California Water Research filed a protest of the May 17 Temporary Urgency Change Petition by the California Department of Water Resources and the US Bureau of Reclamation. We requested that the State Water Resources Control Board require a written report by DWR and USBR on the methodology used for the runoff forecast used in this year’s Drought Contingency Plans, together with an evaluation of the reasons for the errors in this year’s runoff forecast by September 30, 2021. We also requested that the Board hold a workshop to receive input on the subseasonal runoff forecast methodology by November 30, 2021. California Water Research argued that these actions are necessary to protect the public interest and the public trust in future drought years.
The May 17, 2021 Temporary Urgency Change Petition requests waivers of Bay-Delta Water Quality Control Plan requirements. The Petition states:
Although well below average rainfall, the snowpack in March 2021 indicated that sufficient reservoir inflow was likely available to meet requirements. Conditions significantly changed at the end of April 2021 when it became clear that expected reservoir inflow from snowmelt failed to materialize. The May 90% exceedence forecast for the water year Sacramento Valley Four River Index identified a reduction of expected runoff of 685 TAF from those generated only a month earlier in April.
The same forecasts were in DWR’s and USBR’s March 22, 2021 Updated Drought Contingency Plan. The forecasts indicated that sufficient reservoir inflow was likely available to meet the Water Quality Control Plan requirements. The Sacramento River Index forecasts used for the 2021 Drought Contingency Plan are below. The Sacramento River Index is an estimate of the unimpaired flow from four rivers, the Sacramento, Yuba, Bear, and American Rivers, in million acre-feet.
There was supposed to be a 90% probability of seeing at least as much runoff as the “90% Exceedance” forecasts. In retrospect, the increase of 1.1 MAF in the runoff forecast in February was overly optimistic, and was not reduced enough in the March and April forecast.
It has become clear that the forecasting used in the Drought Contingency Plans is inadequate for drought contingency planning with climate change conditions in the watershed. One issue could be using runoff forecasts being based on runoff in analogous water years in the historical record. The issue with using historic hydrologic information has long been noted by the Department of Water Resources engineers. In the 2009 report, Using Future Climate Projections to Support Water Resources Decision Making in California, DWR’s water resources engineers stated:
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.”
DWR’s and USBR’s March 22, 2021 Updated Drought Contingency Plan states only:
The Water Supply Index (WSI) forecasts that are utilized for this March Drought Plan are unique to this water year and informed by precipitation, runoff, and other antecedent hydrologic conditions as they existed on March 1, 2021.
No other information given on the methodology for the Water Supply Index Forecasts. Our protest concluded:
[T]here appear to have been significant errors in the Petitioners’ runoff forecasting, which are likely due to climate change. A written report by the Petitioners on the methodology used for the runoff forecast used in this year’s Drought Contingency Plans will allow stakeholders to evaluate the methodology. A workshop would allow stakeholders to provide input on better subseasonal forecasting, which is of critical importance in protecting the public interest and the public trust. Given the enormous impacts of the Petitioners’ TUCPs, the Water Board should include these requirements in the TUCP Order.
This post was revised on 6/5/2021 to add a graph..