An Assessment of Historical Hydro-Climatic Variability in Two Key Watersheds over the Southern Canadian Prairies

Since human activities and ecosystem health are dependent on adequate, reliable water supplies, extreme hydro-climatic variability, including the occurrence of droughts and excessive moisture, pose a serious threat to society and the environment. The southern Canadian Prairies are a region with high natural hydro-climatic variability, however, recent dramatic shifts between extreme drought and extreme wet conditions have suggested that this variability may be increasing. Using the Oldman and Swift Current watersheds as case studies, this investigation assesses both the occurrence and potential atmospheric causes of hydro-climatic variability and extremes at various temporal and spatial scales during the instrumental period of record. Incorporation of the Standardized Precipitation Evapotranspiration Index (SPEI) reveals considerable decadal-scale variability in hydro-climate over both watersheds with no discernible long-term trends. However, the most recent decade has been associated with a greater frequency of shifts from extreme drought to excessive moisture conditions (and vice versa) than that observed during the previous 100 years. In addition, an assessment of the mid-tropospheric (500 hPa) circulation patterns associated with identified hydro-climatic extremes indicate that major drought episodes over both the Oldman and Swift current watersheds were associated with significantly higher frequencies of circulation types that included distinctive ridging patterns over the Prairie region, and lower incidences of zonal and mid-tropospheric troughing patterns. Excessive moisture conditions had opposite responses. Evidence also suggests that the sequencing of these atmospheric circulation patterns plays an important role in the initiation, persistence, and termination of droughts and excessive moisture in the study regions. Results from this analysis have increased knowledge regarding the occurrence and causes of extreme hydro-climatic variability in the southern Canadian Prairies, which may aid in the better understanding of the frequency and associated impacts of future events.