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 2019, groundwater level data was available for 109 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 above average for the 12-month water year that ended in September 2019, based on records from the long-term rain gauge at the Napa State Hospital. Overall, groundwater levels in fall 2019 remained comparable to levels in recent years. Groundwater levels in spring and fall 2019 were also generally above levels recorded in 2011, the most recent water year with a similar annual precipitation total.

The precipitation total for water year 2019 was 33.29 inches, compared to 2018 which was 19.3 inches, while the average water year total from 1920 to 2015 was 24.86 inches. The most recent water year with a similar precipitation total was 2011.

In the principal aquifer system of the Napa Valley Subbasin, the volume of groundwater in storage increased in spring 2019 (a Wet year) relative to spring 2018 (a Dry year) based on an analysis of groundwater levels measured throughout the Subbasin. The volume of groundwater in storage increased in 2019 by 11,374 acre-feet to a total of 221,358 acre-feet. From 1988 through 2019, the cumulative annual storage change was 15,762 acre-feet in the Subbasin; this reflects 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 increases in saturated thickness and groundwater storage throughout most of the Subbasin between spring 2018 and spring 2019. These increases are consistent with the increase in precipitation between 2018 and 2019. Larger increases in saturated thickness occurred along Dry Creek and in the vicinity of Oak Knoll Avenue. Notably, the increases in groundwater storage were variable over that area. For example, near the dedicated monitoring well at Dry Creek near Highway 29, accumulations 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 2011 to spring 2019, for comparison with the most recent year with a similar precipitation total. Saturated thickness and groundwater storage volumes were slightly greater in spring 2019 compared to spring 2011. 

 

Napa Valley Subbasin
Groundwater Storage Changes, Water Years 1988 - 2019
 

 

Napa Valley Subbasin
Groundwater use and Groundwater storage changes 1988 - 2019 

 

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage Spring 2017 to 2019

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage, Spring 2018 to 2019

 

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage Spring 2011 to 2019

Napa Valley Subbasin Principal Aquifer Change in Groundwater Storage, Spring 2011 to 2019

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness Spring 2018 to 2019

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness, Spring 2018 to 2019

 

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness Spring 2011 to 2019

Napa Valley Subbasin Principal Aquifer Change in Saturated Thickness, Spring 2011 to 2019

 

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 37,098 acre-feet in water year 2019.

Total estimated groundwater use in the Subbasin was 18,005 acre-feet in water year 2019 compared to 2018 which was 17,889 acre-feet. Cumulative changes in groundwater storage show a net increase of 15,762 acre-feet from water year 1988 to 2019.

Groundwater extraction in water year 2019 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 2019 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 2019.

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