Gaps Between Western U.S. Reservoir Inflows and Headwater Precipitation Timing, Amount, and Phase

Snowmelt from mountain headwater basins supplies approximately 80% of the runoff to reservoirs in the western United States (WUS), and is therefore a key component of western hydropower. Yet predicting runoff from hydrometeorological data in complex terrain (and its sensitivity to disturbances and extremes) remains highly uncertain. Basic science uncertainties include how inflows to reservoirs respond to shifts in precipitation timing, amount, phase (rain versus snow), the impacts of evolving forest/land cover due to disturbances, and the often overlooked role of groundwater in the mountain hydrologic cycle. Challenges to address these questions arise in validating and downscaling climate data for use in physics-based or AI-driven watershed models, assessing biases and process fidelity in these models, and scaling process understanding from plot/hillslope/intensive field campaigns to watershed and hydropower-relevant scales. Further, water management restrictions and policies drive reservoir operation guidelines such as stationary rule curves, with the potential to inhibit energy production and water supply.