Quantifying projected changes in runoff variability and flow regimes of the Fraser River Basin, British Columbia

Abstract. Canada's Fraser River Basin (FRB), the largest watershed in the province of British Columbia, supplies vital freshwater resources and is the world's most productive salmon river system. We evaluate projected changes in the FRB's runoff variability and regime transitions using the Variable Infiltration Capacity (VIC) hydrological model. The VIC model is driven by an ensemble of 21 statistically downscaled simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5), for a 150-year time period (1950–2099) over which greenhouse gas concentrations follow the CMIP5 Representative Concentration Pathway (RCP) 8.5. Using mean and standard deviation (variability) metrics, we emphasize projected hydroclimatological changes in the cold season (October to March) over different sub-basins and geoclimatic regions of the FRB. Warming consistent with the RCP8.5 scenario would lead to increased precipitation input to the basin with higher interannual variability and considerably reduced winter snowfall shortening the average snow accumulation season by about 38 %. Such changes in temperature and precipitation will increase cold season runoff variability leading to higher cold season peak flows. In the lower Fraser River, cold season runoff will increase by 70 % and its interannual variability will double compared to the 1990s, presenting substantial challenges for operational flow forecasting by the end of this century. Cold season peak flows will increase substantially, particularly in the Coast Mountains, where the peak flow magnitudes will rise by 60 %. These projected changes are consistent with a basin-wide transition from a snow-melt driven flow regime to one that more closely resembles a rainfall driven regime. This study provides key information relating to projected hydroclimate variability across the FRB, describes potential impacts on its water resources, and assesses the implications for future extreme hydrological events.