Dear DWR, in runoff forecasting, stationarity is dead

For decades, the California Department of Water Resources (DWR) has relied on runoff forecasts from the California Cooperative Snow Surveys (Bulletin 120) for planning State Water Project operations in the late winter and early spring.

In 2021, the Bulletin 120 runoff forecasts, which were based on the last 50 years of historic runoff data, were disastrously wrong. California Water Research filed a protest of DWR’s Temporary Urgency Change Petition, calling for the Water Board to require DWR and the Bureau of Reclamation to write a report on why the 2021 forecasts were wrong. The Water Board did not require the petitioners to write a report on why the 2021 forecasts had such large errors, but did require them to update their forecasting methods, stating:

DWR and Reclamation shall identify and implement needed improvements to forecast methods to avoid significant over- or under- estimates of available water supplies and shall provide updates to the Board on these efforts along with updates on current hydrologic and operational forecasts for the water year on a monthly basis starting in March of 2022 and continuing until the drought emergency is over.

WR 2022-0095 p. 64

DWR’s state climatologist, Michael Anderson, gave a presentation yesterday to the Water Commission on improvements to the Bulletin 120 runoff forecasting. The slides are here: Forecast Improvement to Adapt to Climate Change

Anderson explained that DWR moved from using a 50 year historic record to a 30 year historic record for the 2022 Bulletin 120 forecast.  This is the method they are updating:

So to do a Bulletin 120 runoff forecast, DWR technical staff would pick out years that are similar to the current year’s precipitation & snow patterns.  This year they could pick out years in the historic record that had huge rainfall in December & October.  But those years are no longer representative of the climate. Chaning to the 30 year historic record rather than the 50 year historic record improves the method slightly. But those projections would also be off, because the climate is changing rapidly and the last 30 years are also not predictive of runoff in years like 2014-2016 and 2020-2021.

DWR’s own engineers framed the problem in a 2009 report, Using Future Climate Projections to Support Water Resources Decision Making in California:

In water resources planning, it is often assumed that future hydrologic variability will be similar to historical variability, which is an assumption of a statistically stationary hydrology. This assumption no longer holds true under climate change where the hydrological variability is non-stationary. Recent scientific research indicates that future hydrologic patterns are likely to be significantly different from historical patterns, which is also described as an assumption of a statistically non-stationary hydrology. In an article in Science, Milly et al. (2008) stated that “Stationarity is dead” and that “finding a suitable successor is crucial for human adaptation to changing climate.” (p. 36.)

DWR needs to recognize that stationarity is dead, and move to a physics-based method for runoff forecasting, such as that used by NOAA’s California Nevada River Forecasting Center (CNRFC.)

In a January 31st blog post, NRDC’s Doug Obegi  noted that DWR’s Jan 1, 2022 forecast for inflow to Shasta Reservoir was 4.155 MAF while the CNRFC center predicted 3.085 MAF. That’s a difference of 25%.

In response to a question by a Water Commission member, Michael Anderson did say that “adding on the potential to move outside the distribution” would be done through DWR’s State Water Project allocation process.

John Yarbrough, the Assistant Deputy Director for the State Water Project, gave the presentation on DWR’s SWP allocation process during the drought. Yarbrough did not say that DWR was looking at information from “outside the distribution.” On questioning from one of Commissioners, he said that DWR based their allocations on the following:

What water do we see down south of the Delta – really looking at other flows we can pick up, moving down to San Luis. What’s already in San Luis. What kind of allocation could we support.  The uncertainty is around how much water would be available.
Taking the Bulletin 120 forecast, running it through the operational model… How likely are the different scenarios. By the time we’ve gone through our operational models, a couple of weeks have passed. We look out the window.

While I appreciate the operators looking out the window to ground truth the operations forecasting, the fact that the model is significantly off should be an indication that the entire methodology needs to change.  Stationarity is dead.

Further Reading

Abatzoglou, J., Rallings, A., Bernacchi, L., Viers, J., Medellín-Azuara, J., California’s Missing Forecast Flows in Spring 2021 – Challenges for seasonal flow forecasting. California WaterBlog. July 18, 2021.

Anderson, M. 2010. Water Supply Forecasting for California. Eight Annual Climate Prediction Applications Workshop, UCAR Cooperative Programs for the Advancement of Earth Science. March 2, 2010.

California Department of Water Resources. 2022. California Cooperative Snow Surveys website (DWR Bulletin 120.)

California Department of Water Resources: Chung F., Anderson, J., Arora, S., Ejeta, M., Galef, J., Kadir, T., Kao, K., Olson, A.,
Quan, C., Reyes, E., Roos, M., Seneviratne, S., Wang, J., Yin, H. 2009. Using Future Climate Projections to Support Water Resources Decision Making in California, California Climate Change Center, Report CEC-500-2009-052-F. May 2009.

Des Jardins, D. 2021. TUCP: California Water Research asks Water Board to require report on 2021 runoff forecast errors, June 5, 2021.

Milly, P.C., Betancourt, J., Falkenmark, Hirsch, R., Kundzewicz, Z., Lettenmaier, D., D., Stouffer, R. 2008. Stationarity Is Dead: Whither Water Management? Science 319(5863): 573-574. doi:10.1126/science.1151915

Obegi, D. 2021. Get Ready for Climate Whiplash from DWR’s February Forecast, Natural Resources Defense Council expert blog. January 31, 2022.

State Water Resources Control Board, Order WR 2022-0095, In the Matter of Petitions for Reconsideration of
the Executive Director’s June 1, 2021 Order Conditionally Approving a Petition for Temporary Urgency Changes to License and Permit Terms and Conditions Requiring Compliance with Delta Water Quality Objectives in Response to Drought Conditions… February 15, 2022.

Yarbrough, J. 2022. 2022 SWP Drought Planning. California Department of Water Resources Presentation to the California Water Commission. February 16, 2022.