Characterizing Regional Spatial Variation in Summer Streamflow Response to Climate Warming in the Mountains of the Western US

Warmer air temperatures leading to lower snow accumulation and earlier melt have been shown to reduce summer streamflows for much of the Western US. Predictions of reduced summer low flow from GCMs are spatially variable. Some of this variation is due to spatial differences in the sensitivity of snow accumulation and melt processes to warmer temperatures, such that intermediate elevations - near the seasonal rain-snow boundary - often show larger impacts. Spatial differences in groundwater dynamics can also play a significant role in the sensitivity of summer streamflow to changes in temperature. We use a combined model- (RHESSys - Regional hydro-geologic ecosystem simulation system) and empirical-based approach to illustrate how geologic differences in the rate at which recharge (as either rain or snow) is translated into streamflow can affect the timing and magnitude of reductions in summer low flow. The model-based analysis allows us to develop indicators that include both a priori climate and drainage efficiency to characterize summer streamflow response to both warmer temperatures and changes in the amount of winter precipitation. Our initial modeling analysis is focused on several small study watersheds within the Oregon Cascades that differ both in terms of snow accumulation and melt characteristics and groundwater drainage rates. We emphasize the use of multiple measures of fit between observed and modeled streamflow in the calibration of the hydrologic component of the model, and illustrate the importance of including metrics that directly assess hydrologic model ability to characterize climate change impacts on streamflow. These results can be generalized from these individual watersheds to larger regional scales by using geology as a landscape-scale indicator of groundwater dynamics and topography as an indicator of snow accumulation and melt. Our results show that including geologic-based indicators of groundwater dynamics is as important as including elevation-based differences in snowpack in assessing hydrologic impacts of climate change in the mountains of the western US.