No abstract

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 2 , are predominantly forest and/or mixed agriculture. The fourth basin, the 127 km 2 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.

Persistent cloud-cover in the humid southeastern USA and the low temporal resolution of Landsat sensors limit the derivation of seasonal evapotranspiration (ET) maps at moderate spatial resolution. This article introduces a Landsat Moderate Resolution Imaging Spectroradiometer (Landsat-MODIS) ET fusion model that uses simple linear regression to integrate Landsat-derived reference ET fraction (ET r F) from mapping ET at high resolution with internalized calibration (METRIC) model and the vegetation temperature condition index (VTCI) derived from MODIS images. For a study site in Florida, model-estimated ET and ET estimated using energy budget eddy covariance at a US Geological Survey (USGS) station in Ferris Farm, Florida, were found to be in a good agreement with a root mean squared error of 0.44 mm day -1 , coefficient of determination (R 2 ) of 0.80, Nash-Sutcliffe efficiency of 0.79 for daily ET (ET d ), and 2% relative error for cumulative seasonal ET during the growing season of 2001. At another study site in Alabama, the model underestimated 2008 annual water balance ET for the Fish River Watershed by 39 mm or 4%. Comparisons of model-estimated ET with that obtained using a non-fusion Landsat-only approach at both sites indicated that the fusion of Landsat and MODIS ET values reduces potential errors in ET estimation that would otherwise arise due to insufficient availability of cloud-free Landsat images for METRIC processing. Validation results and application of the model in deriving seasonal/annual ET for different land-cover classes in the Fish River Watershed suggested that the fusion model has the potential to be used in continuously monitoring ET for field-to watershed-level agricultural and hydrological applications in the southeastern USA.