Using 137Cs tracing methods to estimate soil redistribution rates and to construct a sediment budget for a small agricultural catchment in the three gorges reservoir region, China

Soil erosion and associated off-site sedimentation are threatening the sustainable use of the Three Gorges Dam. To initiate management intervention to reduce sediment yields, there is an increasing need for reliable information on soil erosion in the Three Gorges Reservoir Region (TGRR). The purpose of this study is to use Cs-137 tracing methods to construct a sediment budget for a small agricultural catchment in the TGRR. Cores were taken from a pond and from paddy fields, for Cs-137 measurements. The results show that the average sedimentation rate in the pond since 1963 is 1.50 g cm(-2) yr(-1) and the corresponding amount of sediment deposited is 1,553 t. The surface erosion rate for the sloping cultivated lands and the sedimentation rate in the paddy fields were estimated to be 3,770 t km(-2) yr(-1) and 2,600 t km(-2) yr(-1), respectively. Based on the estimated erosion and deposition rates, and the area of each unit, the post 1970 sediment budget for the catchment has been constructed. A sediment delivery ratio of 0.5 has been estimated for the past 42 years. The data indicate that the sloping cultivated lands are the primary sediment source areas, and that the paddy fields are deposition zones. The typical land use pattern (with the upper parts characterized by sloping cultivated land and the lower parts by paddy fields) plays an important role in reducing sediment yield from agricultural catchments in the TGRR. A Cs-137 profile for the sediment deposited in a pond is shown to provide an effective means of estimating the land surface erosion rate in the upstream catchment.