Extending the timescale for using beryllium 7 measurements to document soil redistribution by erosion

Abstract

The need for spatially distributed information on soil mobilization, transfer, and deposition within the landscape by erosion has focused attention on the potential for using fallout radionuclides (i.e., 137 Cs, excess 210 Pb, and 7 Be) to document soil redistribution rates. Whereas 137 Cs and excess 210 Pb are used to estimate medium‐ and longer‐term erosion rates (i.e., approximately 45 years and 100 years, respectively), 7 Be, by virtue of its short half‐life (53 days), provides potential for estimating short‐term soil redistribution on bare soils. However, the approach commonly used with this radionuclide means that it can only be applied to individual events or short periods of heavy rain. In addition, it is also frequently difficult to ensure that the requirement for spatially uniform 7 Be inventories across the study area immediately prior to the study period is met. If the existing approach is applied to longer periods with several rainfall events (e.g., several weeks or more) soil redistribution is likely to be substantially underestimated. These problems limit the potential for using the 7 Be approach, particularly in investigations where there is a need to assemble representative information on soil redistribution occurring during the entire wet season. This paper reports the development of a new or refined model for converting radionuclide measurements to estimates of soil redistribution (conversion model) for use with 7 Be measurements, which permits much longer periods to be studied. This refined model aims to retain much of the simplicity of the existing approach, but takes account of the temporal distribution of both 7 Be fallout and erosion during the study period and of the evolution of the 7 Be depth distribution during this period. The approach was successfully tested using 7 Be measurements from a study of short‐term soil redistribution undertaken within an area of recently harvested forest located near Valdivia in Southern Chile. The study period extended over about 3 months and included the main part of the winter wet season of 2006. The estimates of soil redistribution obtained using the new conversion model were consistent with those obtained from erosion pins deployed within the same study area and were two to three times greater than those obtained using the approach and conversion model employed in existing studies.