Validating the use of caesium-137 measurements to estimate soil erosion rates in a small drainage basin in Calabria, Southern Italy

Recent concern for problems of soil degradation and the offsite impacts of accelerated erosion has highlighted the need for improved methods of estima ting rates and patterns of soil erosion by water. The use of environmental radionuclides, particularly caesium-137 (Cs-137), as a means of estimating rates of soil erosion and deposition is attracting increasing attention and the approach has now been recognised as possessing several important advan tages. However, one important uncertainty associated with the use of Cs-137 measurements to estimate soil erosion rates is the need to employ a calibr ation relationship to convert the measured Cs-137 inventory to an estimate of the erosion rate. Existing calibration procedures are commonly subdivide d into empirical relationships, based on independent measurements of soil l oss, and theoretical models, that make use of existing understanding of the fate and behaviour of fallout radionuclides in eroding soils to derive a r elationship between erosion rate and the reduction in the Cs-137 inventory relative to the local reference value. There have been few attempts to vali date these theoretical calibration models and there is an important need fo r such validation if the Cs-137 approach is to be more widely applied. This paper reports the results of a study aimed at validating the use of a simp le exponential profile distribution model to convert measurements of Cs-137 inventories on uncultivated soils to estimates of soil erosion rates. It i s based on a small (1.38 ha) catchment in Calabria, southern Italy, for whi ch measurements of sediment output are available for the catchment outlet. Because there is no evidence of significant deposition within the catchment , a sediment delivery ratio close to 1.0 can be assumed. It is therefore po ssible to make a direct comparison between the estimate of the mean annual erosion rate within the catchment derived from Cs-137 measurements and the measured sediment output. In undertaking this comparison, account was taken of the different periods covered by the measured sediment output and the e rosion rate estimated using Cs-137 measurements. The results of the compari son show close agreement between the estimated and the measured erosion rat es and therefore provide an effective validation of the use of the Cs-137 a pproach and, more particularly, a profile distribution calibration model, t o estimate soil erosion rates in this small catchment. Further studies are required to extend such independent validation to other environments, inclu ding cultivated soils, and to different calibration procedures. (C) 2001 El sevier Science B.V. All rights reserved.