Interannual Variations in Snow Accumulation and Melt Govern Variability in Groundwater Contributions to Streams Across the Western USA.

In the western USA, decreases in snow accumulation and increases in snowmelt have been linked to rising temperatures. In snow dominated watersheds, earlier snowmelt induced by these warmer conditions is known to generate earlier peak flows and lower summer flows. As summer surface water becomes scarcer, groundwater reservoirs will play an increasingly important role, however how they will be impacted by changing snow dynamics remains unclear. To elucidate how snowpacks govern stream relationships in the western USA, two fundamental questions must be addressed: 1) How do local snow dynamics (snow water equivalent, snow persistence, accumulation, melt rate and melt timing) influence interannual variation of groundwater contribution to stream flow? and 2) How do lithology and climate influence the sensitivity of summer stream flows and groundwater contributions to interannual snow variability? To address these questions, we focus on three watersheds in the western USA 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 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 snowmelt 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 summer stream flows 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 indicates three sources (precipitation, deep groundwater, and shallow subsurface water) are contributing to stream solute concentration, but that 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.