Spatial heterogeneity of low flow hydrological alterations in response to climate and land use within the Upper Mississippi River basin

Abstract

The Upper Mississippi River Basin (UMRB), located in the Midwestern U.S., is a nationally recognized region of concern for water quality and aquatic biological health. At 500,000 km2, geospatial characteristics and climate vary widely within the basin and thus, hydrological behavior and associated effects on freshwater ecosystems may also dramatically vary. So far, previous studies of long-term hydrological behavior in the UMRB have mostly considered changes for the basin rather than changes at finer watershed scales. This study systematically evaluated long term trends in indices of hydrological alteration (IHA) for 118 of 131 watersheds of the UMRB and compared them with 1) variabilities in geospatial features (land use, soil type, and topography), 2) land use changes, 3) climate, and 4) evapotranspiration trends using hierarchical clustering and correlation analysis. Compared to climate trends, land use and evapotranspiration were found to have stronger impacts on trends in low flow IHA indices in the watershed. The results demonstrate that while overall, low flow has been increasing in the basin, there is considerable variability in alterations in low flow across watersheds. Specifically, the results indicate that in forested or mixed-use watersheds, low flow has been decreasing while in agricultural and urban watersheds low flow is increasing. These results together with the literature on water quality in the UMRB, suggest both stressors, water quantity and water quality exist in the UMRB and vary by land use and evapotranspiration, implying that effective watershed management plans are local not regional and should be directed toward reducing the specific local stressors.