Evaluating and Improving Cesium-137 Technology for Estimating Soil Redistribution Using Soil Loss Data Measured During 1954-2015

Soil erosion causes worldwide land degradation. Long-term monitoring of soil erosion is costly and labor-intensive. Multiple models using Cs-137 fallout are developed to retrospectively estimate average soil erosion rates since 1954. However, these models have not been rigorously validated due to the lack of compatible long-term measured soil loss data, and thus their usefulness has been seriously questioned. Compared with 62 years of rare soil loss data measured in two small watersheds of <0.78 ha during 1954 and 2015, all theoretical models (Eqs. (5)-(13)) resulted in overestimations of mean net soil erosion rates by at least 400%. A key assumption of the homogeneous Cs-137 transfer from rainwater to the soil profile during fallout is invalid, and a critical process of the enhanced Cs-137 loss and redistribution during transfer is overlooked. Based on two independent studies, the enhanced Cs-137 uptake by suspended sediment during transfer was responsible for about 8 times more enriched Cs-137 loss in sediment, to which Cs-137 inventory and erosion estimation are extremely sensitive. More experiments are needed to investigate whether this enhanced loss ratio varies with soil texture, rainfall intensity, and slope steepness. A new mass balance model is developed to include the dynamic uptake of Cs-137 by suspended sediment in surface runoff and losses of Cs-137 in both runoff solution and uptake by plants. The new model greatly improves mean net soil erosion estimation with a mean relative error of 33%, which is comparable with those of the empirical models (Eqs. (2)-(4)). The finding of enhanced radionuclide loss with suspended sediment during transfer is, in theory, valid for other continuous fallout radionuclides such as Pb-210 and Be-7, which have been widely used in soil erosion estimation. Taking into account the enhanced radionuclide loss by suspended sediment during fallout will substantially lower the soil loss estimation for all fallout radionuclides.