Implementation of a hysteretic depression model to assess future water availability in the St. Mary and Milk River transboundary basin

The St. Mary and Milk River (SMM) basin is an international transboundary watershed flowing between Canada and the United States. The basin is composed of 2 distinct headwater basins that flow into the Saskatchewan Nelson and Mississippi basin, respectively. A diversion constructed in 1909 conveys water from the higher-yielding St. Mary River into the lower-yielding Milk River. The 1909 Boundary Waters Treaty between the USA and Canada allowed for specific entitlements for each country, allowing for sharing of the combined basin resource between both countries. Lack of storage, conveyance and changing hydrological conditions in the basin have resulted in both countries receiving less than the treaty entitlement, prompting the International Joint Commission (IJC) to study the situation. This research addresses an important part of the IJC study which required implementing hydrological models to simulate natural flow in the SMM basin and understand the reliability of any solution under future climate. The HYPE hydrological model with the HDS module was implemented to model natural flow in the basin. HDS allowed for the explicit representation of the contributing area dynamics of prairie potholes which significantly impact the hydrology of the Milk River. The model was then used to run an ensemble of statistically downscaled future climate scenarios based on the CMIP6 models. Explicitly representing prairie potholes under future climate provided an opportunity to examine how non-contributing area might change in the future. We present an evaluation of historical model performance, a future climate analysis of streamflow in the basin, and the implications of the future climate conditions on apportionment practices in the basin. Results from this research will inform IJC decisions on future practices and infrastructure in this important transboundary basin and may add a new dimension to future practices as the effects of prairie potholes have never been explicitly considered.