A quantitative model for estimating mean annual soil loss in cultivated land using 137Cs measurements

The radioisotope 137Cs has been widely used to determine rates of cultivated soil loss, Many calibration relationships (including both empirical relationships and theoretical models) have been employed to estimate erosion rates from the amount of 137Cs lost from the cultivated soil profile. However, there are important limitations which restrict the reliability of these models, which consider only the uniform distribution of 137Cs in the plough layer and the depth. As a result, erosion rates they may be overestimated or underestimated. This article presents a quantitative model for the relation the amount of 137Cs lost from the cultivate soil profile and the rate of soil erosion. According to a mass balance model, during the construction of this model we considered the following parameters: the remaining fraction of the surface enrichment layer (FR), the thickness of the surface enrichment layer (Hs), the depth of the plough layer (Hp), input fraction of the total 137Cs fallout deposition during a given year t (Ft), radioactive decay of 137Cs (k), and sampling year (t). The simulation results showed that the amounts of erosion rates estimated using this model were very sensitive to changes in the values of the parameters FR, Hs, and Hp. We also observed that the relationship between the rate of soil loss and 137Cs depletion is neither linear nor logarithmic, and is very complex. Although the model is an improvement over existing approaches to derive calibration relationships for cultivated soil, it requires empirical information on local soil properties and the behavior of 137Cs in the soil profile. There is clearly still a need for more precise information on the latter aspect and, in particular, on the retention of 137Cs fallout in the top few millimeters of the soil profile and on the enrichment and depletion effects associated with soil redistribution (i.e. for determining accurate values of FR and Hs). (author)