Abstract–Hydrologic Impacts of Climate Change in Skagit River Basin

Hydrologic Impacts of Climate Change in Skagit River Basin
Christina Bandaragoda, Chris Frans, Erkan Istanbulluoglu,Crystal Raymond and Larry Wasserman

The focus of the Hydrologic Impacts of Climate Change in the Skagit River Basin study is to improve our understanding of the hydrology of the Skagit River system using a coupled glacio-hydrology model and develop projections of naturalized streamflow at Skagit River Hydroelectric Project reservoir locations (Ross, Diablo, Gorge) and at sixteen tributaries using future climate change scenarios. Our methods and scope of work for generating future streamflow projections are a reflection of the collaboration between Seattle City Light, Swinomish Indian Tribal Community, and the Sauk-Suiattle Indian Tribe administered
by contracts with the Skagit Climate Consortium (SC2) and between SC2 and the University of Washington (UW). This project utilized data products from the Integrated Scenarios of the Future Northwest Environment project, which identified a core set of 10 global climate models (GCMs) of the Coupled Model Intercomparison Project Phase 5 (CMIP5; Mote et al., 2015) as the best performing models based on comparisons of observed 20th century climate of the Pacific Northwest. To simulate streamflow, we used the Distributed Hydrology Soil Vegetation Model (DHSVM) – a coupled glacio-hydrology model. The model domain included the entire Skagit River basin at 150m digital elevation model (DEM) resolution, with nested models of 50m resolution of selected subbasins (Thunder Creek and Cascade Creek) that have the major glacier ice cover at their high elevations. The modeling steps included: (a) spin up of the glacier model to develop realistic glacier cover in the glaciated uplands prior to watershed hydrology and streamflow predictions; (b) calibration of DHSVM using select model parameters and climate forcing bias correction in select subbasins; © model validation using historical streamflow observations; (d) projections of streamflow into the future using CMIP5 models; (e) bias-corrections of modeled streamflow to match observations based on monthly mean and (f) low-flow corrections of modeled streamflow to match 90% exceedance probability flows in summer months. The glacio-hydrology model was calibrated using historical meteorological data and observed ice extent using the time frame of 1960–2010.Validation and corrections to the glacio-hydrology model were conducted using empirical data (collected by North Cascades National Park), naturalized flows at reservoir locations (three reservoirs), and observed stream gauges (where and when available at 16 Skagit River tributaries).Future projections were calculated using GCMs for multiple thirty year periods starting from 2010 to 2099.

Our analysis focused on locations and statistics that are applicable for multiple uses in climate change adaptation— planning for hydroelectric project operations for instream flows and hydropower generation along with prioritization of locations for salmon restoration. In this report we highlight changes applicable to mid-century planning (2050). In glaciated high elevation basins, the current conditions of approximately 100 km² of glacier ice are projected to decrease to less than 50 km² by 2050. If global emissions stop increasing by 2040, it is likely that the highest elevation glaciers will continue to store pockets of ice and provide some glacier melt in the summer months. If emissions are not reduced, most models project that Skagit glaciers will disappear by the end of the century. In snow dominated high elevation basins, high flows are projected to increase by 2050 , with the frequency of high flows extending from the current November timing, into December, January and February. By 2050, the low flows will show a wide range of change conditioned on elevation. Low summer flows (August 90% Exceedence