers (LSCE) on the Water Year 2023 Annual Report model results and update on groundwater conditions in Napa County with a focus on the Napa Valley Subbasin and an update on Groundwater Sustainability Plan implementation. This presentation builds on the December 14, 2023 presentation that reviewed groundwater elevation data.
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EXECUTIVE SUMMARY
As in the past nine consecutive annual reports, this Water Year 2023 Annual Report includes an update on groundwater conditions in the Napa Valley Subbasin and elsewhere in Napa County. This is the third 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. To better manage and respond to changing conditions, the NCGSA formed a Technical Advisory Group (TAG) to advise the NCGSA and aid in the implementation of the Napa Valley Subbasin GSP. The five-member committee was first convened on August 11, 2022.
Water Year 2023 (defined as October 1, 2022 through September 30, 2023) experienced lower temperatures and higher precipitation than average throughout Napa County and the Napa Valley Subbasin. Water Years 2020 and 2021 registered as the driest consecutive years since at least the 1890s, as measured by the precipitation gauge at the State Hospital in Napa. Water Year 2022 registered 21.24 inches of precipitation and was classified as a normal (below average) year. Water Year 2023 registered 128 percent of average precipitation, totaling 32.91 inches. Precipitation began in November 2022 and continued through May 2023.
As documented in the Water Year (WY) 2023 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.
There has been substantial groundwater level recovery in the Subbasin representative monitoring site wells. One well, located in the Northeast Napa Management Area, exceeded the Minimum Threshold. All wells monitoring the level of interconnected surface water remained above their Minimum Thresholds with two of the five wells exceeding their respective Measurable Objective. A Minimum Threshold exceedance for depletion of interconnected surface water was measured in the rate of stream depletion at the Napa River at Oak Knoll gaging station. Elevation monuments were surveyed to evaluate land subsidence and found to be below the Minimum Threshold, as well as the associated groundwater levels for land subsidence.
Although overall groundwater pumping in the Subbasin decreased compared with WY 2022, the Sustainability Indicator for reduction in groundwater storage is defined as an Undesirable Result for WY 2023. 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. In WY 2023, groundwater storage increased across most of the basin by 19,214 acre-feet in spring-to-spring measurements and 21,600 acre-feet in fall-to-fall measurements. This contributed to significant groundwater replenishment; however, the Subbasin was significantly affected by persistent drought conditions during WYs 2020, 2021, and 2022. The cumulative groundwater storage recovery still remained below other, similar, wet years.
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. This analysis is critical to ensure careful consideration of potentially changed groundwater conditions before taking steps to implement Projects and Management Actions (PMAs).
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.
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 third Napa Valley Subbasin GSP Annual Report is due April 1, 2024 and covers the period from October 1, 2022 through September 30, 2023. 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.
As in the past nine consecutive annual reports, this Annual Report includes an update on groundwater conditions elsewhere in the county. This is the third Annual Report prepared to support implementation of the Napa Valley Subbasin GSP, adopted by the 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. To better manage and respond to changing conditions, the NCGSA formed a Technical Advisory Group (TAG) to advise the NCGSA and aid in the implementation of the Napa Valley Subbasin GSP. The five-member committee was first convened on August 11, 2022.
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 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.
WATER YEAR 2023 CONDITIONS
WY 2023 (defined as October 1, 2022 through September 30, 2023) saw an end to drought conditions throughout Napa County and the Napa Valley Subbasin. Water Years 2020 and 2021 registered as the driest consecutive years since at least the 1890s, as measured by the precipitation gauge at the State Hospital in Napa. WY 2022 registered 21.24 inches of precipitation and was classified as a normal (below average) year. However, the precipitation pattern and frequency in WY 2022 was far from normal, and Spring 2022 experienced almost no precipitation. WY 2023 registered 128 percent of average precipitation, totaling 32.91 inches, classified as a wet year. Precipitation began in November 2022 and continued through May 2023.
Total water use in the Napa Valley Subbasin in Water Year 2023 is estimated to have been approximately 38,947 acre-feet (approximately 1,355 acre-feet less than WY 2022), 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 less in WY 2023 compared to Water Year 2022. Groundwater extraction by wells totaled approximately 15,270 acre-feet in WY 2023, representing 39% of total water use. The highest level of pumping was in 2021 (22,490 acre-feet) and the second highest year of groundwater pumping was in 2020, when 19,050 acre-feet of groundwater was used. For the first time since WY 2019, the annual extraction volume, given the uncertainty of the methodology, is at the sustainable yield of 15,000 acre-feet for the Subbasin. While increased precipitation increased groundwater storage and provided additional water available for groundwater dependent ecosystems, groundwater use for agricultural and domestic users was similar to other, average, water years. Direct uptake of groundwater by groundwater dependent ecosystems and native vegetation accounted for another 21 percent (approximately 4,570 acre-feet and 3720 acre-feet, respectively) of total water use.
In WY 2023 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. One well, 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. This is attributed to the thin alluvium, the presence of low permeability volcanic rocks, and local faults.
All five monitoring wells co-located at stream monitoring sites did not exceed Minimum Thresholds and two monitoring wells met the Measurable Objective. Modeled results for the depletion of interconnected surface water measured a Minimum Threshold exceedance at the Napa River at Oak Knoll gaging station. The streamflow depletion, as measured as a volume of depletion from June through October, was 3,700 acre-feet, 410 acre-feet above the Minimum Threshold. Investigations into the cause of the exceedance showed the Subbasin had not fully recovered during the winter and spring precipitation events. Summer streamflow was consistent with other wet years, with flow lasting through the summer. The summer flows allowed higher stream depletion to occur throughout the summer and early fall.
Although overall groundwater pumping in the Subbasin decreased compared with WY 2022, the Sustainability Indicator for reduction in groundwater storage is defined as an Undesirable Result for WY 2023. 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. In WY 2023, groundwater storage increased across most of the basin by 19,214 acre-feet in spring-to-spring measurements and 21,600 acre-feet in fall-to-fall measurements. This contributed to significant groundwater replenishment; however, the cumulative storage within the Subbasin remained below other similar wet year storage values.
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. This analysis is critical to ensure careful consideration of potentially changed groundwater conditions before taking steps to implement PMAs or other response actions. Minimum Threshold, Measurable Objectives, and Undesirable Result are summarized in Table 1.
SUPPORTING DOCUMENTS
A. Napa County Groundwater Sustainability, Annual Report - Water Year 2023 Presentation (LSCE, March 2024)