Relating 137Cs and sediment yield from uncultivated catchments: the role of particle size composition of soil and sediment in calculating soil erosion rates at the catchment scale

Soil loss is considered one of the most important consequences of land degradation as it affects the production of agricultural and forested areas, and the natural equilibrium of aquatic ecosystems downstream. For these reasons, the availability of tools and techniques able to identify areas at risk of land degradation is essential. Over the last 3-4 decades, theoretical models, based on the use of 137Cs, an anthropogenic radiotracer, proved to be very effective for this purpose. However, these models require specific information on soil and sediment particle size to provide estimates of soil erosion or deposition and this information is summarised by a particle size correction factor 'P'. Empirical methods of calculation of this factor assume the basic hypothesis that a particle size selectivity takes place in erosion processes and this results in a general enrichment of the fine component in sediments and a corresponding higher radionuclide activity. In this contribution, we demonstrate that this hypothesis is not valid everywhere, and consequently, the P factor cannot be estimated using traditional approaches.