Napa County has been engaged in groundwater management since 1991 when it completed an analysis of the safe yield for the Napa Valley aquifer system and established water use thresholds for groundwater permitting. The County and Napa Valley municipalities then formed the Water Advisory Committee in 1992 and developed coordinated projections of future water supplies, demands, and management strategies and actions to avoid shortfalls. In the years since, the County revised and updated its approach to groundwater management in response to new information with the intent of avoiding overdraft and related concerns including subsidence, groundwater level declines, water quality degradation, and other adverse environmental impacts.
One of the primary tasks of the Groundwater Sustainability Plan (GSP) will be to define a framework for sustainable groundwater management of the Napa Valley Subbasin while also considering the physical setting and beneficial uses and users of groundwater, consistent with the requirements of the Sustainable Groundwater Management Act (SGMA). The framework will be based on six sustainability indicators described by SGMA.
Sustainability Indicators and the corresponding Metric(s) Defined in GSP Regulations
Lowering GW Levels:
Reduction of Storage:
Chloride Concentration Isocontour
Migration of Plumes,
Number of Supply Wells,
Location of Isocontour
Rate and Extent of Land Subsidence
Surface Water Depletion:
Volume or Rate of Surface Water Depletion
The GSP will include descriptions of undesirable results associated with each sustainability indicator and will set corresponding minimum thresholds, measurable objectives, and interim milestones for each sustainability indicator. GSP regulations define a minimum threshold as a numeric value for each sustainability indicator used to define undesirable results.” Measurable objectives are “specific, quantifiable goals that are included in an adopted GSP to achieve the sustainability goal for the basin.” An interim milestone is “a target value representing measurable groundwater conditions, in increments of five years, set by a Groundwater Sustainability Agency (GSA) as part of a GSP. As part of the Napa Valley Subbasin GSP development process, the Napa County GSA and the GSPAC will consider scientific data and input from stakeholders and the community to describe undesirable results in the context of Napa Valley and create management actions and policies that will be implemented to avoid undesirable results.
In 2016, in response to the 2014 Sustainable Groundwater Management Act (SGMA), the County developed a Napa Valley Subbasin SGMA sustainability goal and SGMA management criteria for each of the six sustainability indicators, including numerical minimum thresholds and numerical measurable objectives at 21 representative monitoring sites throughout the Napa Valley Subbasin. The SGMA management criteria developed in 2016 were based on a review of best available historical data and a water budget analysis prepared consistent with GSP regulations. Subsequent annual reports prepared by the County have compared ongoing monitoring data to the SGMA management criteria, as part of the County’s continuing implementation of SGMA. The SGMA management criteria developed for the GSP will be informed by the criteria developed in 2016; however, the Napa Valley Subbasin GSP will also consider more recent stakeholder input, guidance from the Department of Water Resources, and additional data and updated water budget results.
For more detail on all of these indicators, read the Annual Reports.