Regional changes in nitrate loadings in the Upper Mississippi River Basin under predicted mid-century climate

The Upper Mississippi River Basin (UMRB) drains an area of 492,000 km(2) primarily from five states in the central US. Alterations in climate variability and change will lead to changes in hydrologic behavior of the region with implications in the fate and transport of nutrient loadings. This impact assessment study evaluated the long-term changes in annual water yield and nitrogen loadings in the UMRB using the Soil and Water Assessment Tool (SWAT), with mid-century (2046-2065) climate change projections as predicted by the ensemble of ten general circulation models. Predicted precipitation change ranged from -1.6 to 6.3 % at the subwatershed level while the mean temperature was found to be more consistent over the entire basin with an average increase of 2.8 A degrees C. The SWAT-predicted streamflow decreased by 5 % on an average annual basis at the watershed outlet (Grafton, Illinois) with significant monthly variation. Streamflow increased during the winter months of December, January, and February and decreased during the summer with up to a 41 % reduction in July. The nitrogen loading pattern was found to vary widely from a decrease of 3 kg/ha to an increase of 8 kg/ha at the subwatershed level, with an average increase of 1 kg/ha over the entire basin. Portions of the UMRB in Illinois are expected to shift toward adverse conditions for nitrogen loading (and thus greater potential for exporting), whereas portions of the UMRB in Iowa are expected to experience a decrease in nitrogen loads under predicted mid-century climatic conditions for the existing land and nutrient management.