Groundwater Monitoring

Overview

Given the importance of sustainable groundwater resources in Napa County, long-term, systematic monitoring programs are essential to provide data that allow for improved evaluation of water resources, conditions, and to facilitate effective water resources planning.

Groundwater data availability in Napa County varies widely among the subareas. The bulk of the historical and current groundwater level and quality data is located in the Napa Valley Floor Subareas. Recent efforts have expanded the geographic extent of data collection to other subareas.

In 2018, groundwater level data was available for 108 sites throughout the county, including 13 of the 17 groundwater subareas and 61 sites within the Napa Valley Subbasin. In 2018, the Department of Water Resources resumed monitoring efforts at four wells that had been suspended in response to multiple, large wildfires that burned in many areas around Napa County in October 2017. The majority of these sites were privately owned wells volunteered by their owners and monitored by Napa County. Here is a video that highlights the importance of the program and the data collected.

Precipitation was below average for the 12-month water year that ended in September 2018, based on records from the long-term rain gauge at the Napa State Hospital. The precipitation total for water year 2018 was 19.3 inches, while the average water year total from 1920 to 2015 was 24.86 inches. The most recent water year with similar precipitation total was 2014.

 

 
 

Consistent with the Dry water year conditions, the volume of groundwater in storage in the principal aquifer system of the Napa Valley Subbasin decreased in spring 2018 relative to spring 2017. The volume of groundwater in storage declined in 2018 by 9,314 acre-feet to a total of 209,984 acre-feet. From 1988 through 2018, cumulative annual storage increased 4,388 acre-feet, reflecting a basin in balance and the absence of long-term depletions of groundwater storage within the Subbasin.

Maps of saturated thickness and groundwater storage changes in the principal aquifer system show decreases in saturated thickness and groundwater storage throughout most of the Subbasin between spring 2017 and spring 2018. These decreases are consistent with the reduction in precipitation between 2017 and 2018. Larger decreases in saturated thickness occurred along Dry Creek and in the vicinity of Oak Knoll Avenue. Notably, the reductions in groundwater storage were variable over that area. For example, near the dedicated monitoring well at Dry Creek near Highway 29 declines were much smaller than in the active supply well monitored to the west near the Subbasin margin. This latter observation highlights the value of dedicated monitoring wells.

Changes in saturated thickness of the primary aquifer and groundwater storage volume changes were also evaluated for the period from spring 2014 to spring 2018, for comparison with the most recent year with a similar precipitation total. Saturated thickness and groundwater storage were greater in spring 2018 than in spring 2014. Spring 2018 saturated thickness was generally 0 to 10 feet greater than the saturated thickness in spring 2014.

 

Napa Valley Subbasin
Groundwater Storage Changes, Water Years 1988 - 2018
 

 

Napa Valley Subbasin
Groundwater use and Groundwater storage changes 1988 - 2018 

 

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage Spring 2017 to 2018

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage, Spring 2017 to 2018

 

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage Spring 2014 to 2018

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage, Spring 2014 to 2018

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness Spring 2017 to 2018

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness, Spring 2017 to 2018

 

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness Spring 2014 to 2018

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness, Spring 2014 to 2018

 

Total water use in the Napa Valley Subbasin, including groundwater extracted from the Subbasin, surface water from sources within the Napa River Watershed, and imported surface water delivered through the State Water Project, is estimated to have been 37,174 acre-feet in water year 2018. Total estimated groundwater extraction in the Subbasin was 17,889 acre-feet. As noted above, groundwater storage volume decreased in 2018, by 9,413 acre-feet. Cumulative changes in groundwater storage show a net increase of 4,388 acre-feet from 1988 – 2018 in the principal aquifer of Napa Valley Subbasin.

Groundwater extraction in water year 2018 was comparable to amounts used in recent years dating back to 2004. Over a full 30-year period, annual storage changes in the aquifer system have fluctuated between positive and negative values, generally in accordance with the water year type. Cumulative changes in groundwater storage have also fluctuated between positive and negative values, indicating long-term stable groundwater storage conditions, the absence of chronic depletions of groundwater storage, and an overall condition of a basin in balance. Groundwater extraction in the Subbasin in water year 2018 remained within the sustainable yield range of 17,000 to 20,000 acre-feet per year identified in the Basin Analysis Report. Together, the findings presented in this report regarding groundwater conditions at representative monitoring sites, changes in groundwater storage, and groundwater extraction demonstrate that the Napa Valley Subbasin has continued to be managed sustainably through 2018.

Interactive Map

Using the map below, click "show legend" to see map data layers, click on specific areas to find more information and links to subarea groundwater monitoring results, levels and trends. Enter your address, city or zip above the legend to zoom to an area of interest. 

To navigate to groundwater subarea pages, use the links below. Links can also be found in the popups when clicking on certain subareas in the interactive map.

Angwin and Pope Valley | Carneros, Jameson/American Canyon, and Napa River Marshes | Eastern Mountains and Western Mountains | Napa Valley Floor – Milliken-Sarco-Tulucay | Napa Valley Floor – Yountville and Napa | Napa Valley Floor – Calistoga and St. Helena