Nonpoint source runoff modeling A comparison of a forested watershed and an urban watershed on the South Carolina coast

Storm water runoff volumes, flow rates and sediment loads from a forested watershed and an urbanized watershed draining into adjacent estuaries were compared using the distributed parameter (grid cell) agricultural nonpoint source runoff (AGNPS) model. The comparisons were based on 10 simulated rainfall events. Effects of impervious surfaces on runoff and sediment transport were also investigated with the model. The 38 ha forested watershed, representing undeveloped land in coastal South Carolina, was covered with mixed second-growth hardwoods and pines with interspersed cypress wetlands. The urbanized watershed was 15 ha of single-family residential and commercial land and included a four-lane interstate highway segment. Both watersheds had sandy soils and low stream bed slopes ( < 0.5%). This paper shows that the model equations, although intended for agricultural watersheds, also applied to forested and urban land use. The hydrologic submodel was calibrated with 10 rain events ranging from 19–102 mm total rainfall. Simulation results indicated runoff volume was on average 5.5 × (±2.7) and sediment yield 5.5 × (±2.3) greater from the urban watershed than from the forested watershed. The ratio of rainfall volume to runoff volume was on average 14.5% higher in the urban watershed compared to the forested watershed. In the AGNPS model, runoff volumes were governed by the total impervious area and were independent of other impervious surface spatial characteristics (size, shape, location, contiguity). Simulation results indicated eroded sediment from both watersheds originated predominantly within the channels. Adding simulated impervious surface area increased runoff volumes linearly and peak flow rates exponentially, flow rates and sediment loads were controlled by impervious surface spatial characteristics. Maximum sediment loads from the urban watershed occurred when disconnected patches of impervious surface covered 35% of the watershed. Maximum differences between the forested and urban watersheds occurred at low rainfall depths ( < 75 mm). Future nonpoint source runoff modelling should incorporate ground water dynamics, the spatial and temporal variability of rainfall, and accumulation and wash-off of specific pollutants.