Erosional history by combining 210Pbex and 137Cs methods with sediment fingerprinting and measurements

The degradation of soil through erosion poses a major environmental concern on a global scale, adversely affecting the chemical, biological, and physical properties of both soil and water. Precisely identifying the main sediment sources within a watershed is vital for developing targeted management strategies aimed at reducing erosion and improving water quality. Sediment sourcing is a commonly employed approach to achieve this goal. Some sediment sourcing methods not only determine the contributions of different sources but also provide estimates of average erosion/sedimentation intensity and sediment budget components. Therefore, the present study was conducted with the aim to compare the average soil erosion intensity and sediment yield in the past 100-120, 60-70, and 15 years within the control sub-watershed of the Khamsan representative-paired watershed. Furthermore, the contribution of various land uses to sediment yield, such as orchards, irrigated agriculture, rangeland, and rainfed agriculture were compared using geochemical fingerprinting characteristics obtained in 2021 with the corresponding results derived from the ¹³⁷Cs method for the last 60-70-year period. In order to achieve the research objectives, a total of 106 soil samples were systematically collected with appropriate distribution in different land uses for sediment source fingerprinting using radionuclides and other indicators. The findings indicated that during the two recent periods, spanning 100-120 years and 60-70 years, the average gross erosion was estimated at 2.92 and 5.25 t ha^-1 y^-1, respectively, and the average net erosion was estimated at 2.85 and 5.05 t ha^-1 y^-1, respectively. Furthermore, the contributions of rainfed agriculture and rangeland to sediment yield in 2021 were 72.26% and 7.96%, respectively, and using the ¹³⁷Cs method for the 60-70-year period, they were 85.49% and 8.93%, respectively. The study's findings revealed also a decline in sediment yield over the past 60-70 years, compared to the measurements obtained during the last 15 years. This decrease can be primarily attributed to factors that include the abandonment of rainfed agricultural lands, a decrease in rainfall, and a reduction in sediment transport caused by tillage in waterways draining to the sub-watershed outlet.