Temporal variability in the hydrologic regimes of the United States

Discharge records where flows have not been subject to overt anthropogenic controls have been identified for over 1500 streamflow gauging stations throughout the United States in the US Geological Survey Hydro-Climatic Data Network. These stations fall within all 21 water resources regions of the United States. Analysis of runoff in 20 regions, where long-term daily records are available, shows an increasing trend in 16 regions. Further analysis using a stratified subset of 65 sites shows an increase in baseflow at approximately 90% of the sites during the past 50 years, regardless of the size of the drainage area. Because anthropogenic alterations of watershed characteristics cannot explain these hydrologic changes, then meteorological or climatic forces are implicated.