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Napa County Groundwater Rules: WAA, Well Permits and the Subbasin Plan

By Matthew Martinez, Founder, CEO & Managing Broker, Diamond Real Estate Group. California DRE #01869679.

In Napa County, a Water Availability Analysis (WAA) is required for any discretionary project that may use groundwater or increase how intensively a parcel uses it, such as a new winery or a use permit change. The analysis screens the project against a groundwater use limit that depends on location: 1.0 acre-foot per acre per year on the Napa Valley floor, 0.3 in the Milliken-Sarco-Tulucay (MST) groundwater-deficient area, and a parcel-specific recharge figure elsewhere, under the county’s 2015 guidance. New well permits are screened under interim standards from January 2024 that use the same three tiers, and a March 2026 public draft would tighten the valley floor limit to 0.5.

On top of the county rules sits the Napa Valley Subbasin’s groundwater sustainability plan, approved by the state on January 26, 2023, which brings fees, monitoring and the possibility of future pumping limits. This guide explains how the layers fit together for someone buying, planting or permitting a vineyard or winery. For the wider land picture, see our guides to Napa County land for sale and the Napa Valley appellation.

Who regulates groundwater in Napa County

Body	What it does	Where it matters to a buyer
Napa County Planning, Building & Environmental Services (PBES)	Reviews WAAs for discretionary projects under CEQA; issues well permits under the county’s well standards	Winery use permits, vineyard projects needing discretionary approval, new and replacement wells
Napa County Groundwater Sustainability Agency (NCGSA)	Carries out the Napa Valley Subbasin groundwater sustainability plan; sets the groundwater fee	Parcels inside the subbasin: fees on the tax bill, monitoring and any future limits
California Department of Water Resources (DWR)	Sets basin priority and reviews groundwater plans under the Sustainable Groundwater Management Act (SGMA)	The subbasin is rated high priority, so a plan is mandatory

Surface water is a separate system. Diversions from creeks and springs, and many reservoirs, need state water rights; our guide to vineyard water rights covers that side.

When a Water Availability Analysis is required

The county’s guidance, adopted May 12, 2015, requires a WAA for “any discretionary project that may utilize groundwater or will increase the intensity of groundwater use” of a parcel, and when a discretionary groundwater permit is needed. Ministerial projects, such as building permits for a single-family home, aren’t subject to CEQA and generally don’t need one.

The applicant or the applicant’s agent prepares the WAA and submits it to PBES. Projects that need the more detailed second or third tier may need a California registered professional geologist or engineer.

The three tiers

Tier	2015 guidance	March 2026 public draft
Tier 1: water use	Valley floor 1.0 acre-foot per acre per year; MST 0.3 or no net increase, whichever is less; all other areas parcel-specific recharge	Valley floor 0.5 (1 acre-foot a year for domestic use); new Northeast Napa Management Area and MST 0.3 (0.6 for domestic use); other areas parcel-specific recharge, with a streamlined estimate
Tier 2: well and spring interference	Presumed met with no other wells within 500 feet; default drawdown limits of 10 feet (casing 6 inches or less) and 15 feet (larger casing) at neighboring wells; springs within 1,500 feet evaluated	Same 500-foot and 1,500-foot screens; drawdown limits of 10 feet for domestic or public supply wells and 15 feet for agricultural, commercial or industrial wells
Tier 3: groundwater and surface water	Applied when evidence shows it is needed; distance and well-construction tables	Screens wells within 1,500 feet of significant streams; protective standards include well seals to at least 150 feet and, within 500 feet of a significant stream, metering and reporting

The county released the draft on March 30, 2026, with public comment through May 26, 2026. As of our October 2026 review we had not confirmed that it has been adopted, so the 2015 guidance and the January 2024 well standards remain the documents to check. Ask PBES which version applies when you apply.

Estimating a project’s water use

Appendix B of the 2015 guidance gives example water use figures. They are starting points; the county asks applicants to use the most appropriate data for their project.

