Using Runoff Transport Capacity to Predict Watershed Sediment Yield.

A modified version of the Soil and Water Assessment Tool (SWAT) model with two changes to the landscape erosion sub-routine is presented. The first change is to erosion simulated by hydrologic response units (HRUs), and is made by developing a new event erosivity term that uses both daily surface runoff and daily rainfall information. Daily rainfall information is used to derive daily erosivity using a simple power function that varies by season, and daily runoff is used for deriving the fraction of eroded sediment contributing to suspended sediment yield. A second change from the default model is accounting for the amount of sediment deposited within a HRU using a simple, spatially varying, runoff transport capacity (RTC) function that changes with land cover. The modified SWAT was tested in the 37 km² Town Brook agricultural watershed in upstate New York State. A minimal sediment calibration approach was used on a model satisfactorily calibrated for monthly streamflow (R² = 0.67 and Nash Sutcliffe Efficiency (NSE) = 0.67). Sediment calibration for the period from 2000-2003 made by adjusting a single parameter, the runoff transport capacity constant (K_RTC) gave satisfactory predictions of monthly sediment export (t) from the watershed (R²= 0.63 and NSE = 0.52). The model also predicted soil erosion and suspended sediment yield from each HRU while accounting for land cover effects. Model output from a 30 year simulation showed that on average only 20% of the eroded sediment at the HRU left the watershed as suspended sediment. Simulated sediment yields are therefore affected by landscape sediment deposition and are based on an improved understanding of the sediment delivery problem. The modified model has sufficient flexibility for wider application and easy adaptation in studies related to soil erosion and sediment yield, water quality, and non-point source pollution management.