Development of a SWAT ArcView GIS Patch for Accurate Analysis of Soil Erosion and Sediment Yield at Steep Sloping Watershed

Abstract: The watershed scale SWAT model divides the watershed into smaller subwatersheds for rainfall-runoff and pollutant generation at the field level and routing though stream networks. The SWAT model first needs to be calibrated and validated for accurate estimation through adjustment of various input parameters. However, in some instances the SWAT simulated results are greatly affected by the watershed delineation and DEM cell size. In this study, the SWAT ArcView GIS Patch II was developed for steep sloping watershed and its performance was evaluated for various threshold value and DEM cell size scenarios when delineating subwatersheds using SWAT. The SWAT ArcView GIS Patch II was developed using the ArcView GIS Avenue program and Spatial Analyst library. The SWAT ArcView GIS Patch II is better than the SWAT ArcView GIS Patch I by Lim et al. (2007) because it reflects the topographic factor in calculating the field slope length of the HRU in the SWAT model. The simulated sediment value for 321 watershed (threshold value of 200 ha) is greater than that for 43 subwatershed (threshold value of 25ha) by 201% without applying the SWAT ArcView GIS Patch II. However, when the SWAT ArcView GIS Patch II was applied, the difference decreases (12% difference) for the same scenario. The simulated sediment value for DEM cell size of 50m is greater than that for DEM cell size of 10m by 19.80% without the SWAT ArcView GIS Patch II. However, the difference becomes smaller (3.41% difference) between 50m and 10m DEM scenarios. As shown in this study, the SWAT ArcView GIS Patch II can reduce differences in simulated sediment values for various watershed delineation and DEM cell size scenarios. Without the SWAT ArcView GIS Patch II, variations in the SWAT simulated results using various watershed delineation and DEM cell size scenarios could be greater than those from input parameter calibration. The results obtained in this study show that the SWAT ArcView GIS Patch II needs to be used when simulating hydrology and water quality for steep sloping watersheds (especially if average slope of the subwatershed is greater than 25%) for accurate simulation of hydrology and water quality using the SWAT model. The SWAT ArcView GIS Patch II is available at http://www.EnvSys.co.kr/~swat for free download.