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 below 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 similar precipitation total was 2014.
Groundwater level trends in the alluvial aquifer system of the Napa Valley Subbasin of the Napa-Sonoma Valley Groundwater Basin are stable in the majority of wells with long-term groundwater level records. Spring 2019 groundwater levels were generally somewhat higher compared to spring 2018, which was a Dry year. Overall, the increased recharge potential in 2019 due to Wet year conditions (33.29 inches of rainfall), 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.
Although designated as a groundwater subarea for local planning purposes, the majority of the Milliken-Sarco-Tulucay (MST) Subarea is not part of a groundwater basin as mapped by the California Department of Water Resources (DWR). Groundwater level declines observed in the MST Subarea as early as the 1960s and 1970s have stabilized since about 2009. Groundwater level responses differ within the MST Subarea and even within the north, central, and southern sections of this subarea, indicating that localized conditions, whether geologic or anthropogenic in nature, might be the primary influence on groundwater conditions in this local subarea. An expanding recycled water distribution system in the MST subarea, supplied by the Napa Sanitation District, delivered over 297 acre-feet of recycled water to users in the MST Subarea in water year 2019. Increased use of this new source of water along with continued land use permitting constraints are expected to aid in maintaining stable groundwater level conditions in the MST subarea in the future.
In the past several years, Napa County has developed a more focused understanding of the geology that controls the occurrence and availability of groundwater. With this updated geologic information and with assistance from community members serving on the Groundwater Resources Advisory Committee (2009-2011), the County expanded the number and distribution of wells that it monitors through a concerted effort to improve the understanding of groundwater resources countywide. Between 2011 and 2015 the number of County monitored wells increased from 47 to 100. In 2019, Napa County monitored 97 wells. One new well was added to the network in the Calistoga subarea near the margin of the Napa Valley Subbasin where surficial alluvial deposits transition volcanic rock formations. Part of the monitoring network expansion in recent years occurred through the construction of ten dedicated monitoring wells at five sites in Napa Valley, designed specifically to provide data on the interactions between groundwater and surface water.
2019 Groundwater Level Monitoring Sites
Despite the recent monitoring network expansion, there are additional areas where data are lacking and additional wells are needed to fill data gaps. To request more information on options for including your well in the County’s monitoring program, please contact Jeff Sharp, Principal Planner, at (707) 259-5936.
The Basin Analysis Report for the Napa Valley Subbasin provides an updated sustainability goal for the Subbasin based on the requirements of SGMA (LSCE, 2016c). The Basin Analysis Report meets the functionally equivalent standard for alternatives to a Groundwater Sustainability Plan (GSP) in part by updating sustainability criteria for the Napa Valley Subbasin in conformance to the definitions provided in SGMA. To evaluate the condition of the Subbasin in relation to the sustainability goal, the sustainability criteria include measurable objectives and minimum thresholds developed to avoid the six undesirable results identified in SGMA (LSCE, 2016c). For SGMA purposes a “measurable objective” is a “specific, quantifiable goals for the maintenance or improvement of specified groundwater conditions” (Section 351). SGMA additionally defines a “minimum threshold” as “a numeric value for each sustainability indicator used to define undesirable results” (Section 351).
2019 Groundwater Sustainability Indicators
Groundwater levels recorded in 2019 were above the minimum thresholds established as sustainability criteria for 19 of 20 Sustainable Groundwater Management Act (SGMA) Representative Wells with water level criteria. The reduction of groundwater levels below the minimum threshold at one of twenty SGMA Representative Wells, 08N06W10Q001M. The water level in that well was two feet below the minimum threshold in September 2019; however, subsequent measurements at the same well were above the minimum threshold in November and December 2019. Two other wells in the vicinity, NapaCounty-224 and NapaCounty-225, did not experience similar groundwater level conditions in fall 2019, and is likely the result of a short-term localized groundwater condition and does not reflect a changed condition in the Napa Valley Subbasin. Subsequent monitoring has found that water levels in well 08N06W10Q001M increased throughout the winter of 2018-2019, even before substantial rainfall occurred. In response to the fall 2019 groundwater level in well 08N06W10Q001M, the County is reviewing conditions in the vicinity of the well including water use and the location and operation of nearby wells.
The measurable objectives established in the Basin Analysis Report for the Napa Valley Subbasin provide a reasonable margin of operational flexibility under adverse conditions where applicable and utilize components such as historical water budgets, seasonal and long-term trends, and periods of drought. Groundwater elevations serve as the proxy for multiple sustainability indicators where reasonable. For representative monitoring sites where, long-term periods of record are not available, as in the case of the dedicated monitoring wells constructed in 2014, which were developed specifically to monitor groundwater-surface water interactions, measurable objectives established at these facilities will be reviewed and reevaluated as appropriate, as the collection of available data for each site expands to better reflect true long-term variability at those locations.
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.
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.
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.