Comparing WEPP and Hydro Physical Models to Estimate Soil Erosion and Sediment Production: A Case Study of Chardavol Watershed

Extended abstract 1- Introduction Soil erosion and sediment production include fundamental limitations in the use of soil and water resources. The sediment yields watersheds in addition to the loss of soil fertility, causing water quality reduction. Therefore, evaluation processes that govern their behavior to better understand and assess the systems of watershed management practices are important. The role of soil erosion and sediment productuion in soil fertility decline and waste as well as in filling reservoirs dams is a global problem. This is due to the destruction of the environment and human factor on landuse cover. WEPP erosion model is a physical model that estimates the amount of erosion and loss of soil using the approach of local development assessment. WEPP model can assess the amount of runoff and erosion process in a watershed.  Nearly 2 billion tons of resource soil is destroyed, and much damage equivalent to 18.5 × 1012 Rails enters the country of Iran. Therefore, to prevent this destruction of natural resources, suitable management for preventing erosion and sediment movement is needed. While erosion is a natural process, human activity is the cause of erosion. Also, erosion caused a devastating impact on the landscape and natural resource. These effects included decrease in the agricultural productivity resulting in ecological collapse. Water and wind erosion are the two primary causes of land degradation; both of them together are responsible for about 84% of the global extent of degraded land.   2- Methodology In order to achieve the purpose of the research, a case was studied and identified based on the WEPP and Hydro physical Models. In this study, to estimate soil erosion and sediment production, the WEPP and Hydro physical models were used. In the research, to prepare database for estimating soil erosion and sediment production, suitable map, topography map, slop, and land use were produced. In the second step, in the environment of GIS software, all the layers were corrected and analyzed.  Many data were collected through field work. In this study, WEPP and Hydro physical models were utilized for estimating soil erosion and sediment production in the watershed of Chardavol in the northern Ilam province. In the WEPP model, for data collection, geomorphology methods were used to determine the work units. In WEPP Model, the type of data collection run in six classes was classification, which contained the files of soil, climate, management, slope, drainage, and tank. In the soil class, information such as texture, action exchange capacity, organic matter, some pebbles, and effective hydraulic flow was used. In file management, information such as the type of landuse cover, vegetation, soil and plant phonological characteristics were dominant. Klyghen sowftware was used to build the climate file. Information required in this file included daily rainfall and temperature. For the stream slope and the scope of slope, the software of geo-WEPP was used.   3- Results According to the results of weighting maps and the data about the potential of sedimentary basins, the basin A8, with a potential production of 13.76 tons of sediment per square hectar, was the highest rate of sedimentary and erosion. Also, the lowest sedimentary was located in the sub-basin A3 with a potential production of 2.64 tons of sediment per hectar. The highest erosion rates in the three sub-basins in the southeastern part of the basin Chardavol were located in this part of the basin due to slopes and high rainfall, low vegetation and poor pastures. The rate of erosion was very high due to natural and human destructive factors. These areas have the highest amount of sediment in the study area. In this study, the lowest rate of erosion in western and northern study areas is located in the areas covered with dense forest and semi-dense and high vegetation with protective programs.   4- Discussion and Conclusions The results of the study based on the WEPP model showed that the amounts of sediment in the three methods of domain, watershed and flow direction were 7.64, 6.01, and 11.87 ton/ha/year, respectively. According to the estimated results, the domain and flow direction methods with 7.64 and 11.87 ton/ha/year were in good agreement with the actual amount (10.5), and were suitable for the erosion and sediment estimation in Chardavol watershed. Also, the results obtained in the Hydro physical method indicated that the amount of sediment was 8.066 ton/ha/year (Table 1).   Table 1: Comparing WEPP and hydro physical models for estimating soil erosion and sediment production Type Parameter model WEPP Hydro physical Model   Hydrometric Station Watershed Flow direction Domain Sediment (t/ha/yr) 6/01 11/87 7/64 8/066 10/5