Assessing hydrological sensitivity to future climate change over the Canadian southern boreal forest

This study develops a physically based hydrological process model using the Cold Region Hydrological Modelling (CRHM) platform to simulate the water budget storages and fluxes over the Canadian southern boreal forest (SBF). Evaluation of the CRHM-based model in a well-gauged SBF basin, White Gull Creek (WGC), Saskatchewan, Canada, indicated quite good performance in reproducing historical observations of streamflow, snow water equivalent (SWE), evapotranspiration (ET) and soil moisture without parameter calibration from streamflow. The entire SBF was then evenly divided into 2243 virtual basins, each of which was structured and parameterized with the same land cover, soil and hydrological parameters as the WGC basin, but with local latitude and topography in order to examine the sensitivity of governing hydrological processes to future climate variability and perturbation. Hydrological sensitivity in the virtual basins was assessed by examining the differences be-tween hydrological simulations driven by 4-km gridded convection-permitting Weather Research and Fore-casting (WRF) outputs in the current period (ctrl, 2001-2013) and a Pseudo Global Warming period (pgw, 2087-2099). The WRF simulation in the pgw period was forced by a perturbation of the same boundary con-ditions from ERA reanalysis data as for the ctrl period, and the perturbation was based on the ensemble-mean of projected changes from the CMIP5 RCP 8.5 emission scenario. Results showed that temperature would increase by 4.5 & DEG;C to 7 & DEG;C over the SBF but increases in annual precipitation of 15-24% would more than compensate for the effects of warming on runoff generation and result in greater streamflow volumes. Annual streamflow vol-umes would increase by 64 mm (35%) and 95 mm (16%) in the west and east, and by 48 mm (17%) in the central SBF. Annual snowfall and maximum SWE would decrease by 89-109 mm (-29%) in the east, 3-8 mm (-6%) in the west, and 31-50 mm (-20%) in the central SBF. Annual mean soil moisture storage would decrease by 54-56 mm (27%) in the west and central, and by only 37 mm (14%) in the east SBF. Decreases in soil moisture would be caused by reduced soil freezing and enhanced thawing under future warming which would increase soil water loss from ET, subsurface runoff and percolation into groundwater storage. The larger sensitivity of streamflow and snow processes in the east SBF is partly due to the wetter climate and the larger increase in annual precipitation, the later also buffered the sensitivity of soil moisture to warming. These results show that the SBF would switch to a higher water yield region, dominated by rainfall-runoff fed streamflow over a longer snow-free season, and provide first-order guidance for sustainable water management of the SBF in the future.

Automated summary

This study develops a physically based hydrological process model to simulate water budget storages and fluxes over the Canadian southern boreal forest. The model shows good performance in reproducing historical observations without calibration. The study also assesses the sensitivity of hydrological processes to future climate variability.