Variability and Trends in Annual Runoff Efficiency in the Conterminous United States

Abstract:

Variability and trends in annual runoff efficiency (RE)—computed as the ratio of annual runoff (streamflow per unit area) to annual precipitation—in the conterminous United States (CONUS) are examined for the period from 1951 through 2012. Changes in RE are analyzed using measured runoff and precipitation aggregated to United States Geological Survey 8-digit hydrologic cataloging units (HUs). Results indicate that, in the central CONUS, RE has predominantly increased (determined using Kendall’s Tau statistic) likely due to changes in climate. Some HUs in the central CONUS, however, indicate changes in RE that cannot be attributed to climate variability. In addition, many HUs in the western CONUS show decreases in RE that do not appear to be related to climate changes. Some of the RE increases in the central CONUS that cannot be explained by climate occur in agricultural areas with substantial artificial drainage (i.e. tile drains, canals, ditches). The decreases in RE in the central CONUS that are not explained by climate coincide with HUs where water is withdrawn for irrigation and subsequently consumed by evapotranspiration. These results indicate that climate and anthropogenic effects associated with agriculture have affected changes in RE in the CONUS.