Evaluating Sediment Deposition Prediction by Three 137Cs Erosion Conversion Models

Although the Cs-137 technique has been widely used to estimate soil redistribution in past decades, most Cs-137-conversion models have not been rigorously validated. The objective of this study was to explicitly evaluate the sediment deposition components of three widely used Cs-137 models using 34 yr of soil loss data from a plot (200 x 80 m). The average slope of the plot is approximately 4% in the upper section and 1% in the lower section. The primary soil (fine, mixed, thermic, Udertic or Pachic Paleustoll) is silt loam with 23% sand and 56% silt. Winter wheat (Triticum aestivum L.) was raised primarily under conventional tillage. Sediment load was measured with a pump sampler at the outlet. Bulk soil cores were taken in a 10-m grid to estimate Cs-137 inventory. The Cs-137 depth profiles were measured at eight locations to determine net deposition depth. The proportional model (PM) and two mass balance models (mass balance model 1 [MBM1] and mass balance model 2 [MBM2]) were evaluated. The measured average deposition depth in the depositional area of the lower section was 5.83 cm, and the predicted deposition depths in the area were 4.12, 2.02, and 1.64 cm for PM, MBM1, and MBM2. The results indicated that the simple PM appeared to predict deposition depths better than the two sophisticated mass balance models under the study conditions. However, the true capability of the two mass balance models needs to be further evaluated under more complex conditions that include the critical period of Cs-137 peak fallout in the 1950s and 1960s.