Groundwater Sustainability Plan

A Groundwater Sustainability Plan (GSP) is a 20-year plan to ensure that groundwater is managed sustainably within a groundwater basin. One of the primary objectives of a GSP is to define a framework for sustainable groundwater management with consideration of the physical setting and beneficial uses and users of groundwater, consistent with the requirements of the Sustainable Groundwater Management Act (SGMA). A GSP must also determine how the basin will achieve sustainable groundwater management within a 20-year period through monitoring and management actions. In general, sustainability means avoiding all of the following effects due to groundwater conditions occurring throughout the basin: 

  1. Chronic lowering of groundwater level indicating a significant and unreasonable depletion of supply if continued over a 50-year period, 
  2. Significant and unreasonable reductions in groundwater storage, 
  3. Significant and unreasonable seawater intrusion, 
  4. Significant and unreasonable degraded water quality including contaminant plume migration, 
  5. Significant and unreasonable land subsidence that substantially interferes with surface land uses, and
  6. Depletions of interconnected surface waters that have a significant and unreasonable adverse impacts on beneficial uses of the surface water.

Sustainability Indicators and the corresponding Metric(s) Defined in GSP Regulations

GW-icon-1Lowering GW Levels:

Groundwater Elevation
GW-icon-2Reduction of Storage:

Extraction Volume
GW-icon-3Seawater Intrusion:

Chloride Concentration Isocontour
GW-icon-4 Degraded Quality:

Migration of Plumes,

Number of Supply Wells,


Location of Isocontour

GW-icon-5 Land Subsidence:

Rate and Extent of Land Subsidence
GW-icon-6 Surface Water Depletion:

Volume or Rate of Surface Water Depletion

(Source: DWR)

“Significant and unreasonable” effects must be determined by the Napa County Groundwater Sustainability Agency (GSA) with consideration of all of the beneficial uses and users of groundwater an interconnected surface water within the Napa Valley Groundwater Subbasin.

The Department of Water Resources (DWR) and the State Water Resources Control Board (State Board) are responsible for implementing the Sustainable Groundwater Management Act (SGMA). DWR has developed GSP regulations and guidance documents for developing and implementing GSPs. DWR is also responsible for reviewing GSPs adopted by GSAs and determining whether the plan and the ongoing implementation meet SGMA requirements. 

The State Water Resources Control Board is authorized to intervene and require reporting of groundwater extraction and adopt an interim GSP for all or part of a basin that lacks a GSA-adopted GSP or if the GSP is not being implemented in manner likely to achieve the sustainability goal for the basin. 

GSP Requirements

GSPs must describe and identify:

  • Who is involved in the Groundwater Sustainability Agency (GSA)
  • The basin’s geology and hydrogeology
  • How the GSA will define and measure sustainability
  • Programs and projects that get the basin to sustainability
  • How the GSP will be implemented


The Napa County Groundwater Sustainability Agency (GSA) has developed a GSP for the Napa Valley Subbasin with help from a Groundwater Sustainability Plan Advisory Committee (GSPAC), professional technical consultants, a facilitation team and Agency staff. The GSPAC reviewed draft sections of the GSP in monthly public meetings and submitted a final draft of the GSP to the GSA on November 1, 2021.  

The GSA adopted the Groundwater Sustainability Plan on January 11, 2022 and submitted the GSP  to DWR on January 31, 2022. DWR has 20 days to complete their initial review prior to making the GSP public on their SGMA portal. DWR will then offer a 75-day public comment period. DWR has two years to determine whether the GSP meets the requirements of SGMA.

The GSP must be updated every five years. Annual reporting is required to demonstrate implementation actions and progress towards either achieving or maintaining groundwater sustainability. 

Click on the GSP Schedule diagram below for a larger view on page 1 and a transcript on page 2 (PDF):

Napa Valley Subbasin GSP Timeline diagram and transcript Opens in new window