Trends and Conditions

Napa County has several groundwater basins, the largest of which is the Napa Valley Subbasin.

Napa County and other public agencies have been studying our groundwater since the mid-1900s. Although water levels drop during the dry-season and during dry years, they fully recover when we have higher than average rainfall. Infiltration of precipitation has been shown to provide significant groundwater recharge in Napa County, particularly in unconsolidated geologic settings like much of the Napa Valley floor (Kunkel and Upson 1960, LSCE and MBK 2013). Recently, long term precipitation records in Napa County have been used to develop a water year rating system that defines Very Dry, Dry, Normal, Wet, and Very Wet water year types (LSCE 2016, 2NDNATURE 2013). Overall, groundwater levels in the main Napa Valley Subbasin have been relatively stable for decades. Groundwater conditions outside the Napa Valley Subbasin are more variable, such as in the Milliken-Sarco-Tulucay (MST) area.

Monitoring is important

The County monitors a network of wells to detect short-term and long-term trends in groundwater storage, use, levels, and quality. Well measurements are taken at least twice each year – before and after the dry season. This monitoring began in 1962. Over time, the County continues to add wells to gain a better understanding of groundwater and fill data gaps in the monitoring network. Most of the wells monitored by the County are located in the Napa Valley Subbasin and in the Milliken-Sarco-Tulocay area.

Groundwater conditions in a given year are strongly correlated to annual rainfall amounts. The County uses a network of rainfall gauges to gather rainfall information. 

Today, the County’s well monitoring network includes 97 wells. In addition, the County evaluates data from about 12 other wells monitored by other agencies. In total, the County tracks groundwater conditions in 64 wells located in the Napa Valley Subbasin. See a map of the wells here.

The groundwater monitoring network includes:

  • Wells on public land that the County owns and maintains
    • Includes 10 wells at 5 sites designed to monitor groundwater-surface water interactions
  • Wells on private land that the CA Department of Water Resources monitors
  • Wells on private property that the County monitors with permission from the landowners

The Napa County Groundwater Monitoring Plan (2013) describes the County’s approach to tracking groundwater levels and quality throughout the County. 

Watch this video to learn more about groundwater monitoring in Napa County, and how the results are used.


Current Groundwater Level Monitoring Sites

Map showing 2020 Groundwater Level Monitoring Sites

Overview of Trends and Conditions

Trends and conditions for each subarea in the County are tracked separately. Find information about the latest trends and conditions by visiting the webpage for each groundwater subarea:

Follow this link to find annual reports with additional details on groundwater trends and conditions.