Using 137 Cs and 210 Pb ex to trace soil erosion rates for a small catchment in the mid-hills of Nepal

Purpose Soil erosion is a major threat to agricultural sustainability in the hills of Nepal. However, little is known about the soil erosion rates at a small catchment scale in this region. Materials and methods In this study, the soil redistribution rates in a typical small hilly catchment in the Ramechhap District of Nepal were evaluated using the 137 Cs and 210 Pb ex tracer techniques. Nine representative slopes under three land uses and a reference site from the gentle sloping forest land within the catchment were selected as sample plots. A downhill transect was built in each sample plot, and soil samples were collected at 5-m intervals along the transects. Results and discussion The results showed that the spatial variations in radionuclide inventory and soil erosion rates measured by 137 Cs measurements were highly consistent with equivalent estimates from 210 Pb ex measurements, suggesting the potential for using 137 Cs and 210 Pb ex measurements to estimate soil redistribution rates. All 137 Cs and 210 Pb ex inventories on the slopes were less than the reference values, indicating that net soil loss has occurred at all of the sampling points. The net erosion rates from sloping farmland (37.08 t ha −1 year −1 ) and shrub land (32.52 t ha −1 year −1 ) were significantly higher than those from forest land (14.91 t ha −1 year −1 ). Furthermore, the soil erosion rates obtained from 210 Pb ex measurements were higher than those obtained from 137 Cs measurements, which implies that soil erosion in the catchment was accelerated during the last two decades. The net soil loss from the study catchment was estimated to be 31.29 t ha −1 year −1 , which is mainly contributed by shrub land (55.08%) and sloping farmland (37.68%). Conclusions These results indicate that shrub land and sloping farmland are vulnerable to erosion, which should be specifically considered in the sustainable management of small agricultural catchments in the mid-hills of Nepal, especially in dry valley regions.