Modeling Storm Water Runoff and Soil Interflow in a Managed Forest, Upper Coastal Plain of the Southeast US.
The Forest Service-Savannah River is conducting a hectare-scale monitoring and modeling study on forest productivity in a Short Rotation Woody Crop plantation at the Savannah River Site, which is on Upper Coastal Plain of South Carolina. Detailed surveys, i.e., topography, soils, vegetation, and dainage network, of small (2-5 ha) plots have been completed in a 2 square-km watershed draining to Fourmile Creek, a tributary of the Savannah River. We wish to experimentally determine the relative importance of interflow on water yield and water quality at this site. Interflow (shallow subsurface lateral flow) can short-circuit rainfall infiltration, preventing deep seepage and resulting in water and chemical residence times in the watershed much shorter than that if deep seepage were the sole component of infiltration. The soil series at the site (Wagram, Dothan, Fuquay, Ogeechee, and Vaucluse) each have a clay-rich B horizon of decimeter-scale thickness at depths of 1-2 m below surface. As interflow is affected by rainfall intensity and duration and soil properties such as porosity, permeability, and antecedent soil moisture, our calculations made using the Green and Ampt equation show that the intensity and duration of a storm event must be greater than about 3 cm per hour and 2 hours, respectively, in order to initiate interflow for the least permeable soils series (Vaucluse). Tabulated values of soil properties were used in these preliminary calculations. Simulations of the largest rainfall events from 1972-2002 data using the Green and Ampt equation provide an interflow: rainfall ratio of 0 for the permeable Wagram soil series (no interflow) compared to 0.46 for the less permeable Vaucluse soil series. These initial predictions will be compared to storm water hydrographs of interflow collected at the outflow point of each plot and refined using more detailed soil property measurements.