Projected Hydroclimatic Changes in Two Major River Basins at the Canadian West Coast Based on High-Resolution Regional Climate Simulations

The impact of anthropogenic climate change on water resources and flood and drought risk is of great interest for impact modeling and to inform adaptation strategies. Here an analysis of hydroclimatic changes in the Fraser and Athabasca River basins in western Canada is presented, based on an ensemble of climate projections, which have been dynamically downscaled to 10-km resolution using the Weather Research and Forecasting Model in two configurations. The GCM ensemble comprises four independent integrations of the Community Earth System Model under the representative concentration pathway 8.5. Basin-integrated changes in the seasonal cycle of hydroclimatic variables, and the variability of water supply and flood and drought risk, are considered. It is found that fall and winter precipitation generally increase by 20%-30% toward the end of the century, while changes in summer precipitation are smaller and associated with high model uncertainty. Furthermore, a reduction in snowfall and an increase in evapotranspiration are projected. However, projected impacts on water resources east and west of the Rocky Mountains are quite different: in basins closer to the coast (west of the Rocky Mountains) higher temperatures lead to a transition from predominantly solid to liquid precipitation and a significantly weaker spring freshet, followed by drier summers. In the lee of the Rocky Mountains the spring freshet remains largely unaffected and in summer the increase in evapotranspiration (ET) is compensated by increasing precipitation, so that water balance changes appear to be small. It is further found that a shift in runoff seasonality near the coast may lead to significantly increased flood risk in fall.