A hydrogeologic framework for characterizing summer streamflow sensitivity to climate warming in the Pacific Northwest, USA

摘要

Summer streamflows in the Pacific Northwest are largely derived from meltingsnow and groundwater discharge. As the climate warms, diminishing snowpackand earlier snowmelt will cause reductions in summer streamflow. Mostregional-scale assessments of climate change impacts on streamflow usedownscaled temperature and precipitation projections from general circulationmodels (GCMs) coupled with large-scale hydrologic models. Here we develop andapply an analytical hydrogeologic framework for characterizing summerstreamflow sensitivity to a change in the timing and magnitude of recharge ina spatially explicit fashion. In particular, we incorporate the role of deepgroundwater, which large-scale hydrologic models generally fail to capture,into streamflow sensitivity assessments. We validate our analyticalstreamflow sensitivities against two empirical measures of sensitivityderived using historical observations of temperature, precipitation, andstreamflow from 217 watersheds. In general, empirically and analyticallyderived streamflow sensitivity values correspond. Although the selectedwatersheds cover a range of hydrologic regimes (e.g., rain-dominated, mixtureof rain and snow, and snow-dominated), sensitivity validation was primarilydriven by the snow-dominated watersheds, which are subjected to a wider rangeof change in recharge timing and magnitude as a result of increasedtemperature. Overall, two patterns emerge from this analysis: first, areaswith high streamflow sensitivity also have higher summer streamflows ascompared to low-sensitivity areas. Second, the level of sensitivity andspatial extent of highly sensitive areas diminishes over time as the summerprogresses. Results of this analysis point to a robust, practical, andscalable approach that can help assess risk at the landscape scale,complement the downscaling approach, be applied to any climate scenario ofinterest, and provide a framework to assist land and water managers inadapting to an uncertain and potentially challenging future.