Performance Evaluation of Watershed Management Measures in Reducing Soil Erosion in Treated and Control Sub-Watersheds of Khamsan Representative Watershed using Cs-137 Method

Introduction and Objective Soil is a part of every ecosystem and one of the most valuable and important sources of food production in every country, which plays a significant role in the continuation of human life. Today, soil erosion is one of the most important threats to water and soil resources and has become one of the most important human environmental problems. Soil supports food security and environmental quality, both of which are essential for human life. The purpose of this research was to evaluate the performance of watershed management measures in the treated sub-watershed of Khamsan representative watershed. The lake of Gavshan Reservoir has receded at the maximum level to the outlet of the Khamsan watershed. This has caused soil erosion from this watershed to enter the Gavshan dam lake directly and with the shortest distance. Therefore, it is very important to evaluate the performance of the measures taken in this watershed, especially based on the distribution map of erosion and sedimentation. Materials and Methods The first step of measuring soil erosion using cesium-137 is to select a control or reference area. Slope and land use maps were used to determine the final sampling points in this research. After measuring the activity of cesium-137 (becquerels per kilogram of soil), the presence of cesium-137 per square meter of soil sample is needed to calculate soil erosion. The activity of cesium-137 in reference points was compared with the activity of cesium-137 remaining in the research points. If the cesium-137 activity of the soil samples was lower than the activity in the reference profile, soil loss and erosion occurred. The increase in the activity of cesium-137 in the soil samples compared to the reference profile was indicative of soil transfer to the sampling point and considered as a sedimentation site. Results  and Discussion The results of the comparison of the same points in the rangelands in both treated and control sub-watersheds showed that sedimentation conditions prevail over erosion. The amount of sedimentation in the rangelands with watershed management measures (contour trenching, seeding and exclosure) in the treated sub-watershed with a slope of 20-30% was more than the rangeland with the same characteristics in the control sub-watershed. In rangeland with watershed management measures, erosion conditions prevailed in the slope of more than 30-60%, but erosion in protected conditions was less compared to the same point in the control sub-watershed.Conclusion and Suggestions The results of the amount of erosion and sedimentation in both treated and control sub-watersheds in rangeland with a slope of 20-30% showed that sedimentation conditions prevail. However, the amount of sediment deposition in the rangeland with watershed management measures in the treated sub-watershed showed an increase of 1445.65% compared to the rangeland without protective measures in the control sub-watershed, which indicates the positive effect of watershed management measures in the treated sub-watershed rangelands with a slope of 20-30%. This finding shows the special effect of contour trenching on increasing sediment trapping, reducing the volume and speed of runoff, and thus reducing the power of sediment transfer. Also, in the slope of 30-60%, although erosion conditions prevailed in both treated and control sub-watersheds, watershed management measures in rangeland in the treated sub-watershed showed a 28.57% reduction in soil erosion compared to the same point in the control sub-watershed. Considering the positive performance of watershed management measures, especially contour trenching, it is suggested to use these measures in other watersheds similar to Khamsan representative watershed.