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 (Delta Water Unavailability Method or 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. Based on the current output of the methodology, 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 indicating that water supplies are not available for their use.
The Water Unavailability Methodology may continue to be updated to inform diverters and other interested persons how the State Water Board is determining water unavailability within the Delta watershed during the summer irrigation season. The Water Unavailability Methodology improves upon the approach used during the drought years of 2014 and 2015 and is a first step in anticipated ongoing improvements to the State Water Board’s process for determining water unavailability in the Delta watershed. The methodology has been refined in response to public comments and may continue to evolve in response to public comments and as the year progresses. The State Water Board expects to further develop the methodology to assess water unavailability for pre-1914 appropriative and riparian claimants and for use during the upcoming wet season. 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.
Available Methodology Resources
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 released a set of resources related to the Water Unavailability Methodology for public review and comment.
Available resources include the Water Unavailability Visualization Tool (see below) and the resources shown in the following table:
|Resource (Date Last Updated)||Resource Description||Other Versions|
|Summary Report (6/15/2021)||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.||5/12/2021|
|Technical Appendix A, Methodology Spreadsheet Description (6/15/2021)||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).||5/12/2021|
|Technical Appendix B, Demand Data Development Process (6/15/2021)||The appendix describes how user-reported data from 2018 and 2019 was collected and processed for the water demand dataset used in the methodology.||5/12/2021|
|Appendix C, Summary of Public Comments (6/15/2021)||The appendix summarizes the substantive technical, factual, or legal comments received during the public review and comment period, including during the May 21, 2021 public workshop.||N/A|
|Water Unavailability Methodology Spreadsheet (6/16/2021)||The spreadsheet contains the supply and demand datasets and analyses used to implement the methodology and create the visualizations.||5/12/2021|
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.
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.
For a summary of the substantive technical, factual, and legal public comments received during and after the workshop, see Appendix C, Summary of Public Comments.
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. Staff will respond by sending a link to access them.
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.