An Investigation of Flood Risk under a Changing Climate in the Souris River Basin

The Souris River Basin, a 61,770 square kilometer basin straddling the Canadian-American border of the Northern Great Plains, has experienced a substantial increase in precipitation and flooding risk since the 1970s. In 2011, the cumulative effect of saturated soils with above normal winter precipitation followed by heavy rainfall events in early summer led to flooding in communities such as Minot, North Dakota, Souris, Manitoba, and Estevan, Saskatchewan. While the 2011 flood event was determined to be rare, there is still concern about the potential for flooding caused by heavy precipitation events, particularly in the summer months, that lead to extreme flooding. There is also uncertainty about how much the existing reservoir operations agreement for the basin will be impacted by heavy precipitation events under the future climate regime.

The 2011 flood led the International Joint Commission to form the International Souris River Study Board (ISRSB) in 2017. One of the objectives of the ISRSB is to use numerical modeling to evaluate different operating scenarios at each of the reservoirs. The U.S. Geological Survey (USGS) has supported the study board by providing expertise on stochastic and hydrologic modeling.

The USGS generated synthetic 10-day average streamflow for 15 sub-basins of the Souris River Basin using a water balance model. One hundred stochastically generated precipitation, potential evapotranspiration, and temperature time series were input into the water balance model to generate 100 synthetic streamflow traces for all sub-basins. The synthetic streamflow data suggest, assuming maintenance of historic conditions, the risk of heavy rainfall events that can lead to large floods is generally low. As part of the ISRSB’s study activities, climate change scenarios are being run through the water balance model and compared to historic and synthetic streamflows. The synthetically-generated streamflow data will be compared to streamflow generated with at least three climate-change scenarios to determine what hydrometeorological changes the basin might experience and how hydrologic conditions change relative to historic conditions. This study is intended to characterize (1) how flood risk could change, (2) determine what and how much risk results from climate change, and (3) determine how the dynamic relationship between heavy precipitation events and antecedent soil moisture drive flooding under an evolving climate. Additionally, existing reservoir operations will be evaluated to determine potential risks that are new to the management of the basin. Initial results are intended to provide an understanding of the likelihood of flooding due to heavy precipitation under a new climate regime.