Evaluating soil erosion and sediment deposition rates by the ¹³⁷Cs fingerprinting technique at karst gabin basin in Yunnan Province, southwest China

Abstract. Soil erosion is a global environmental problem that can lead to the loss of nutrients in topsoil layers, particularly in fragile karst environments where the low contents of organic carbon and steep slopes used to be key pedological and geomorphological factors. Researching the erosion and deposition rates in small watersheds is important for designing efficient soil and water conservation measures. In this research, the Dapotou closed catchment, a representative depression in karst gabin basin, located in the Yunnan province, Southwest China, was selected to assess the variation of soil erosion and soil sediment mobilisation at different hillslope positions using the ¹³⁷Cs tracing technique. The results showed that the soil erosion rates in the shoulders, backslopes and footslopes were 0.87, 0.35 and 0.49 cm a⁻¹, respectively, meanwhile the soil sediment rate in depression bottom was 2.65 cm a⁻¹. The average annual soil erosion modulus of the complete hillslope was 632 t km⁻² a⁻¹, which confirmed in the serious gradation according to karst soil erosion standards. The soil deposition modulus reached up to 3180 t km⁻² a⁻¹. The sediment delivery ratio summarized 0.82 in the whole catchment according to the square of hillslope and depression bottom. To identify which factor could play the most important role, a Principal Component Analysis was conducted. The results showed ¹³⁷Cs concentration of different soil depth at different hillslope positions were significant correlated with soil organic matter (SOM) and total nitrogen (TN) (P < 0.05). As the typical karst geomorphological types, these findings are expected to provide data support for the whole watershed soil erosion management and ecological restoration in this fragile karst ecosystem.