Can An Ecohydrologic Analysis Distinguish Land-Use and Climate-Change Effects on Watershed Hydrology?

Impacts of climate change on watershed hydrology are subtle compared to differences among years of drought and surplus rainfall, and not easily separated from effects of land use change. In the Midwest, trends such as increasing baseflow can be attributed to changes in agricultural practices as easily as to climate change. In a 25-year small watershed study in Iowa, when annual hydrologic budgets were accrued between major droughts (inter-drought periods), an eco-hydrologic water budget analysis suggested effects of agricultural practices and climate trends could be separated. Both effects resulted in increased stream discharge as a fraction of precipitation, but unsatisfied evaporative demand increased under conservation tillage, yet decreased with time. We investigated records from larger Midwest watersheds to determine whether this interpretation could inform a wide-scale analysis. We selected records from four agricultural watersheds with minimal development of dams, urbanization, and irrigation, and with glacial geology that limits deep groundwater recharge. Hydrologic and weather (daily max/min temperatures and precipitation) records extending prior to 1932 were analyzed. All four watersheds showed significant increases in discharge, baseflow, and ratio of precipitation to potential evapotranspiration (PPT:PET) ratios (p<0.05). All four also showed significant trends of decreasing PET and/or increasing precipitation. All four showed trends of increased fractional discharge, three being significant. Unsatisfied evaporative demand showed a decreasing trend in all four watersheds. Although only one was significant, extension of small watershed results suggests climate change impacts on watershed hydrology in the Midwest exceeds that of conservation tillage practices at current extent.