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Overall, the Napa Valley Subbasin remains full. The sustainable yield is estimated at between 17,000 and 20,000 acre-feet (AF) per year. (An acre-foot is equal to 325,851 gallons.) The stability of water levels over the past 10 years indicates that the amount of groundwater extraction has been balanced by the amount of groundwater recharge.
Over 100 wells are monitored at least twice a year to track groundwater conditions. Of these, 24 wells in the monitoring network were measured in the spring of 2020 to calculate the change in groundwater storage for the 2020 water year. Consistent with a Very Dry water year, the amount of groundwater in storage decreased in 2020 from 221,358 AF to 196,651 AF.
The volume of groundwater in storage in spring 2020 is below the average (209,407 AF) and median (210,929 AF) volumes calculated from 1988 to 2020. The amount of groundwater storage changes from year to year depending on how much rainfall was received the previous year. Rainfall during water year 2020 (12.19 inches) was the driest year recorded since 1977 (11.23ches). Since 1950, the median annual rainfall at the Napa State Hospital has been 22.84 inches. In 2020, rainfall at the State Hospital was 53% of the median.
Total water use from all sources (State Water Project, surface water, groundwater, and recycled water) within the Napa Valley Subbasin was estimated at 38,073 AF in 2020. Of the total water use, 17,933 AF (47%) came from groundwater pumping. The other 20,140 AF (53%) came from reservoirs, State Water Project, and recycled water.
About 71% of the groundwater used within the subbasin in 2020 was for agricultural irrigation (12,675 AF). Another 27% was used for unincorporated residential and business use (4,840 AF). The remaining 2% (418 AF) went to urban uses in the City of St. Helena.
The amount of water used by groundwater dependent ecosystems was estimated at 4,184 AF.
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Both planning efforts will propose projects and management actions to achieve their respective goals. While the two efforts are on slightly different timelines, there is opportunity for collaborating on potential joint projects that address both groundwater and drought resiliency. The Draft Napa Valley DCP is expected to be available in Spring 2022 while the adopted GSP is currently available.
Napa County has been monitoring groundwater conditions since the 1960s, when it collaborated with the U.S. Geological Survey on a study of groundwater resources in Napa Valley. Since 2009, the County has implemented additional programs to implement the General Plan and better understand, assess, and maintain groundwater sustainability. To further General Plan implementation, the County appointed a Groundwater Resources Advisory Committee (GRAC) in 2011. One of the Committee’s products was the 2013 Napa County Groundwater Monitoring Plan (GMP), to document and enhance the groundwater monitoring efforts that had previously been conducted under the County’s Comprehensive Groundwater Monitoring Program. The GMP recommended annual reports on groundwater conditions and modifications to the countywide groundwater monitoring program as needed. The first report was issued in 2015, analyzing 2014 water year data.
For the 2020 water year report (published in 5/2021), groundwater monitoring was conducted at 107 wells across Napa County that are representative of various geologic and geographic conditions. These included 60 wells within the Subbasin, as well as 47 wells located in other areas of the County. The cost for preparing the report this year was $42,000.
In the spring of 2020, groundwater depths in the alluvial aquifer of the Napa Valley Subbasin ranged from 7 feet to approximately 50 feet below the surface. Groundwater level trends in the alluvial aquifer system of the Napa Valley Subbasin are stable in most wells with long-term groundwater level records. Changes in groundwater levels from one year to the next generally reflect the amount of rainfall received.
Differences in the subsurface geology of the Napa Valley Subbasin are important to the interpretation of groundwater levels, particularly for wells constructed entirely or partially within the alluvium in Napa Valley. Wells located entirely within the alluvium have relatively shallow spring depths to groundwater. Wells built partially or entirely in geologic formations below the alluvium may experience different depths to groundwater. As a result, two wells located very near to each other may have very different depths to groundwater depending on how these wells are constructed and whether they are tapping into the alluvial aquifer or a deeper, confined aquifer.
Please visit the County’s Water Conservation website for ways to reduce water use in your home and garden.