Assessing the Impact of Climate Change on the Hydrology of the Upper Mississippi River Basin

Recent observations and modeling suggests acceleration of the hydrological cycle at high latitudes in the Northern Hemisphere and that extreme intense precipitation events were more frequent over the last 30 years of the twentieth century. Assessments of local and regional impacts of changes in the hydrological cycle in future climates call for improved capabilities for modeling the hydrological cycle and its individual components at the subwatershed level. We have evaluated the impact of climate change on stream flow in the Upper Mississippi River Basin by use of a regional climate model (RCM) coupled with a hydrologic model - Soil and Water Assessment Tool (SWAT) - and by use of an ensemble of GCMs producing output for the IPCC 4th Assessment Report coupled to SWAT. Both the RCM and the GCM ensemble reproduce quite well the annual flow and interannual variability of observed streamflow of the UMRB for the 20th Century. Individual low-resolution GCMs give poor simulation of annual streamflow, but the one high-resolution GCM tested gave good results. The RCM driven by a single GCM (HadCM2) results for the decade of the 2040s gave a 21% increase in future precipitation, which resulted in a 51% increase in surface runoff, 43% increase in groundwater recharge, and 50% net increase in total water yield in the UMRB on an annual basis. Although there is inconsistency among GCMs, the ensemble-mean precipitation increased of 6% due to climate change. ET calculations give positive changes for all models, likely due to warm-season temperature increases. Substantial decreases in snowfall suggest that warming is strong in winter. Runoff decreases substantially for most models, possibly due to enhanced drying of soils between rains. Total water yield varied widely among models, with the ensemble mean showing almost no change from the contemporary climate.