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8-2-1 Water Supply, Use and Conservation Assessment Guidelines

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Version: 01/07/2020


The purpose of this policy is to provide guidance to applicants and their representatives on how to prepare a Water Supply, Use, and Conservation Assessment (henceforth, the “Assessment”). The Assessment may be a standalone document, or supplemental to a hydrogeologic study, Zero Net Use report, or other water supply related report. These guidelines are intended for discretionary and ministerial projects.

Discretionary projects that are dependent on groundwater or surface water will typically require an Assessment with the use permit application. The Assessment will inform the environmental review process and conditions of approval.

An Assessment may also be required for non-discretionary projects, such as development of an accessory dwelling unit in a Class 4 groundwater area where zero net increase in groundwater use is required.


Sonoma County General Plan, Water Resource Element Goals WR-2 and WR-4, Objective WR-4.1, WR-4.2, and WR-4.3, and Policies WR-2c, WR-2d, WR-2e, WR-4b, and WR-4f.


The following information is required for all Assessments unless indicated otherwise by Permit Sonoma:

  1. Introduction

    1. Name and credentials of the individual preparing the Assessment
    2. Project description;
    3. Study area description. Generally, the study area will be the project parcel, but may extend to the local groundwater basin or recharge area, subject to approval by the Director;
    4. Photographs of the site.
  2. Site Plan

    Prepared in accordance with Sonoma County’s Site Plan Requirements for Planning Applications (PJR-129) and includes the following:

    1. Delineation of the study area;
    2. Water-related infrastructure including wells, pools, reservoirs, ponds, water tanks, rainwater catchment surfaces, stormwater features, leach fields, wetlands, springs, streams, and drainage ways;
    3. Delineated irrigated landscape and irrigated agricultural areas.
  3. Water Use Estimates

    1. Estimate existing and proposed water uses of the project, parcel, or study area, as applicable to the Assessment;
    2. Water use estimates should be from default water use rates and/or methods (Appendix A). Note, similar methods and assumptions of water use should be used for existing and proposed conditions;
    3. Distinguish water uses, including but not limited to: residential domestic, employee, project-related irrigation, landscape irrigation, agricultural irrigation, other water uses;
    4. Identify the water source(s) for each use;
    5. Provide tables of annual and monthly water use for each use and each source.

      Note: Water use estimates by month may be required of discretionary permit applications.
  4. Water Conservation

    1. Describe water conservation measures applicable to the proposal that guarantee with reasonable certainty that water conservation will be achieved:
      1. Plumbing fixtures;
      2. Graywater or process water reuse;
      3. Landscape alterations;
      4. Other.
      Note: Water conservation measures reliant on adjustment of personal habits will not be approved.

The following information should be provided, as applicable to the Assessment

  1. Water Wells

    1. Provide all available well completion reports for existing wells, abandoned wells, or attempted wells on the project parcel;
    2. Provide available yield tests for wells that provide water for the project or parcel.
  2. Surface Water And Springs

    1. Describe surface water diversion and storage systems for the project or parcel;
    2. Provide available surface water rights or registration documentation;
    3. Provide available yield tests for springs that provide water for the project or parcel;
    4. If proposed, provide preliminary design details and grading plans for surface water impoundments or diversions.
  3. Rainwater Capture

    1. Preliminary design specifications related to rainwater catchment. Rainwater catchment systems shall meet the specifications of California Plumbing Code chapters 15 and 16;
    2. Maximum available rainwater volume shall be estimated using the formula below:

      Max Volume = 0.6 x 0.62 x Roof Catchment Area x Ave Annual Precip

      Max Volume is in gallons
      Roof Catchment Area is the contributing area of the roof in square feet
      Ave Annual Precip is in inches as available through the Sonoma County Cannabis Viewer
      0.6 is a factor of safety multiplier to account for drought years and system efficiency
      0.62 is a unit conversion

      Note: Rainwater storage tanks of larger size may be proposed, but additional volume beyond the (Max Volume) may not be used to credit water uses during the dry season (April 1 - October 31)
  4. Trucked Water

    1. Provide the trucking company name, source, and letter or contract with the supplier.
  5. Recycled Water

    1. Describe offsite recycled water source, availability, and letter or contract with the supplier;
    2. Describe graywater or process water reuse.
  6. Enhanced Groundwater Recharge Plan

    1. Prepared and certified by a Qualified Professional;
    2. Numeric estimate of recharge to groundwater under existing and proposed conditions;
    3. Conceptual design details and grading plan;
    4. Increased recharge to groundwater may be used to offset increased groundwater use at a ratio of 2 to 1.


Qualified Professional
A professional geologist, engineer, or landscape architect, acting within the scope of their license. A qualified professional must hold a valid license in the State of California.

Approved By

Approved By: Tennis Wick, Director

Reviewed By Department Manager:  John Mack, Division Manager

Reviewed By County Counsel:  Christa Shaw, Deputy County Counsel IV

Lead Author:  Robert Pennington, Professional Geologist

Appendix A

Default Water Use Guidelines


Type of Water Use, Volume in Acre-feet per Year

Primary dwelling indoor water use (no landscaping) 0.2
Primary dwelling with unspecified landscaping 0.5
Accessory dwelling indoor water use (no landscaping) 0.1
Accessory dwelling with unspecified landscaping 0.25


Type of Water Use, Volume in Acre-feet per Year

Lawn 3.6 per acre
Other Landscaping 1.8 per acre
Pools and decorative water features 0.1 per 1000 square-feet


Type of Water Use, Volume in Acre-feet per Year

Vineyard irrigation 0.6 per acre
Irrigated pastures 3.6 per acre
Irrigated orchards 1.8 per acre
Cannabis: Indoor or mixed light 4.0 per acre canopy
Cannabis: Outdoor 2.0 per acre canopy
Livestock (sheep or cows) 0.05 per acre


Type of Water Use, Volume

Process water

2.15 acre-feet per 100,000
gallons of wine produced

Tasting room visitation 3 gallons per visitor
Events with onsite catering 15 gallons per visitor

Commercial and Industrial Uses

Type of Water Use, Volume

Food processing 31.0 acre-feet per
employee per year
Brewery process water 7 gallons of water per
gallon of beer produced
Printing/Publishing 0.60 acre-feet per
employee per year


15 gallons per shift

Other Uses

See Sonoma County Onsite Wastewater Treatment System Regulations and Technical Standards (OWTS Manual), or another published reference.

Site-Specific Water Use Rates

Site-specific landscape water use should be estimated in accordance with Sonoma County’s Water Efficient Landscape Ordinance (WELO).

Water use rates may deviate from the above listed default use rates with site-specific data or published reference, and approval of the review authority.