Groundwater Discharge to Streams in the Eastern United States

The importance of groundwater discharge to the understanding of quantity and quality of total streamflow along with impact to ecological habitats has long been recognized, yet seldom adequately quantified over large spatial and temporal scales. Hydrograph separation data from 849 stream gages in the Appalachian Plateaus were used to calibrate a Soil-Water-Balance model for the region and to assess spatial and temporal trends in groundwater discharge. Base-flow and runoff components of streamflow show close relations with annual precipitation and increased during the period of analysis from 1900 to 2011, especially since 1970. Increases in base flow account for most of the observed increases in mean annual streamflow, and the percentage of precipitation that resulted in base-flow discharge to streams varied annually by up to a factor of two. The evaluation has been extended into other physiographic provinces in the Mid-Atlantic states by inclusion of an additional 433 stream gages. Base flow is the dominant component of streamflow at approximately 65 percent of the sites in the eastern United States. The temporal distribution of base-flow index (ratio of base flow to streamflow) values from the complete data set (1,282 gages) shows the importance of base flow in sustaining streamflow in both wet and drought conditions, especially in watersheds underlain by carbonate rocks. The base-flow index, however, is independent of annual climate trends and indicates that changes in the components of streamflow are probably in response to shifts in seasonal precipitation or widespread changes in land-use practices.