MIXING MODELS AND HYDROGRAPH SEPARATION REVEALS WESTERN US WATERSHEDS’ SUMMER FLOWS AND GROUNDWATER PROPORTIONS RESPOND QUICKLY TO INTERANNUAL SNOW VARIABILITY

Over the last 50 years, decreases in snow accumulation and increases in snow melt during the mid-to-late snow season have been linked to the rising temperatures observed across the western US. In snow dominated watersheds, earlier snow melt induced by these warmer conditions is known to generate earlier peak flows and lower summer flows. Our understanding of snowpack and stream relationships in the western US will be improved by investigating two fundamental questions: 1) How do local snow dynamics (snow water equivalent, snow persistence, accumulation, melt rate and melt timing) influence interannual variation of summer flow volume and groundwater proportion? and 2) How do lithology and climate influence the sensitivity of summer stream flows to interannual snow variability? To address these questions, we focus on three watersheds in the western US that vary in snow dynamics and lithology: Lookout Creek (Oregon Cascades), Sagehen Creek (California Sierra Nevada), and Coal Creek (Colorado Rockies). Using 20 years of stream flow, stream chemistry, and Snow Data Assimilation System (SNODAS) data, we explore how snow dynamic-streamflow interactions vary based on watershed characteristics. Two end-member chemical hydrograph separation indicates that proportional groundwater contributions are highest in the summer and lowest during the snow melt season and that years with more snow have higher summer low-flow volumes and lower proportions of groundwater in the summer flows. We infer that source waters which contribute to streamflow are dynamic: shallow subsurface storage contributes to a greater degree in wet years while deep groundwater inputs dominate in dry years. End-member mixing analysis also indicate three sources (precipitation, deep groundwater, and shallow subsurface water) are contributing to stream solute concentration, but that the shallow subsurface flow contributes differently depending on the climate regime. Preliminary data indicate that 1) summer flows and groundwater proportions often respond within the same year to interannual variability in snow dynamics, and 2) watersheds underlain by less permeable bedrock are more sensitive to these changes than those underlain by more permeable bedrock.