A Geoclimatic Framework For Characterizing Summer Streamflow Vulnerability To Climate Warming In The Pacific Northwest, USA

Summer streamflows in Pacific Northwest are largely derived from melting snow and groundwater discharge. As the climate warms, diminishing snowpack and earlier snowmelt will cause spatially non-uniform reductions in summer streamflow. Most assessments of impacts of a changing climate on streamflow make use of downscaled temperature and precipitation projections from General Circulation Models (GCMs). Projected climate simulations from these GCMs are often too coarse for climate change planning purposes, and do not capture smaller scale topographic controls, leading to uncertainty in predicting streamflow. This uncertainty is further amplified when downscaled climate predictions are coupled to macroscale hydrologic models (i.e. Variable Infiltration Capacity (VIC) model) which do not capture the contributions from deep groundwater. Our earlier studies have demonstrated the important role of deep aquifers in mediating streamflow response to climate change, and the need to explicitly incorporate this process into sensitivity assessments. To address this need, we developed and applied an analytical framework for characterizing summer streamflow sensitivity to a change in the timing and magnitude of recharge (rain or snowmelt) in a spatially explicit fashion. Two patterns emerge from this analysis: first, areas with high streamflow sensitivity also have higher summer streamflow. Second, the level of sensitivity and spatial extent of highly sensitive areas diminishes over time as the summer progresses. Results of this analysis point to a robust, practical, and scalable approach that can help prioritize risk at the landscape scale, complement the downscaling approach, and provide a framework to assist land and water managers adapt to an uncertain and risky future.