Evaluating the Rate of Soil Erosion and Sedimentation of the Khasoyeh Watershed Using the USPED Model and GIS and its Comparison to Water Erosion Types

Soil erosion is one of the main factors of land degradation and desertification in large areas of the earth's surface, which is the result of the interaction of human and natural factors causing the destruction of natural ecosystems such as rangeland, forests, and agricultural ecosystems. Factors such as topographical settings, land use and land cover change, intensity of rainfall, soil properties, and wind can also be accelerated by human activities of intensive agriculture, deforestation, and tillage on steep slopes. Many models have been suggested for estimating water erosion such as USLE, RUSLE, WEEP, and EPM.  According to previous studies, the global model of soil erosion has been used in the zoning of areas with high erosion risk, but very few studies have used the USPED model, which unlike the RUSLE model that only identifies the erosion areas, this model identifies also the amount of sediment. In a study conducted by Zakerinejad and Maerker, 2015, they assessed and zoned water erosion in the Mezayjan watershed in Fars province by combining the USPED model and the Strean Power Index (SPI). The results indicated the high capability of the proposed model. It is also used to identify high erosion risk areas. The Khasuye watershed is one of the sub-watersheds located in the south of the country and in the territory of Fars province, which is affected by the severe types of water erosion leading to the loss of surface fertile soil, filling of reservoirs of dams and the reduction of water quality. Therefore, planners can consider the preparation of the potential map of the rate of erosion and sedimentation in the identification of susceptible areas in order to prioritize the implementation of soil protection and watershed management operations.

The parameters of the USPED model were estimated using different data sources by ArcGIS 10.8. The rainfall erosivity factor (R value) was estimated from the annual rainfall data of six meteorological stations located throughout the study area. The soil erodibility factor (K value) was derived from the soil samples collected from our study area. The topographic factor was calculated by analyzing a digital elevation model (DEM) with a spatial resolution of 12 m from TanDEM-X. The crop factor (C) weas derived from Sentinel-2 satellite images and conservation practice factor (P) was considered 1 because of no special soil practice in the study area. Since the spatial resolution of the applied factor maps is different, it requires resampling of the factor maps. So, in this study, the nearest neighbor re-sample sub-tool of the data management toolbox in the ArcGIS platform was used.

The study used the GIS–USPED interface model for analyzing the spatial distribution of water erosion in Khasoue watershed in Fars province. Also, according to the classification result tables of the erosion and sedimentation model of the Khosoye watershed, about 39% of the studied area is in the low and insignificant erosion class, which is mostly in the central and low slope areas of the basin, and about 24% corresponds to the low and very low sedimentation class.  It is possible that these areas are better next to areas with low erosion and low areas with a higher slope of the basin (Table 2). It should be noted that this interval is due to the change of vegetation and physiographic factors and human activities. The lowest rate of erosion is in the center of the basin, and the highest rate of erosion is in the western and northern steep areas of the studied basin. The final results obtained from the application of the USPED erosion-sedimentation model in the studied basin showed that the amount of erosion and sedimentation varies from 0.1 to more than 30 tons per hectare per year, that signify erosion areas with a negative number and sedimentation areas with a positive number (without sign) area.

Among the several fundamental approaches for calculating soil erosion rate, the USPED model is a critical tool for conducting improved conservation planning. The implementation of this erosion model can help to identify the places that are affected by erosion, especially the inaccessible areas, and by identifying these places, the necessary management can be applied to control and reduce soil erosion in these areas.