Drought Information Banner

Water Unavailability Methodology for the Delta Watershed

Due to critically dry conditions throughout the Sacramento-San Joaquin Delta (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.

In May 2021, 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. To see which water rights and claims are currently curtailed, see the Delta Watershed Curtailment Status List.

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

Resource (Date Last Updated) Resource Description Prior Versions
Summary Report
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. 9/27/2021
Technical Appendix A: Methodology Spreadsheet Description
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). 9/27/2021
Technical Appendix B: Demand Data Development Process
The appendix describes how user-reported data from 2018 and 2019 was collected and processed for the water demand dataset used in the methodology. 9/27/2021
Appendix C: Summary of Public Comments
The appendix summarizes the substantive technical, factual, or legal comments that were received prior to the release of the July 23, 2021 version of the summary report. 7/23/2021
Technical Appendix D: Assessment of Water Unavailability Issues Within the Legal Delta
The appendix provides additional background information used to evaluate water unavailability in the Legal Delta portion of the Delta watershed. 8/20/2021
Water Unavailability Methodology Spreadsheet

The spreadsheet contains the supply and demand datasets and analyses used to implement the methodology and create the visualizations.

*The Water Unavailability Methodology is run frequently in order to determine curtailments based on the most up-to-date water supply forecasts. To obtain a copy of the spreadsheet for a specific date, please send an email to Bay-Delta@waterboards.ca.gov with ‘Request for Water Unavailability Methodology Spreadsheet’ in the subject line.


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.

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

Public Comments

On May 12, 2021, 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.  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.

Previous Events

  • October 20, 2021 - Staff Workshop on Potential Changes to the Water Unavailability Methodology (Notice | Presentation Slides | Video)
  • June 1, 2021 – Board Meeting Informational Item: Update on Water Unavailability Methodology (Video)
  • May 21, 2021 – Staff Workshop on Water Unavailability Methodology (Notice | Video)

  How to Report an Unauthorized Diversion

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: