Climate change impacts on the hydrology of the Upper Mississippi River Basin as determined by an ensemble of GCMs

We used 20th century simulations by nine GCMs to drive a streamflow model, the Soil and Water Assessment Tool (SWAT), to examine components of the hydrologic budget for the Upper Mississippi River Basin (UMRB). Statistical tests revealed that streamflow data produced by members of the GCM ensemble were serially uncorrelated at all lags and formed unimodal distributions and that GCM ensemble results may be used to assess annual streamflow in the UMRB. Despite the fact that all low-resolution global models produce large differences from observations of streamflow and hydrological components simulated by the streamflow model, the nine-member ensemble performed quite well. Results of statistical tests indicate that, of all models used, only a high-resolution global model – the only high-resolution model tested - correctly simulates the observed streamflow, despite the fact that its low-resolution sister model has no advantage over the other seven low-resolution models. Output from an ensemble of seven global climate models provided future scenario climates to drive SWAT. Results showed only modest increases in precipitation (+6%) and streamflow (+3%) but substantial reduction in snowfall (-37%) for the UMRB for the end of the 21st century. The low-resolution of global models contribute to biases in some but not all hydrologic components, most notably evapotranspiration, potential evapotranspiration, and baseflow. Ensemble results indicate an increase in baseflow (+12%) and decrease of runoff (-20%) for the future scenario. Such changes would likely decrease sediment loading of streams, but influences on the fate of fugitive nitrates is indeterminate from this preliminary study.