Analysis of Recent Changes in Extreme Hydrologic Events in the Upper Missouri River Basin

In 117 years of record-keeping (1898-2014), 9 of the highest 10 annual runoff years in the Upper Missouri River Basin (UMRB) have occurred in just the last 40 years (1975-2014). This study explores two primary aspects of the increased frequency of high runoff years: antecedent soil moisture conditions and trends in observed precipitation, with a particular interest as they pertain to the 2011 Missouri River floods—the largest event since 1898. Observations show that there have been spatial and temporal changes to precipitation that are altering regional hydrologic physical mechanisms that govern extreme flow events. We seek to quantify the impact of wet antecedent conditions and anomalous precipitation on extreme runoff by initializing a calibrated physics-based hydrologic model to various historical antecedent moisture conditions and driving the model with an ensemble of historical meteorological forcings. Anomalously high soil moisture in the lowest reach of the UMRB exhibited a significant impact on the extreme runoff in 2011, coupled with anomalous cold-season (October to March) precipitation. Further, we calculate trends in temperature and precipitation using approximately 200 long-term stations, with data extending back 1901, finding a warming trend, increased mean seasonal precipitation and increasing precipitation contribution from extreme events. Hydrologic simulations utilizing these trends reveal that increased delivery of cold-season precipitation is the most important contributor to increased runoff driven by heightened seasonal runoff efficiency, whereas temperature and extreme precipitation trends showed less impact. We find that cold-season precipitation has increased by 12% over the last 40 years, which highlights a significant sensitivity in runoff for the UMRB; this sensitivity is of note as CMIP5 simulations from the NCAR-CESM1 model project an increasing precipitation trend through the end of the 21st century.