Climate Model Selection For The Reclamation Central Valley Project (cvp) Long-Term Operation Study

In 2021, the Bureau of Reclamation (Reclamation) and the California Department of Water Resources (DWR) jointly requested Reinitiation of Consultation on the Coordinated Long-Term Operation of the Federal Central Valley Project (CVP) and the State Water Project (SWP), henceforth referred to as the 2021 LTO. The motivation for the reinitiation was the extensive Western drought that required further analysis beyond the 2019 LTO to determine appropriate operations under the drier and warmer conditions.

Extensive and ongoing work on climate change in California has shown that warming temperatures and changing weather regimes are likely to have a significant impact on CVP/SWP water resources. To describe future system management under these potential future conditions, it is necessary to incorporate projections of future climate effects within the LTO analysis. It is also prudent to incorporate climate change into the analysis in such a way that recognizes the inherent uncertainty associated with climate projections in order to plan for a broad range of potential future CVP/SWP operational conditions.

The overall goal of this climate analysis is to inform the development of inputs necessary for water operations, temperature, and temperature dependent fish morality (TDM) models used within the core 2021 LTO analysis. This requires determining climate scenarios that are likely representative of future climate conditions within California. Consistent with many of the previous efforts mentioned above, this determination is made by evaluating the accuracy of the general circulation models (GCMs) over the historical period in comparison to observationally informed datasets such as the Parameter-elevation Regressions on Independent Slopes Model (PRISM). Though good historical performance is not necessarily an indicator of future performance, notably poor performance in the historical period calls into question the reliability of a GCM for predicting future conditions.

The climate analysis is founded on a joint Reclamation/DWR understanding of climate change science relevant to California water management. The analysis builds upon previous climate work, incorporating lessons learned from previous studies and improved best practices. This work is representative of current knowledge and data; future efforts should continue to build upon the present analysis to further refine understanding of climate effects within California.