What is the Annual Groundwater Report and what are its most recent findings?

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.

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1. How is the GSP related to the Napa Valley Drought Contingency Plan?
2. What is the Annual Groundwater Report and what are its most recent findings?
3. What is the recharge rate of the Napa Valley Sub-basin and what does that mean?
4. What can I do to reduce water use?
5. Return to the Groundwater FAQ page