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 2017, groundwater level data were available for 107 sites throughout the county, including 13 of the 17 groundwater subareas and 61 sites within the Napa Valley Subbasin . 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 well above average for the 12-month water year that ended in September 2017, based on records from the long-term gauge at the Napa State Hospital in Napa. This marked the first year since water year 2011 that the Napa State Hospital gauge recorded above average precipitation. The precipitation total for water year 2017 at the Napa State Hospital gauge was 45.5 inches, while the average water year total from 1920 to 2015 was 24.86 inches.

 
 

In the principal aquifer system of the Napa Valley Subbasin, the volume of groundwater in storage increased in both spring 2016 and spring 2017 relative to the prior year. The magnitude of the increase in 2016 was 1,586 acre-feet greater than the increase in 2017 despite much more precipitation occurring in water year 2017. This result is consistent with the finding that the Subbasin has been at a relatively full condition with respect to groundwater storage capacity.

Maps of saturated thickness and groundwater storage changes in the principal aquifer system show increases in saturated thickness and groundwater storage, primarily from St. Helena southward in the Subbasin between 2015 and 2016. A small area of less than two feet of saturated thickness decrease is mapped near Rutherford. The greatest increases in saturated thickness and groundwater storage occurred along the western margin of the Subbasin along Dry Creek. The change in saturated thickness and groundwater storage from spring 2016 to spring 2017 were also broadly positive, with no areas showing a decrease in saturated thickness greater than two feet.

 

Napa Valley Subbasin
Groundwater Storage Changes, Water Years 1988 - 2017
 

 

Napa Valley Subbasin
Groundwater use and Groundwater storage changes 1988 - 2017 

 

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage Spring 2015 to 2016

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage, Spring 2015 to 2016

 

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage Spring 2016 to 2017

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage, Spring 2016 to 2017

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness Spring 2015 to 2016

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness, Spring 2015 to 2016

 

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness Spring 2016 to 2017

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness, Spring 2016 to 2017

 

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 34,793 acre-feet in water year 2016 and 34,142 acre-feet in water year 2017. Total estimated groundwater use in the Subbasin was 17,039 acre-feet in water year 2016 and 15,831 acre-feet in water year 2017. As noted above annual groundwater storage changes were positive in both water years 2016 and 2017, at 6,056 acre-feet and 4,470 acre-feet, respectively. Cumulative changes in groundwater storage show a net increase of 13,702 acre-feet from water years 1988 to 2017 in the principal aquifer of Napa Valley Subbasin.

Groundwater use in water years 2016 and 2017 was comparable to amounts used in recent years dating back to 2004. Over the 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 stable groundwater storage conditions and the absence of chronic depletions of groundwater storage. Groundwater use in the Subbasin in water years 2016 and 2017 remained below 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 use demonstrate that the Napa Valley Subbasin has continued to be managed sustainably through 2017.

Interactive Map

Using the map below, 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