Measuring Sediment Movement at Low Erosion Rates Using Cesium‐137

Abstract New and innovative methods for measuring soil loss and its impact on productivity are needed to accurately assess the value of soil lost by erosion. Cesium‐137 ( 137 Cs), a radionuclide from nuclear weapons tests which is strongly adsorbed to clay, can be used to trace sediment movement. Measurements of 137 Cs concentrations in uneroded soils across the southern United States indicated that the 137 Cs input was proportional to the average annual precipitation. Distribution of 137 Cs within the profile was related to soil properties rather than to rainfall. Cesium‐ 137 measurements on a small native grass watershed in Oklahoma showed considerable spatial variability in the 137 Cs concentrations, but the variability was random and was not correlated with changes in slope. Cesium‐ 137 concentrations measured on a 10‐m grid in an adjacent small watershed that had been cultivated for 8 yr showed differences with slope positions even though only 17.8 Mg/ha sediment had been removed from the watershed during an 8 yr measurement period. Also, 137 Cs concentrations in the cultivated watershed were significantly lower than in the uncultivated watershed. These data show that 137 Cs measurements can be used at low erosion rates if enough samples are analyzed. Further research is needed to determine the number of samples needed for different watersheds and to refine the method.