What is the recharge rate of the Napa Valley Sub-basin and what does that mean?

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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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