GROUNDWATER DISCHARGE TO STREAMS IN MOUNTAINOUS REGIONS OF THE EASTERN US

Groundwater discharge to streams in the eastern US is often represented by a single static value, which assumes a hydrologically inactive, infinite reservoir of water storage in the subsurface. Using hydrograph separation data from 849 stream gages in the Appalachian Plateaus, we show the importance of temporal and spatial differences in streamflow, its components base flow and surface runoff, and base-flow index (ratio of base flow to streamflow) for the period 1900 to 2011. Using the observed differences, the intent of our study is to describe the spatial and temporal changes in groundwater discharge to streams, and ultimately provide bounds for continuous prediction of the portion of streamflow comprised by base flow across a range of hydrologic conditions.

For the Appalachian Plateaus region, annual data anomalies confirm the close relation of annual precipitation to base-flow and runoff components of streamflow, both of which increased during the period of analysis. Increases in base flow account for most of the observed increases in mean annual streamflow from 1900-2011. The percentage of precipitation that resulted in base-flow discharge to streams varied annually by up to a factor of two depending on prevailing climate conditions. 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.