3. Quantifying Long-term Hydrologic Response in an Urbanizing Basin*

This paper describes long-term hydrologic response within a rapidly developing watershed in the western suburbs of Washington, DC, within the Chesapeake Bay drainage basin. Data consist of up to 24 years of observed rainfall, basin discharge, and land use/land cover (LULC) from four headwater basins of the Occoquan River in northern Virginia. Basin outlets are monitored for storm and nonstorm flows, respectively, using flow-proportional, volume-integrating storm samplers and continuous stream gaging. Landsat-derived measures of impervious surface are supplemented with regional land use mapping to characterize LULC in each basin. Three of the four study basins, ranging in size from 67 to 400 km, are predominantly forest and/or mixed agriculture. The fourth basin, the 127 km Cub Run watershed, has urbanized rapidly over the past 20 years with approximately 50 percent of current land area classed as urban (18 percent impervious surface). Results indicate a greater hydrologic response in the urbanizing Cub Run basin, compared to the three adjacent basins. Cub Run basin has higher annual and seasonal storm volumes per surface area than non-urban basins after 1983, when estimated impervious surface area in Cub Run basin reached approximately nine percent. In all four study basins, storm discharge per surface area is more responsive to rainfall during winter and spring due to generally denuded dormant season landscapes. Only during the summer and fall is long-term storm runoff in Cub Run basin higher than non-urban basins. This result is significant, as it supports expected biophysical reductions in interception, infiltration, and evapotranspiration during the growing season due to higher imperviousness. Results of this study also support literature regarding increased storm volumes in catchments above 10 percent imperviousness. Increased unit-area storm discharge in Cub Run basin during the growing season has important implications for seasonal NPS pollutant flux which are addressed in the following chapter.