Significant Changes Proposed to Napa County Water Availability Analysis - Public Comment Period Open Until May 26, 2026

POSTED BY‍ ‍Joshua S. Devore and Nick Conti‍ ‍

‍ ‍

Napa County has released for review its 2026 proposed updated Water Availability Analysis (“WAA”) Public Draft that introduces an updated framework to manage groundwater resources and water use across Napa County. This blog post summarizes key changes contained in the 2026 WAA Public Draft, which makes several critical changes to groundwater use and screening criteria throughout Napa County.

There are four key updates provided by the 2026 WAA Public Draft:

(1)   Updated Definitions to Groundwater Management Areas. First, the 2026 WAA established four areas with tailored requirements: (1) Napa Valley Floor, (2) Northeast Napa Management Area, (3) Groundwater Deficient Area, and (4) All Other Areas. The Northeast Napa Management Area is a new designation located within the Napa Valley Subbasin, generally located along Silverado Trail between downtown Napa and Soda Canyon.

(2)   Updated Groundwater Use Criteria (Tier 1). Screening criteria and limits for groundwater use have been updated. For Napa Valley Floor parcels, groundwater usage has been capped at 0.5 acre-feet/acre/year. This is a decrease from the 1.0 ac.ft/ac./yr. standard that was in place before the recent drought, but an increase from the current 0.3 ac.ft./ac./yr. standard that had been imposed on a temporary basis in 2022 and currently under the January 2024 Interim Well Permit Standards.

Significantly, the 2026 Draft proposes an additional, new limit of 1 acre-feet/year total for domestic uses, regardless of the size of the property. This proposal could significantly impact residential development that is dependent on groundwater.

For the new Northeast Napa Management Area and Milliken-Sarco-Tulucay (“MST”) groundwater-deficient Area, the groundwater usage has been capped at 0.3 acre-feet/acre/year, with a 0.6 acre-feet/year limit for total domestic uses on the parcel, or no net increase if domestic usage is already above 0.6 ac.ft./year.

(3)   Improved Well Interference Analysis (Tier 2). Clarifies methods for evaluating impacts to neighboring wells and springs. New or replacement domestic wells located within 500 feet of neighboring wells should be sited as far as feasible from neighboring wells. Tier 2 evaluations are required where neighboring off-site wells are located within 500 feet of the applicant’s existing or proposed well, but they are not required where the neighboring well within 500 feet is owned by the applicant. Wells within 1,500 feet of natural springs must be evaluated to assess potential connectivity between the spring and the aquifer system from which the groundwater is planned to be produced (Tier 2 analysis is required).

(4)   Enhanced Surface Water Protections (Tier 3). Provides additional analysis for groundwater extraction near significant streams (defined by Napa here). New or replacement wells located in the vicinity of significant streams may require protective standards, such as well seal depths of 150 feet or greater. Tier 3 evaluations (to assess whether the well may result in stream depletion) are required for wells located both inside and outside the Napa River Watershed within 1,500 feet of a significant stream. These provisions are based on the County’s “Public Trust” obligations to manage navigable waters.

As noted above, the 2026 WAA suggests capping all new domestic wells at no more than 1 acre-feet per year. The impact of this effort will be substantial, as it will sharply limit the available water use for, and thus the size of, new residential housing developments, including guest houses or accessory dwelling units (“ADU”). The accompanying Technical Supporting Documentation does not explain the basis for the 1 ac.ft./year limit.

Napa County has opened a public comment period that runs through May 26, 2026, encouraging community members and stakeholders throughout Napa County to review the draft documents and provide feedback. Additionally, there is an upcoming Planning Commission Public Meeting taking place on Wednesday, May 6, 2026 at 9:00 A.M., at the Board of Supervisors Chambers (1195 Third Street, Third Floor, Napa, CA 94559).

For further questions regarding Napa County’s 2026 Water Availability Analysis Public Draft, or any other water or land use matters, please reach out to Joshua S. Devore and DP&F’s Land Use team.