Simulating Watershed Outlet Sediment Concentration using the ANSWERS Model by applying Two Sediment Transport Capacity Equations

Soil erosion is one of the major sources of environmental deterioration and has many impacts on soil and water resources quality. Scientists generally use computational models to simulate the amount of sediment yield from watersheds. One of the leading equations used in these models is the overland flow sediment transport capacity. Owing to the lack of such equations, the streamflow equations have been used widely instead. Some of these equations relatively resulted good estimations of runoff flow sediment concentration. In this study the Areal Non point Source Watershed Environment Response Simulation (ANSWERS) model code was changed and the original equation of the model replaced with a new equation. Although the new equation underestimates sediment concentration, the original model resulted in closer agreement between observed and simulated sediment concentration in different rainfall events. Results of this study suggested that adding some components considering fine particles of soil such as silt and clay to the new and original equations, may improve the accuracy of prediction of sediment concentration by the ANSWERS model. Although, both equations revealed that tend to underestimate sediment concentration; however, the original equation overestimated sediment concentration whenever the runoff coefficient exceeded 0·3 under relatively moderate rainfall intensity. Furthermore, the results showed that soil moisture conditions, rainfall depth and rainfall intensity affect underestimation or overestimation of the model; and initial soil moisture is a key factor in simulation of sediment concentration. Wet and dry soil conditions caused overestimation and underestimation of sediment concentration for the original model, respectively. Nevertheless, no specific erosion model is available now which can simulate sediment yield accurately.