Partitioning the Contributions of Sheet and Rill Erosion Using Beryllium‐7 and Cesium‐137

Abstract

An understanding of the relative contributions of sheet and rill erosion to the total soil loss associated with individual events and of changes in their relative importance during an event is important for developing more effective soil erosion prediction models and for predicting the physical and geochemical properties of the eroded sediment in relation to those of the in situ soil. It is difficult to obtain information on variations in the relative importance of sheet and rill erosion during an event using traditional monitoring techniques. The use of environmental radionuclides, particularly beryllium‐7 (Be‐7) and cesium‐137 (Cs‐137), as sediment tracers offers an alternative approach to obtaining this information. Because of their contrasting depth distributions, the two radionuclides provide a means of distinguishing sediment eroded from the soil surface by sheet or interrill erosion and that eroded from greater depths by rill erosion. The approach was tested using simulated rainfall experiments undertaken on two small erosion plots located in Yichuan County, Shaanxi Province, China, representative of cultivated land and uncultivated land previously under forest. The results confirmed that the development of rills within the plots was clearly marked by changes in the Be‐7 activity of the sediment eroded from the cultivated plot and changes in the Be‐7 and Cs‐137 activity of the sediment eroded from the uncultivated plot. By relating the changes in the Be‐7 and Cs‐137 activity of the eroded soil to information on the depth distribution of the two radionuclides in the soils of the two plots and the mass of soil eroded, it was possible to estimate the relative contributions of the two erosion processes to the total soil loss and their changes through an event. This information provided a basis for investigating the influence of temporal variations in the relative contributions of sheet and rill erosion during an event on variations in the physical and geochemical properties of the sediment eroded from the plot during the event.