Quantifying 137Cs uptake by suspended sediment and plant interception during transfer from rainwater to soil using an improved mass balance model

Authors: Zhang, Xunchang; Busteed, Phillip; YU, BOFU - Griffith University; YANG, WANHONG - University Of Guelph; CHAPPELL, ADRIAN - Cardiff University

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2026
Publication Date: 6/25/2026
Citation: Zhang, X.J., Busteed, P.R., Yu, B., Yang, W., Chappell, A. 2026. Quantifying 137Cs uptake by suspended sediment during transfer from rainwater to soil using an expanded erosion conversion model. Geoderma. 472. Article 117919. https://doi.org/10.1016/j.geoderma.2026.117919.
DOI: https://doi.org/10.1016/j.geoderma.2026.117919

Interpretive Summary: Decades-long continuous erosion monitoring in water runoff-soil erosion plots or small watersheds is costly and labor intensive. A cost-effective alternative erosion estimate using fallout radioactive isotope caesium-137 (Cs-137) has been explored since the 1970s. Caesium-137 was released to the atmosphere during the aboveground nuclear bomb tests in the 1950s and 1960s and was brought back to the earth's surface in precipitation. The fallout Cs-137 deposit is held tightly by fine soil particles and organic matters and can be used as a tracer to track soil movement or erosion. However, the Cs-137 method has shortcomings in its central assumption of no Cs-137 interception by plant canopy and no Cs-137 redistribution during deposition or transfer from rainwater to soil. To improve the method, we corrected the shortcomings in the assumption by accounting for Cs-137 interception and redistribution during deposition in an improved method. The new method substantially improved its accuracy in soil erosion estimation. The new method would be useful to soil and water conservationists and hydrologists for estimating long-term soil erosion rates and for laying out site-specific conservation plans to combat soil erosion in agricultural land.

Technical Abstract: Decades-long erosion monitoring in runoff plots or small watersheds is costly and labor intensive. A cost-effective alternative erosion estimate using fallout caesium-137 has been explored since the 1970s. This caesium-137 technique has been widely used to estimate the net (loss and gain) soil redistribution for the period from 1954 to the sampling year with a one-time field visit. However, the caesium-137 technique has shortcomings in its central assumption of no caesium-137 loss or redistribution during transfer from rainwater to soil. To improve the technique, we elucidate the caesium-137 transfer and redistribution processes by quantifying caesium-137 uptake in suspended sediment and caesium-137 interception by plants during fallout. An improved caesium-137 mass balance model was developed and optimized using legacy soil loss data from runoff plots along with caesium-137 inventories measured during the fallout period of 1954-1976. Predicted soil loss proved highly sensitive to caesium-137 redistribution in runoff and interception by plants during transfer. The two processes can be effectively simulated by two key parameters: caesium-137 uptake by sediment (Psi) and caesium-137 interception by plants. The optimized caesium-137 uptake by sediment (Psi) value was negatively related to soil loss rates. When caesium-137 uptake by sediment (Psi) is > 1, it simulates caesium-137 redistribution in runoff. When caesium-137 uptake by sediment (Psi) is < 1, it rectifies the uniform erosion assumption by accounting for rill incision from concentrated flows and can be estimated as the proportion of interrill erosion (i.e., 1 minus the rill erosion proportion). More experiments are needed to quantify caesium-137 uptake by sediment (Psi) under different rainfall and topographic conditions to improve parameter estimation and therefore erosion prediction. It is worth noting that the processes and estimates found in this study are broadly applicable to other radionuclides commonly used for erosion prediction.