Use	2015 WAA estimate
Primary residence	0.5 to 0.75 acre-foot a year
Second residence or farm labor dwelling	0.20 to 0.50 acre-foot a year
Vineyard irrigation	0.2 to 0.5 acre-foot per acre a year
Vineyard frost protection	0.25 acre-foot per acre a year
Vineyard heat protection	0.25 acre-foot per acre a year
Winery process water	2.15 acre-feet per 100,000 gallons of wine
Winery domestic and landscaping	0.50 acre-foot per 100,000 gallons of wine
Employees	15 gallons per shift
Tasting room visitors	3 gallons per visitor
Events with on-site catering	15 gallons per visitor

An illustration

Take a hypothetical 40-acre parcel on the valley floor with a home and 30 planted acres. Using the top of the ranges, the home is 0.75 acre-foot, irrigation 15 acre-feet (30 x 0.5) and frost protection 7.5 acre-feet (30 x 0.25), about 23 acre-feet a year in all. Under the 2015 Tier 1 criterion the parcel’s screen is 40 acre-feet (40 x 1.0); under the 2026 draft’s 0.5 it would be about 20.

The same farming could pass one screen and fail the other, which is why buyers adding a winery or new plantings should run the numbers against both documents. A project that exceeds Tier 1 isn’t automatically denied, but it needs more analysis and may need to cut water use. Our guide to vineyard development covers the rest of the planting approvals.

New and replacement wells

The county’s January 2024 interim well permit standards apply the WAA tiers to new wells, with separate columns for the MST, the Napa Valley Subbasin and the rest of the county. They cite the Governor’s drought Executive Orders N-7-22 and N-3-23, CEQA, the public trust doctrine and the subbasin plan.

Domestic wells. The standards assume individual domestic users pump less than 2 acre-feet a year, and a Water Use Declaration can satisfy the requirement for certain domestic users.
Interference screens. A well less than 500 feet from a neighboring well or 1,500 feet from a natural spring triggers a Tier 2 study, and one less than 1,500 feet from a significant stream triggers Tier 3, prepared by a licensed professional at the applicant’s cost.
Use above the limit. Where existing use is already above 0.3 acre-foot per acre, or above the parcel’s recharge, the standards call for no net increase.
Replacement wells. No analysis is needed for a replacement well that is farther from neighboring wells, springs and streams and doesn’t increase use.

The county now lists these standards among its archived well permit materials, so confirm with Environmental Health which standards it applies before you budget for a well. For permits, testing and septic on a homesite, see our guide to wells, septic and groundwater in Napa County.

The Napa Valley Subbasin plan

The Napa Valley Subbasin covers 45,928 acres. DWR rated it medium priority in 2014 and re-designated it high priority in 2019; the county says that change was not due to groundwater conditions. High and medium priority basins must have a groundwater sustainability plan and reach sustainability within 20 years.

Adopted and approved. The NCGSA adopted its plan on January 11, 2022 and submitted it on January 31, 2022. DWR approved it on January 26, 2023.
Goal. The absence of undesirable results within 20 years, which the 2026 WAA draft describes as no later than 2042.
Sustainable yield. The plan’s executive summary puts it at about 15,000 acre-feet a year, measured against a seven-year average of net extraction.
Streams. It sets interim objectives for streamflow depletion at two gauges, Pope Street and Oak Knoll Avenue.
Possible actions. Vineyard and winery water conservation, managed aquifer recharge, more recycled water, new well permit conditions, a Northeast Napa management area and, as a later tier, pumping reductions that don’t apply to de minimis users.

The county says domestic wells pumping under 2 acre-feet a year are considered de minimis and could be exempt from most SGMA requirements, including metering, reporting and fees. The NCGSA has released a draft periodic evaluation of the plan, with comments due November 9, 2026, and the county says the next plan update is due in 2027.

The groundwater sustainability fee

The NCGSA adopted a fee structure on December 9, 2025 (Resolution 2025-05), and on August 11, 2026 it temporarily reduced all rates by about 55% for the 2026-27 fiscal year (Resolution 2026-03), using $1.25 million from the GSA fund balance and the county General Fund. Because the cut is temporary, budget for the possibility that later years return toward the maximum authorized rates. The fee applies only to groundwater users on parcels inside the subbasin and is levied on the property tax bill for most parcels.

