Understanding the 2011 Upper Missouri River Basin floods in the context of a changing climate

Study Region: Upper Missouri River Basin. Study Focus: The semi-arid Upper Missouri River Basin (UMRB) has experienced notable volatility in high and low streamflow extremes in recent decades, punctuated by the record 2011 flood. This study provides a new perspective into the relative importance of precipitation and antecedent moisture conditions in driving extreme streamflow. Ensemble streamflow simulations demonstrate that precipitation is largely the dominant driver for high streamflows. Applying the observed atmospheric forcing in 2011 with initial conditions of antecedent hydrologic conditions from 64 historic years consistently produces large streamflow events exceeding the 85th percentile of historical peak flows. This study attributes the individual roles of atmospheric conditions and antecedent soil moisture on extreme streamflow production. It uses a novel modeling framework that provides a greater understanding for the role that heterogeneity in basin-scale hydrologic features have in extreme streamflow generation. New hydrologic insights for the region: A detailed analysis of the record 2011 flood event shows that streamflow generated over the region's easternmost sub-basin is acutely sensitive to antecedent moisture. Yet, the 2011 record streamflow cannot be explained by a single factor or as the result of long-term trends, with the basin responding to several independent factors; significantly high (p < 0.05) antecedent moisture and significant cold-season precipitation. Perhaps most importantly was the record-setting May precipitation, which limited the ability of ensemble streamflow simulations initialized on 1-March from reliably predicting the record June streamflows. The recent volatility of UMRB streamflow may be a harbinger of future decades based on our analysis of climate projections that indicate increased hydroclimate variability by the latter half of the 21st century.