Impacts of projected climate change on extreme monthly river flow in the Upper Mississippi River Basin

A previously calibrated Soil and Water Assessment Tool (SWAT) model simulated Upper Mississippi River Basin (UMRB) streamflow at Grafton, IL, for contemporary (1981–2010) and future-scenario (2041–2070) climates. Driving data used Max Planck Institute (MPI) and Geophysical Fluid Dynamics Laboratory (GFDL) GCM simulations for historical and mid-century RCP8.5 scenarios that were dynamically downscaled by RegCM4 and bias corrected as part of the NA-CORDEX program. The work advances previous studies by examining drivers of the changes and using more recent climate simulations. Both MPI-RegCM4 and GFDL-RegCM4 produced similar results for monthly streamflow during the contemporary period, although with a low bias. In the future-scenario, GFDL-RegCM4 yielded higher streamflow with greater exceedance of the observed contemporary climate's 90th, 95th, 98th percentiles than MPI-RegCM4, especially during the high-water season. The MPI-RegCM4 tended to simulate a drier and hotter future scenario than GFDL-RegCM4 with lower evapotranspiration and water yield, although the MIP-RegCM4 scenario simulation also yielded increased exceedances of the 95th and 98th percentiles. Both models projected greater interannual variability for monthly streamflow in the future scenario compared with contemporary climate. The more frequent extreme high flows and the increased interannual variability pose important, emerging challenges for river managers if human-caused climate change continues.