A modelling framework for assessing the future water quality of Lake Diefenbaker under climate change

ABSTRACT The South Saskatchewan River Basin is one of the Canada's largest river basins, containing approximately 75% of Canada's irrigated agriculture. Water quality and climate change are key concerns in this heavily managed watershed that is fed by snow and ice melt from the Canadian Rockies, and experiences both floods and droughts. This study applies a loosely coupled modelling framework to Lake Diefenbaker, the largest reservoir in the South Saskatchewan River Basin, to predict future water quality at the reservoir's two outflow dams. Model scenarios are developed in a two-dimensional water quality model of Lake Diefenbaker built in CE-QUAL-W2. The framework incorporates data from the cold-region hydrology Modélisation Environmentale Communautaire – Surface and Hydrology (MESH) model, the watershed nutrient export SPARROW model, and climate data from Coupled Model Intercomparison Project Phase 6 (CMIP6). Results predict that reservoir water quality at the outflow locations will deteriorate by the end of the century. Results differ between the thermal layers and the two dams. The epilimnion will generally see the greatest response to changes in climate and watershed inputs, with epilimnion temperatures increasing by up to approximately 3 °C. Total phosphorus and total nitrogen are predicted to increase, while dissolved oxygen will decrease in response to the warming conditions. Approximately 6–7 weeks less ice cover is predicted by 2100.