Sensitivity of Projected Streamflow Changes to Future Scenarios in Three Hydrologic Regimes in BC

Climate change projections are inherently uncertain; therefore we accept the projected range and opt to investigate the sensitivity to changes in the main drivers of the hydrologic system, temperature and precipitation. British Columbia (BC) is a hydrologically diverse province where the sensitivity to climate change and resulting hydrologic change varies with region. Three watersheds with unique hydro-climatic settings are analyzed. The Upper Campbell River watershed (1,200 km²) has a mixed rainfall and snowmelt (hybrid) hydrologic regime. The Upper Peace River basin (101,000 km²) is a snowmelt-dominated watershed and the Mica Basin (21,100 km²) the headwater of the Columbia River Basin, has a mixed snowmelt-glacier melt runoff regime.

Streamflow is assessed using an implementation of the Variable Infiltration Capacity (VIC) hydrologic model, parameterized with common historical forcing data (temperature, precipitation and wind), vegetation and soils in all basins. Streamflow changes are estimated based on eight Global Climate Models (GCMs) from the CMIP3 suite, downscaled using the Bias Correction Spatial Downscaling (BCSD) technique, run under three emissions scenarios (A2, A1B and B1), for a total of 23 combinations (B1 was not available for UKMO_HADGEM1). These 1/16^th degree (~32 km²) transient scenarios of temperature and precipitation are used to drive the VIC model at the same scale, from 1950 to 2099 for each basin. To test the sensitivity, streamflow changes for the 2050s (2041-2070), as a percentage of the 1961-1990 values, are plotted against temperature (absolute) and precipitation (percentage) anomalies for all seasons, and relationships are quantified by multiple-linear regression.

Sensitivity to climatic drivers differed between hydrologic regimes. A hybrid regime, like the Campbell, moves toward a rainfall-dominated regime under projected changes in temperature and precipitation. Here a 1% increase in winter precipitation combined with a 1^oC increase in winter temperature, results in a 25% (±6%) increase in winter streamflow (r²=0.91, p<0.001) and summer streamflow decreases of up to 80%, result from decreases in winter precipitation and increases in winter and spring temperature. In the snowmelt – glacier melt regime of the Mica, projected increases in winter and spring are responsive, but the basin remains snowmelt dominated with a strong spring freshet. Changes in glacier melt contributions are unclear at the seasonal time step. Winter streamflow increases with increased fall precipitation and winter temperatures. An 8% (±2%) increase in spring runoff results from a 1% increase in winter precipitation and a 1^oC increase in spring temperature (r²=0.90, p<0.001). In the Peace, projected winter temperature increases from one GCM were a degree smaller than the other scenarios and in the summer and fall were a degree larger than the other scenarios. Due to these outliers, the forcing signal is less clear in this basin than the others. However, these outliers might identify thresholds in temperature that trigger strongly declining flows in summer and fall in this basin and require more investigation. The Peace remains snowmelt dominated in the 2050s under all scenarios.

This study identifies the major climatic drivers of streamflow change in three watersheds in BC and the timing of their responses. It forms a foundation for assessing uncertainty in future projections within the most sensitive components of the system and aims to make the physical mechanisms behind these projected changes more clear, to enable efficient planning and management under changing climate conditions.