Evaluation of Agricultural Nonpoint Source (AGNPS) model for small watersheds in Korea applying irregular cell delineation

The Agricultural Nonpoint Source (AGNPS) model was tested in two small agricultural watersheds in Korea. The model was calibrated for 412.5 ha located in the Balhan watershed. The rainfall amount distinguishing between antecedent moisture conditions (AMC) I and II was changed to calibrate the runoff volume. A validation was performed for 274.1 ha located in the Banwol watershed, with similar land use and soil characteristics as the 412.5 ha in the Balhan watershed. The input data were extracted from multiple GIS layers using the Geographic Resources Analysis Support System (GRASS)–AGNPS interface. The AGNPS model was modified to treat the undisturbed forest areas as irregular cells instead of the uniform cell division currently used by AGNPS. Simulated results from the irregular cell-based scheme (ICS) and the uniform grid scheme (UGS) of ANGPS were compared with the observed data. The ICS increased runoff volume and decreased peak flow rate and sediment yields from the watershed compared to the UGS. The ICS significantly reduced the number of cells in a watershed and provided better agreement for surface runoff and peak flow rate compared to the UGS.