Where’s Water?: Integrating Geology with Hydrology to Improve Predictions of Future Streamflows in the Western US

Gordon Grant, Research Hydrologist, USDA Forest Service, Pacific Northwest Research Station and Courtesy Professor, College of Earth, Ocean, and Atmospheric Sciences, Oregon State University

The western US is emerging as one of the more vulnerable regions to climate change, with changes in the magnitude, seasonality, and timing of streamflow among the most sensitive variables.  As the climate warms, diminishing snowpack and earlier snowmelt will cause reductions in summer streamflow, trends that are already apparent in the hydrologic record. Peak winter streamflows are also expected to change.  Most regional-scale assessments of climate change impacts on streamflow use downscaled temperature and precipitation projections from general circulation models (GCMs) coupled with large-scale hydrologic models.  But such “top down models can miss important dynamics due to the interaction between climate changes and the intrinsic geologically-mediated drainage efficiencies of diverse landscapes. To address this, we have developed an alternative strategy that uses simple hydrologic theory to derive landscape-level predictions of streamflow sensitivities (both peak and low flow) to climate warming.  We test these predictions against the historical record, and compare this “bottom up” method against the “top down” forecasts, revealing strengths and weaknesses of both approaches.