Modeling 21st century peak streamflows in the Stillaguamish Watershed using dynamically downscaled general circulation model…

Climate change is projected to increase river flooding in the Puget Sound region of Washington State by reducing snowpack and yielding more intense storm events. Pairing meteorological forcings from general circulation models (GCMs) with a physically based hydrologic model is a robust method of assessing watershed response to projected climate. Before GCM forcings can be applied to regional hydrologic models, some form of downscaling or regionalization is required. Dynamical downscaling is a means of incorporating mesoscale atmospheric processes within GCM-informed boundary conditions. Here I apply climate projections, dynamically downscaled using the Weather Research and Forecasting model (WRF), to the Stillaguamish watershed in northwest Washington State using the physically based Distributed Hydrology Soil Vegetation Model (DHSVM). I simulate hourly streamflow for 12 high emissions scenarios (i.e., Representative Concentration Pathway 8.5) throughout the 1,724- square-kilometer basin from 1970 through 2099 and analyze climate and hydrologic trends, with a particular emphasis on peak flows. My projections indicate that as the climate warms, snowpack will recede to higher elevations and the basin will shift from transitional to raindominant, leading to an increase in average winter streamflows and a decline in spring and summer streamflows. Peak streamflow magnitude in the Stillaguamish River will increase by about +26.5% across the assessed return intervals (2 to 100 years) and flow durations (3 hours to 1 week) by the 2080s. Similarly, modern day peak flow magnitudes will recur more frequently. Flow generating mechanisms also change, with rain-on-snow events decreasing in likelihood while extreme precipitation events become more common and more severe. These shifts have wide ranging implications for flooding within the developed lowlands of the watershed, and for threatened salmonid populations which are culturally and economically critical to the region.