Models that forecast water that isn’t there hurt all beneficial uses

Assemblymember Adam Gray has called for an audit of the Department of Water Resources and the State Water Resources Control Board, in part to answer the question,

What happened in the Water Year 2021 (October 1, 2020 to September 30, 2021) to lead the state to miscalculate by some 700,000 acre-feet the amount of water that would flow out of the Sierra Nevada and into our reservoirs?


While the state was grossly miscalculating Sierra runoff, predictions by local agencies such as the Turlock and Merced Irrigation Districts and NOAA’s California-Nevada River Forecast Center were quite close to the actual runoff in their calculations. What factors resulted in vastly different predictions, and what collaboration has DWR and SWRCB conducted with local agencies to improve the state’s modeling and data collection?

The State Auditor doesn’t have the expertise to dig into technical issues, but could consider some of the governance and collaboration issues. To my knowledge, the Department of Water Resources hasn’t collaborated with local agencies, nor with other stakeholders on improving the runoff forecasts. Although the Water Board held a workshop on Sacramento River Temperature Management, and included panels on forecasting, we didn’t get the technical details that are needed to really examine DWR’s forecasting models and understand the reasons for errors.

Assemblymember Gray also asks:

What models were used by DWR and SWRCB to prepare for the 2021 drought? Importantly, what models will the DWR and SWRCB use to prepare for near constant, systemic drought going forward? If new and more accurate models are not adopted, how will current models be adapted to perform more accurately? Who is responsible to make certain this happens?

There have been major issues with DWR’s CALSIM II planning model, which I testified about in the WaterFix hearing. The model has a known problem with creating water — it was estimated at over 200,000 acre-feet a year of non-existent flow from the Colusa Basin Drain between April and August in severe droughts. The graph below, from a presentation at the California Water and Environmental Modeling Forum by Walter Bourez, shows the errors. CALSIM II is being used for the water supply modeling for most of the Prop 1 storage projects.

While we do not agree with all the statements in Assemblymember Gray’s letter, we do agree that models that forecast water that isn’t there are becoming catastrophic with climate change.  DWR needs to stop relying on press releases and provide real technical information to stakeholders in a true collaborative process.

Finally, the Delta Independent Science Board has advocated for better forecasting models as part of adaptation to a rapidly changing climate. The DISB sent a letter to the Delta Stewardship Council in 2019, which concluded,

History may view this and coming decades as the time when critical decisions about the Delta ecosystem and water management throughout the state should have been made. The time for action is now.

This letter started the Delta Science Program’s development of the Science Needs Assessment. A draft of the Science Needs Assessment was developed in January 2021, but the Independent Science Board was struggling with being defunded for the entirety of FY 2020-21.

The Delta Independent Science Board is critically important for climate adaptation, and the independence and authority of the DISB needs to be fully restored.

Further reading

Delta Independent Science Board. 2019. Letter to Delta Stewardship Council Subject: Urgency & Opportunities for Improving Delta Interagency
Science & Technical Integration. February 11, 2019.

Delta Stewardship Council. 2020. Building an Effective Delta Science Enterprise. Briefing paper for October 5-6 2020 Long Term Science Needs Assessment Workshop.

Delta Stewardship Council. 2021. Draft Science Needs Assessment.

Des Jardins, D. 2022. Fatal errors in DWR’s runoff forecasting: comments to the State Water Resources Control Board. California Water Research blog. March 18, 2022.

Des Jardins, D. 2016. Testimony for the WaterFix Water Right Change Petition Hearing, Part 1A.

Draper, A., Bourez., W. 2004. CALSIM II Sacramento River Basin Hydrology
Enhancements. Presentation to California Water and Environmental Modeling Forum.

Gray, A. [Assemblymember]. 2022. Letter to Rudy Salas, Chair, Joint Legislative Audit Committee RE: Audit Request of the Department of Water Resources and State Water Resources Control Board, March 22, 2022.

Kukilich, T. 2021. Delta Independent Science Board funding slashed. The Brentwood Press. June 17, 2021.



One thought on “Models that forecast water that isn’t there hurt all beneficial uses

  1. “Models that forecast water that isn’t there hurt all beneficial uses (and users?) …
    You mean like what happens in our Water Berry Ponzi Scheme in the Monterey Bay here for how many decades already run down @ (2009 ) and (2011 )?

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