Climate Impacts on Source Contributions and Evaporation to Flow in the Snake River Basin Using Surface Water Isoscapes (δ2H and δ18O)
Rising global temperatures are expected to decrease the precipitation amount that falls as snow, causing greater risk of water scarcity, groundwater overdraft, and fire in areas that rely on mountain snowpack for their water supply. Streamflow in large river basins varies with the amount, timing, and type of precipitation, evapotranspiration, and geologically mediated drainage properties of watersheds; however, these controls vary in time and space making it difficult to identify the areas contributing most to flow and when. In this study, we address this problem by separating the evaporative influences from source values of water isotopes from the Snake River Basin in the western United States (US) to relate source area to flow dynamics. We developed isoscapes (δ2H and δ18O) for the Snake Basin and found that surface water isotopes of small watersheds are primarily controlled by longitude and elevation via Rayleigh distillation. To examine temporal variability in source contribution to flow, we present a six-year record of water isotopes from King Hill, Idaho with evaporative influences removed. During periods of low flow, source water values were isotopically lighter and evaporation exerted greater influence, indicating longer surface transport times and restriction of flow-contributing areas to the highest elevation, far eastern fringe of the basin. Our findings present a potential tool for identifying watersheds that are vulnerable to warming temperatures and a framework for understanding flow contributions in other continental-interior basins of the western US where river evaporation may obscure source water isotopic signatures.