Assessment of the WEPP capability in coupling with GeoWEPP for predicting spatial distribution of soil erosion in Taleghan's pair catchments

protection of water and soil is the fundamental work for development natural resources in any country. Lake of available data of soil erosion is a limitation for land managers to apply soil and water conservation in the watershed scale. Over the past decades several erosion models have been invented under different environmental conditions, but, very limited numbers can be used under watershed scale that are enable to considering soil conservation measures. Among the existing methods, WEPP model, has been known as a process oriented model that can be used in different spatial scale namely: hillslope, watershed and large basin. The WEPP model requires a large amount of input data and the model efficiency is highly related into the input data. The main aim of the present investigation was the assessment of WEPP model and spatial extension of the GeoWEPP for estimating of soil erosion and sediment yield in the paired watershed.

The study area is located into the Taleghan watershed and the pair catchments of Zaidasht with area of 198.4 ha. All environmental data including: soil attributes, vegetation, climate data were extracted by field and laboratory data analysis for the five erosion plots and whole of the watershed. Model calibration was conducted based on the both infiltration and erodibility parameter through Nash- Sttcliff and R2 as well as RMSE coefficients.

After optimizing the base value of hydraulic conductivity to about 50% for runoff, the efficiency of the model using the Nash-Sutcliffe coefficient was 0.64, which means that the estimated runoff of each plot is close to the observed values. For the amount of sediment, using the basic values of the model relationships, the model did not provide the right result and the best results were obtained with an increase of 100% for rill erodibility and 150% for interrill erosion, and a decrease of 9% for shear stress and 12% for conductivity. has obtained hydraulics and the efficiency of the model using the Nash-Sutcliffe coefficient was 0.51%, the estimated values are close to the observed values of the plots. Also The results showed that in 2016 , the sediment on the submontane level was 0.2 and 0.1 ton/ha/yr for the control and the sample, respectively and In the watershed, 1.1 and 0.3 ton/ha/yr were obtained for control and sample, respectively.Therefore, the WEEP model in the study area will be effective when the parameters of erodibility and hydraulic conductivity are measured with high accuracy. On the other hand, the results of GeoWEPP were obtained in the control and sample basins, respectively, 3.3 and 2.5 ton/ha/yr.

Based on the field observations and forms of erosion, GeoWEPP spatial extension was able to present a realistic picture of the erosion situation at the basin level Considering that this extension can remove the limitation of the model implementation in terms of area, therefore it is recommended to estimate the sediment in the scale of the watershed and in a distributed manner.