Abstract
Spatially distributed models of soil erosion and sediment delivery have been increasingly used in studies of catchment sediment dynamics in recent years. Distributed models permit both the spatial heterogeneity of catchment land use, soil properties and topography, and the spatial interaction of soil erosion and sediment transport processes to be represented and can, therefore, provide spatially distributed predictions of soil redistribution rates for complex threeโdimensional terrain. However, validation of such distributed models has been severely hampered by the lack of information on the spatial distribution of sediment mobilization and deposition within catchments, and has commonly been restricted to comparison of predicted and measured sediment outputs from catchments. The use of the fallout radionuclide ^137^Cs for documenting soil redistribution affords a means of assembling distributed information on mediumโterm rates and patterns of soil redistribution within a catchment which can be used for model testing. The present study uses the ^137^Cs technique to investigate erosion and sediment delivery processes within a cultivated field in Devon, UK, and the results have been used to test four spatially distributed soil erosion and sediment delivery models, namely the widely used AGNPS and ANSWERS models, a topographyโbased sediment delivery model, and a topographyโdriven soil erosion model. Copyright ยฉ 2003 John Wiley & Sons, Ltd.