Quantifying Long-Term Hydrologic Response in an Urbanizing Basin

Long-term hydrologic response is described within a rapidly developing watershed west of Washington, D.C. Data consist of up to 24 years of observed rainfall, basin discharge, and land use/land cover from four headwater basins of the Occoquan River. Three of the four study basins, ranging in size from 67 to 400 km2 , are predominantly forest and mixed agriculture. The fourth basin, the 127 km2 Cub Run watershed, which is the focus of this study, has urbanized rapidly over the past 20 years (current impervious surface approximately 18%). Results indicate that Cub Run basin has higher annual and seasonal storm discharge per surface area than nonurban basins after 1983, when impervious surface in Cub Run basin reached approximately 9%. Only during the summer and fall is long-term storm runoff in Cub Run basin higher than nonurban basins. Long-term results support expected biophysical reductions in interception, infiltration, and evapotranspiration due to higher imperviousness, indicating that these reductions persist throughout the growing season, unlike adjacent nonurban areas.