Groundwater dynamics mediate low-flow response to global warming in snow-dominated alpine regions

In mountain environments, spatial and temporal patterns of snow accumulation and melt are dominant controls on hydrologic responses to climate change. In this paper, we develop a simple conceptual model that links the timing of peak snowmelt with geologically mediated differences in rate of streamflow recession. This model demonstrates that within the western United States, spatial differences in subsurface drainage rates can exacerbate summer streamflow losses associated with diminishing snowpacks. Application of a process-based hydrologic model to four watersheds in the Western Cordillera further reveals that contingent on timing of snowmelt, slower draining watersheds are likely to have more water in summer but paradoxically are subject to the greatest summer water losses under a 1.5 degrees C warming scenario. A slow draining watershed located in the young volcanic arc of the High Cascades in Oregon shows 4 times the summer streamflow reduction when compared with faster draining watersheds with similar timing of peak snowmelt. On the other hand, watersheds where snowmelt occurs late in the season but have little groundwater influence show high relative sensitivities to snowpack changes due to warming, as shown by a high-elevation granitic Sierran watershed. Our results highlight the importance of geological factors in interpreting hydrologic response to climate change and argue for a geoclimatic framework to guide the design of monitoring networks that will become the basis for assessing climate change impacts in mountain regions throughout the globe.