User class	FY 2026-27 rate	Maximum authorized
Agricultural, base rate	$17.53 per planted acre	$38.58
Agricultural, additional rate	$26.91 per groundwater-irrigated acre	$60.16
Self-supplied user (mainly domestic wells)	$28.17 per parcel	$62.58
Public water system	$58.47 per acre-foot	$129.87

Dry-farmed acres pay only the base rate, and a parcel with planted acres and a home pays both the agricultural fee and the parcel fee. Properties served by Calistoga, St. Helena, Napa or Yountville water are outside the fee, and households at or below 80% of county median income, adjusted for household size, could apply for a waiver that must be renewed each year.

The MST groundwater-deficient area

The Milliken-Sarco-Tulucay (MST) area is the county’s designated groundwater-deficient area, and it has the strictest WAA limit: 0.3 acre-foot per acre per year or no net increase, whichever is less. Much of the Coombsville appellation sits in it; our Coombsville guide covers the neighborhood.

The county and Napa Sanitation District have worked on a recycled water pipeline to the area. A Community Facilities District formed in December 2012 finances it, joining is voluntary, and the county estimates a maximum of $2,000 a year for 20 years for a residential parcel with less than an acre of vineyard. Recycled water itself costs $1.57 per 1,000 gallons, plus on-site connection costs.

The county page says it will decide whether to proceed after bids, owner commitments and funding are in place, so confirm the current status. See our guide to recycled water for vineyards.

Water due diligence for a Napa vineyard or winery

Locate the parcel on the county’s maps: inside the subbasin, the MST, the proposed Northeast Napa area, or elsewhere.
Collect the well records: well completion reports, permits, pump tests, any meter data and recent water-quality results.
Read the use permit and any prior WAA for water limits, monitoring and reporting conditions; our guide to the Napa winery use permit explains where these sit.
Check the tax bill for the groundwater fee and confirm the planted and irrigated acreage it is based on.
Map nearby wells, springs and significant streams to see whether a new well would trigger Tier 2 or Tier 3.
Model your plan against both the 2015 criteria and the 2026 draft, with a hydrogeologist if you plan new plantings, a winery or more visitors.
Confirm surface water rights for any pond, reservoir or creek diversion.

Our vineyard due diligence guide puts water alongside soils, vines and permits. If you also look across the county line, our guide to Sonoma County groundwater rules explains a different system, with three basin agencies and a well ordinance still affected by litigation.

Questions people ask

Do I need a Water Availability Analysis to build a house in Napa County?

Generally not. The 2015 guidance says ministerial projects such as building permits for single-family homes aren’t subject to CEQA and generally don’t need a WAA. A new well still needs a permit and is screened under the county’s well standards.

What is the WAA limit on the Napa Valley floor?

Under the 2015 guidance, 1.0 acre-foot per acre per year. The March 2026 public draft would lower it to 0.5. Check which version PBES applies when you file.

Will the subbasin plan limit pumping from my well?

Not today. The plan lists pumping reductions as a possible later action, alongside conservation and recharge, and says they wouldn’t apply to de minimis users pumping under 2 acre-feet a year. Larger agricultural users should watch the 2026 periodic evaluation and the 2027 update.

How much is the Napa groundwater fee?

For 2026-27, $17.53 per planted acre plus $26.91 per groundwater-irrigated acre for agriculture, and $28.17 per parcel for self-supplied users such as domestic wells, inside the subbasin only. Those are temporarily reduced rates; the maximum authorized parcel rate is $62.58.

Is Coombsville in a groundwater-deficient area?

Much of it is in the MST, where discretionary projects face a limit of 0.3 acre-foot per acre per year or no net increase, whichever is less.

Can I replace an old well without a new analysis?

Under the January 2024 standards, a replacement well that sits farther from neighboring wells, springs and streams and doesn’t increase use doesn’t need the analysis. It still needs a permit, so confirm current requirements with the county.

Buying vineyard land with the water picture clear

Water is often the deciding question in a Napa vineyard or winery purchase. This is general information, not legal or technical advice; talk to a hydrogeologist or a California water attorney about a specific parcel. When you are weighing a property, our vineyard and winery team can help you gather the well, permit and fee records early.

Sources & notes

Last reviewed October 2026 by Matthew Martinez.