The effect of climate change on the simulated streamflow of six Canadian rivers based on the CanRCM4 regional climate model

Abstract. The effect of climate change on the hydro-climatology, particularly the streamflow, of six major Canadian rivers (Mackenzie, Yukon, Columbia, Fraser, Nelson, and St. Lawrence) is investigated by analyzing results from the historical and future simulations (RCP 4.5 and 8.5 scenarios) performed with the Canadian regional climate model (CanRCM4). Streamflow is obtained by routing runoff using river networks at 0.5° resolution. Of these six rivers, the Nelson and St. Lawrence are the most regulated. As a result, the streamflow at the mouth of these rivers shows very little seasonality. Additionally, the Great Lakes significantly dampen the seasonality of streamflow for the St. Lawrence River. Mean annual precipitation (P), evaporation (E), runoff (R), and temperature increase for all six river basins in both future scenarios considered here, and the increases are higher for the more fossil-fuel-intensive RCP 8.5 scenario. The only exception is the Nelson River basin, for which the simulated runoff increases are extremely small. The hydrological response of these rivers to climate warming is characterized by their existing climate states. The northerly Mackenzie and Yukon River basins show a decrease in the evaporation ratio (E/P) and an increase in the runoff ratio (R/P) since the increase in precipitation is more than enough to offset the increase in evaporation associated with increasing temperature. For the southerly Fraser and Columbia River basins, the E/P ratio increases despite an increase in precipitation, and the R/P ratio decreases due to an already milder climate in the northwestern Pacific region. The seasonality of simulated monthly streamflow is also more affected for the southerly Fraser and Columbia rivers than for the northerly Mackenzie and Yukon rivers as snow amounts decrease and snowmelt occurs earlier. The streamflow seasonality for the Mackenzie and Yukon rivers is still dominated by snowmelt at the end of the century, even in the RCP 8.5 scenario. The simulated streamflow regime for the Fraser and Columbia rivers shifts from a snow-dominated to a hybrid or rainfall-dominated regime towards the end of this century in the RCP 8.5 scenario. While we expect the climate change signal from CanRCM4 to be higher than that from other climate models, owing to the higher-than-average climate sensitivity of its parent global climate model, the results presented here provide a consistent overview of hydrological changes across six major Canadian river basins in response to a warmer climate.