GROUNDWATER CONTRIBUTIONS TO STORM WATER RUNOFF IN LARGE, URBAN WATERSHEDS

Storm runoff in large, moderately urbanized watersheds takes both surface and subsurface flow paths to stream channels. The purpose of this study was to determine groundwater contributions to stormflow in the NE and NW branches of the Anacostia Watershed. Hydrograph separation techniques were used to determine the relative contributions of two dominant sources, overland flow and groundwater inputs to stormwater discharge. Electrical conductivity was used as a tracer to separate multiple storm hydrographs at six gauged sites located within the watershed. The timing and quantity of discharge from each source was used to track flow processes in the watershed. Results indicate that upstream tributaries generate proportionally more runoff per impervious surface area than the watershed does as a whole. Significant runoff is also generated from the small tributaries in the heavily-urbanized downstream portions of the channel system. Additionally, the hydrograph separations suggest that groundwater discharge is a major mechanism of stormwater runoff. Groundwater contributes more to storm discharge in the Piedmont NW branch watershed, possibly due to the re-infiltration of overland flow runoff in the wide forested valleys that border the stream.