About the Subbasin
The Department of Water Resources has categorized and prioritized the Napa Valley Subbasin as one of 46 high priority basins statewide. It is therefore subject to the Sustainable Groundwater Management Act.
Napa Valley Subbasin
The Napa Valley Subbasin is the only groundwater basin in Napa County that is subject to the Sustainable Groundwater Management Act (SGMA) rules and regulations at this time. View a map of the basin.
The 71.8 square-mile Napa Valley Subbasin sits in the larger 426 square-mile Napa River watershed and underlies Calistoga, St Helena, Yountville, and Napa. The extent of the Napa Valley Subbasin generally aligns with the floor of the Napa Valley. The Subbasin consists of sediments that have been eroded from the surrounding mountains and deposited by the Napa River over millions of years. These sediments are permeable – they can soak up and hold a lot of water. The sediments are shallow near the base of the nearby hills and in the Calistoga area. The sediments can be up to several hundred feet thick in the center of the valley. Beneath these sediments lie older bedrock.
Groundwater within the Subbasin is used by farmers, commercial users, rural residents, and as part of the municipal water supply for the City of St Helena. There are over 2,600 wells that take water from the Subbasin. Importantly, groundwater also supports aquatic ecosystems and habitats for a variety of plants and wildlife.
Why is the Napa Valley Subbasin subject to Sustainable Groundwater Management Act?
Napa Valley Subbasin is categorized by the Department of Water Resources (DWR) as one of 46 high priority groundwater basins statewide. Medium and high priority basins are subject to the Sustainable Groundwater Management Act (SGMA) requirements. The Department of Water Resources (DWR) prioritized all basins in the state according to the following criteria:
- Population size
- Anticipated population growth rate
- Number of public supply wells
- Total number of wells
- Irrigated acreage
- The degree to which people rely on groundwater as their primary source of water.
- Any documented groundwater issues, such as overdraft, subsidence, saline intrusion, and other water quality degradation.
- Any other information determined to be relevant by DWR, including threats to local habitat and streamflows.
The Napa Valley Subbasin is categorized as high priority primarily due to the amount of irrigated lands, the density of wells, projected population growth, and the degree to which people rely on groundwater in the Subbasin.
DWR’s basin prioritization process is not a determination of whether groundwater basins are being managed sustainably. Rather, it is a way for DWR to determine the reliance on groundwater in individual basins across California and whether those basins should be subject to the requirements of SGMA.