Tesemma Z., Shook K., Princz D., Razavi S., Wheater H., Davison B., Li Y., Pietroniro A. and Pomeroy J.W. (2020). Diagnosis of Historical and Future Flow Regimes of the Bow River at Calgary Using a Dynamically Downscaled Climate Model and a Physically Based Land Surface Hydrological Model - Centre for Hydrology Report #18. University of Saskatchewan

This report assesses the impacts of projected climate change on the hydrology, including the flood frequencies, of the Bow and Elbow Rivers above Calgary, Alberta. It reports on investigations of the effects of projected climate change on the runoff mechanisms for the Bow and Elbow River basins, which are important mountain headwaters in Alberta, Canada. The study developed a methodology and applied a case study for incorporating climate change into flood frequency estimates that can be applied to a variety of river basins across Canada. This research was carried out by scientists from the University of Saskatchewan Centre for Hydrology, under contract to Natural Resources Canada and Alberta Environment and Parks with contributions from the City of Calgary, Environment and Climate Change Canada and the Global Water Futures program.

high resolution, enhanced version of Environment and Climate Change Canada’s MESH (Modélisation Environnementale Communautaire - Surface Hydrology) land surface hydrological model was set up at a spatial resolution of approximately 4 km by 4 km to correspond to the resolution of dynamically downscaled Weather Research Forecast (WRF) atmospheric model outputs for current and future climates in the region. This convection-permitting WRF product used ERA-Interim reanalysis product boundary conditions over 2000 - 2015 to produce realistic, high resolution weather simulations. Other available meteorological forcings were evaluated at the lower resolution of approximately 10 km by 10 km for which MESH is normally run. Prior to this study, MESH did not consider the impact of slope and elevation on meteorological forcings below the resolution of the data, which is not a reasonable assumption in mountains. Here, incoming solar radiation was calculated as a function of terrain slope and aspect. Also, precipitation, temperature, pressure, humidity and longwave radiation were corrected for elevation. The necessary cold regions processes (blowing snow, intercepted snow, sublimation, frozen soil infiltration, slope/aspect impacts on melt rates, glacier ice melt) and water management processes needed to simulate the natural and reservoir-managed streamflow hydrographs in the basin were modelled. Most model parameter values were set based on remote sensing, land surveys and the results and understandings from previous regional hydrological investigations, however forest root depth and stomatal resistance, and soil hydraulic conductivity and channel routing model parameters were calibrated using measured (2006 - 2015) streamflows on the Bow River at Banff, and evaluated (2000 - 2005) at the same stream gauging station. The pseudo global warming (PGW) approach to dynamical downscaling of future warming projection under RCP8.5 (2086 - 2100), used WRF bounded by ERA-Interim outputs that were perturbed by the mean outcomes of an ensemble of Coupled Model Intercomparison Project Phase 5 (CMIP5) climate model projections.

The simulation results show that, by the end of the century, snowmelt runoff events are projected to increase by up to six events per year, an approximately 20% increase, in the highest elevations of Central Ranges of the Canadian Rockies, primarily in Banff National Park (BNP), and to decrease by up to fourteen events per year, a decrease of approximately 100% in the lower elevation foothills. Snowmelt runoff itself would virtually cease at the middle to lower elevations of the basins. Rain-on-snowmelt events are projected to become more frequent at all elevations (100%-200% increase), particularly in Banff National Park, the Kananaskis and Elbow river iv headwaters, and the agricultural lands in the eastern part of the basins, but less frequent in the foothills where they will drop by 50%. The future reduction in frequency of rain-on-snowmelt events in the foothills is associated with a substantial shortening of the snow-covered period and its increase at medium to high elevations and on the plains is due to more frequent rainfall in winter on the plains and spring in the high mountains. Compared with the historical period, rainfall- runoff events are projected to become more frequent and widespread. They currently cause more than four events per year only in the foothills and eastern part of the basins, this will decline dramatically in the agricultural areas as soil become drier. However, overall, there will be an increase of four events per year for the Bow River Basin, particularly in the foothills, but also in the high mountains, as the warmer climate increases the proportion of precipitation falling as rain. Glacier contributions to runoff will decline dramatically at high elevation locations with concomitant deglaciation, providing notable declines in late summer streamflow above Banff. This is projected to cause a reduction in annual streamflow volumes of less than 2% for the Bow River at Calgary and will have no impact on the Elbow River.

A novel way was devised to use bias correction from streamflow observations to reduce the uncertainty of modelled and projected flow duration curves. The effects of climate change on future streamflow is likely to reduce the highest streamflows and to increase the medium and low flows. A detailed examination of historical floods in June of 2005 and 2013 and how such events may shift under future climates showed increases in flood event flow volumes for the Bow River at Banff, but reductions in flood event flow volumes at Calgary in both the Bow and Elbow rivers. These shifts can be attributed to changes in the precipitation regime, and to reduced rain-on-snow runoff and antecedent snowmelt runoff from the Front Ranges – both are consequences of warmer conditions. The increase in rainfall runoff components of the events that causes higher flow volumes at Banff is unable to compensate for the decrease in snowmelt runoff and rain-on- snowmelt runoff components in the Front Ranges and so overall, the flood event flow volumes are smaller at Calgary.

A companion report, Centre for Hydrology Report No. 17 incorporates future climate uncertainty from RCMs into subsequent WRF-MESH modelling exercises and should be considered along with this foundational report as part of the comprehensive case study of how to estimate future flood streamflows using coupled climate and hydrological models.

https://research-groups.usask.ca/hydrology/documents/reports/chrpt18_historicalfutureflowregimesbowriver_finalreport_2020.pdf