Examining the role of climate and land use change on hydrologic trends in the Upper Mississippi River Basin

Positive streamflow trends in the Upper Mississippi River Basin (UMRB) during the 20th century have been well documented. During this period, the UMRB experienced relatively wetter climatic conditions, while the basin's land cover changed significantly through a net extensification of croplands. In this study we first examine the observed changes in the regional climate of the UMRB using gridded precipitation, minimum, maximum, and average daily temperatures, and wind speed data. Increasing trends in precipitation, annual minimum daily temperature, and annual average temperature have been observed while maximum daily temperature has decreased. This led to a general decrease in the potential evapotranspiration in the basin. The climate data is then used to force the Variable Infiltration Capacity (VIC) macro-scale hydrology model, and the influences of land use and climate change on regional hydrology in the UMRB are explored. Two modeling scenarios for the period between 1918 and 2007 have been conducted representing: the dynamic changes in cropland extent and a static condition representing the year 1918. Model simulations suggest that agricultural extensification after 1918, through the conversion of lower biomass perennial vegetation to higher biomass annual vegetation in most parts of the basin, has increased evaporative losses, slightly decreasing annual runoff at the outlet when compared to the 1918 land surface state. Spatial hydrological fluxes are investigated in relation to the initial vegetation before land cover change. More significant differences are observed locally where changes in cropland extent have increased mean annual runoff up to 6% in forested regions of the basin and decreased runoff up to 8.5% in areas where croplands replaced grasslands. The observed wetting trends in the climate have produced increases in regional runoff as well as evaporation regardless of land cover change. These results suggest that the observed increasing trends in streamflow in the UMRB could be attributed to climate rather than changes in the land cover.