A Saturation Excess Erosion Model

Abstract

Scaling up sediment transport has been problematic because most sediment loss models (e.g., the Universal Soil Loss Equation) are developed using data from small plots where runoff is generated by infiltration excess. However, in most watersheds, runoff is produced by saturation excess processes. Therefore, scaling up requires a hydrology model that accurately predicts the location and extent of runoff source areas. These runoff predictions can then be used for simulating sediment concentrations. We base sediment predictions on a simple, well-tested distributed saturation excess hydrology model, which calculates surface runoff, interflow, and baseflow. Surface runoff originates from bottom lands that become saturated during the rainy season or from severely degraded lands with little or no storage capacity. Baseflow and interflow are generated from the remaining parts of the landscape. Interflow comes from the shallow soils over an impermeable surface and base flow results from percolation below the impervious layer. To obtain the sediment concentrations, we assume that during surface runoff, there is a linear relationship between runoff velocity and sediment concentration, but baseflow and interflow are sediment free. Thus only the runoff component of stream discharge is involved in active erosive work compared to baseflow and interflow that contribute minimally to watershed sediment yield. To show the general applicability of the Saturation Excess Erosion Model (SEEModel), the model was tested for watersheds located 10,000 km apart, in the United States and in Ethiopia. In the Ethiopia highlands, we simulated the 113 ha Anjeni watershed, the 400 ha Enkulal watershed and the 180,000 km2 the Blue Nile basin. In the Catskill Mountains in New York State, the sediment concentrations were simulated in the upper 493 km2 Esopus Creek watershed. Daily discharge and sediment concentration were well simulated over the range of scales with comparable parameter sets. The Nash Sutcliffe values for the daily stream discharge were greater than 0.80 and the daily sediment concentrations had Nash Sutcliffe values of 0.65 using only two calibrated sediment parameters and the subsurface and surface runoff discharges calculated by the hydrology model. The model results suggest that correctly predicting both amount of surface runoff and subsurface flow is an important step in simulating the sediment concentrations.