Drought Information Banner

Water Unavailability Methodology for the Delta Watershed

The Sacramento-San Joaquin Delta (Delta) watershed is currently experiencing extremely dry conditions following the dry conditions in 2020. The 2020-2021 period is projected to be the second driest two-year period on record—second only to the extreme drought years of 1977 and 1976. Due to critically dry conditions throughout the Delta watershed that are likely to result in insufficient supplies to meet all water demands, water quality objectives, and flow requirements in 2021, the State Water Board is currently evaluating water unavailability for water right holders in the Delta watershed.

The State Water Board developed the Water Unavailability Methodology for the Delta Watershed (Water Unavailability Methodology for short) to identify when supply and demand data indicate that water is unavailable for diversion by water right holders. The Water Unavailability Methodology identifies which diverters in the Delta watershed may face insufficient water supplies based on their priorities of right.

Currently, the Water Unavailability Methodology is being used to support the issuance of curtailment orders to water right holders and claimants in the Delta watershed pursuant to an emergency regulation that was adopted by the State Water Board on August 3, 2021 and became effective on August 19, 2021. To see which water rights and claims are currently curtailed, see the Delta Watershed Curtailment Status List on the Delta Watershed Drought webpage.

Earlier this summer, the State Water Board used the Water Unavailability Methodology to assess water unavailability for water right holders and claimants in the Delta watershed for the purposes of issuing notices of water unavailability indicating that information showed water supplies are not available for their use. On June 15, 2021, the State Water Board issued notices of water unavailability to all post-1914 appropriative water right holders in the Delta watershed. On July 23, 2021, the State Water Board sent additional notices of water unavailability to some pre-1914 appropriative claimants.   See notices of water unavailability under the "Previous Events and Actions" section

The Water Unavailability Methodology has been refined in response to public comments and may continue to be updated to reflect the best available information. The Water Unavailability Methodology improves upon the approach used during the drought years of 2014 and 2015. Additional improvements will likely be possible in the future with better data and improved tools.

  Please see our Frequently Asked Questions for more information regarding water unavailability in the Delta watershed and the related Water Unavailability Methodology.

Methodology Resources

Resources related to the Water Unavailability Methodology include the Water Unavailability Visualization Tool (see below) and the resources shown in the following table:

Resource (Date Last Updated) Resource Description Prior Versions
Summary Report
(8/20/2021)
The summary report outlines the methods used to develop the tool and describes the data sources, assumptions, and the bases for the assumptions used to develop the methodology. 7/23/2021
6/15/2021
5/12/2021
Technical Appendix A: Methodology Spreadsheet Description
(8/20/2021)
The appendix describes the data sources used to develop the methodology, calculation steps used in the analysis, and outputs used to create the visualizations. It is presented as a data dictionary for the Water Unavailability Methodology Spreadsheet (see below). 7/23/2021
6/15/2021
5/12/2021
Technical Appendix B: Demand Data Development Process
(7/23/2021)
The appendix describes how user-reported data from 2018 and 2019 was collected and processed for the water demand dataset used in the methodology. 6/15/2021
5/12/2021
Appendix C: Summary of Public Comments
(7/23/2021)
The appendix summarizes the comments received, including during the May 21, 2021 public workshop. 6/15/2021
Technical Appendix D: Assessment of Water Availability Issues Within the Legal Delta
(8/20/2021)
The appendix provides additional background information used to evaluate water unavailability in the Legal Delta portion of the Delta watershed. N/A
Water Unavailability Methodology Spreadsheet
(8/20/2021)
The spreadsheet contains the supply and demand datasets and analyses used to implement the methodology and create the visualizations. 7/23/2021
6/16/2021
5/12/2021

Water Unavailability Visualization Tool

The Water Unavailability Visualization Tool compares water supply and demand data for the Sacramento and San Joaquin River watersheds at regional and watershed scales. The tool is updated weekly to track ongoing supply conditions and monthly to reflect monthly supply forecasts. The monthly supply forecasts include four supply forecast scenarios: optimistic, neutral, pessimistic, and extremely pessimistic. Currently, supply conditions in the Delta watershed are extremely dry and trending drier than the extremely pessimistic scenario.

  Click here to watch a brief demonstration of the Water Unavailability Visualization Tool.

Visualizations have been made available for public use below:

Public Comments

On May 12, 2021, the State Water Board issued a Notice of Public Workshop and Opportunity for Public Comment on the Water Unavailability Methodology for the Delta Watershed. With that notice, the State Water Board first released the Water Unavailability Methodology for public review and comment.

On May 21, 2021, State Water Board staff held a workshop to receive public input on the Water Unavailability Methodology.  During the workshop, staff provided an update on the current and forecasted Delta watershed conditions for this year and presented an overview of the updated Water Unavailability Methodology. A recording of the workshop is available on the State Water Board’s YouTube channel. Comments received informed subsequent updates to the methodology.

A summary of public comments received can be found in the “Methodology Resources” section above. If you would like to request a copy of written public comments submitted to the State Water Board on the Water Unavailability Methodology for the Delta Watershed and related resources, please send an email to Bay-Delta@waterboards.ca.gov with “Request for Methodology Comments” in the subject line.

  How to Report an Unauthorized Diversion

Notices of water unavailability (also referred to as “notices”) are used to inform diverters that, based on the best available information, there is insufficient water available to divert under their priority of right. Notices inform affected diverters that water is either currently unavailable or will be unavailable in the near future for their diversion. These notices play an important role by offering the opportunity for voluntary compliance prior to the initiation of any formal enforcement action by the State Water Board. Diverting unavailable water can result in penalties for injuring more senior water right holders and public trust resources.

The State Water Board relies on the public to help identify unauthorized diversions, diversions by junior water right holders that impact senior water right holders, diversions in violation of permit and license conditions, diversions that constitute the waste or unreasonable use of water, and diversions that cause adverse impacts to public trust resources, such as fish and wildlife.

Report an Environmental Concern logo

  If you have information that an unauthorized diversion may be occurring, you can submit a complaint through the CalEPA Environmental Complaint website.

  Please visit the Complaints Program webpage for more information on water rights enforcement complaints.

  Stay informed

Email Subscription List

Subscribe to the Delta Drought email list to receive notifications and the latest updates.

Email Address: (required)

Your Full Name: (required)

(e.g. John Smith)

Check your email account for a confirmation email to complete your subscription.

Contact Us

If you have specific questions or concerns related to the Water Unavailability Methodology for the Delta Watershed you can reach us at:

For inquiries on other drought-related tools please contact: