Evaluation of annualized agricultural nonpoint source model for a watershed in the Siwalik Hills of Nepal

A study was undertaken to determine the predictive capability of Annualized Agricultural NonPoint Source (AnnAGNPS) model with respect to runoff volume, peak flows, and sediment yield from a 130.8 ha watershed in the Siwalik Hills of Nepal. AnnAGNPS simulations are compared with two years of field observations from the study watershed. The model predicted the runoff volume within the range of acceptable accuracy. The event-based peak flows were overpredicted by 105% with R2 of 0.85 during calibration and 162% with R2 of 0.77 during validation. Similarly, the model overpredicted the event-based sediment yield by 59% with R2 of 0.63 during calibration and 92% with R2 of 0.59 during validation. These results show that the model performs well in simulating runoff volumes compared to the peak flows and sediment yields. It is observed that there is a need to modify or improve the estimation methods of the peak flow and sediment yield to increase the performance of the model which can aid watershed management in local conditions.