In this study, GeoWEPP (WEPP v2006.5) was used to estimate the sediment yield and runoff in Orcan Creek watershed of Kahramanmaras. 1|]# The sediment yield and runoff results from the GeoWEPP model were compared with the observed monthly data collected from the sample watershed to evaluate the performance of the model.2.?Material and Method2.1. GeoWEPPGeoWEPP was developed as a collaborative project conducted by the Agriculture Research Service, Purdue University, and the USDA National Soil Erosion Research Laboratory [17]. To predict sediment yield and runoff at watershed scale, GeoWEPP integrates WEPP model and TOPAZ (TOpography PArameteriZation) software within the ArcView 3.2 (ArcView 2000) [17] (Figure 1).
In GeoWEPP, necessary input files (land cover, land use, slope, climate, soil, and management) are generated within WEPP and topographic data are parameterized by using TOPAZ based on DEMs [18]. Finally, watershed outputs are produced by using GIS functions in ArcView [7].Figure 1.Logic flowchart of the GeoWEPP.2.1.1. TOPAZIn GeoWEPP, hillslope profiles are generated by utilizing TOPAZ, which parameterizes topographic data based on DEMs. TOPAZ determines the channel network based on the steepest down slope path, considering 8 adjacent cells of each raster cell (pixel) [19]. The channel network can be adjusted by changing values of Mean Source Channel Length (MSCL) and Critical Source Area (CSA). The MSCL defines the shortest channel length and the CSA is the minimum drainage area [7, 19].
After defining the channel network, TOPAZ generates the sub-watersheds which represent the watershed (Figure 2).
Figure 2.The flow accumulation for sub-watersheds by using TOPAZ.2.1.2. ArcViewThe GeoWEPP model has a feature of being run in ArcView. The watershed outputs are generated as grid layers representing soil loss as a percentage of the tolerable soil loss (TSL). In the grid layers, areas that generate soil loss values greater than or less than the GSK-3 TSL are highlighted. The runoff and sediment yield data AV-951 for each pixel can be produced in text files or in grid outputs. Text files indicate average annual rainfall and number of storms, total runoff, soil loss, and sediment yield for each sub-watersheds and for the entire watershed [7].
2.1.3. WEPP Input FilesWEPP model requires four input files including slope, climate, soil, and management files to describe hillslope geometry, meteorological characteristics, soil properties, and ground cover, respectively.Slope FileThe slope file is generated based on necessary hillslope parameters such as slope gradient, shape, width, and orientation along its length. GeoWEPP utilizes TOPAZ to produce sub-watershed profiles based on DEM data (Figure 3).