Effects of Snow Water Storage on Hydrologic Partitioning Across the Mountainous, Western United States

In the montane western United States, where the majority of downstream water resources are derived from snowmelt, a warming climate threatens the timing and amount of future water availability. It is expected that the fraction of precipitation falling as snow will continue decreasing and the timing of snowmelt will continue shifting earlier in the year with unknown impacts on partitioning between evapotranspiration and streamflow. To assess this, we employ a Snow Storage Index (SSI) to represent the annual temporal phase difference between daily precipitation and daily modeled surface water inputs (SWI, the sum of rainfall and snowmelt), weighted by the respective amounts. We coupled the SSI metric with a Budyko-based framework to determine the effect of snow water storage on relative hydrologic partitioning across snow-influenced watersheds in the western U.S. Greater snow water storage was positively correlated with greater hydrologic partitioning to streamflow, particularly in the North Cascades/Cascades (r²: 0.62), Blue Mountains (r²: 0.56), Canadian Rockies (r²: 0.55), Idaho Batholith, (r²: 0.48), and Columbia Mountains/Northern Rockies (r²: 0.45). The weekly SWI:P ratio was an equally strong predictor for hydrologic partitioning, particularly in mid-spring (e.g., March/April) in the same mountainous areas (r²: 0.62–0.74, across the same eco-regions). The retention of snow water storage and subsequent release of stored water in summer months resulted in increased hydrologic partitioning to streamflow. If SSI decreases with future warming, the volume of water partitioned streamflow will decrease non-uniformly across the western U.S. with substantial implications for ecosystems and agricultural, industrial, and domestic water supplies.