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 estimating rates and patterns of soil erosion by water. The use of environmental radionuclides, particu- larly caesium-137 ( 137Cs), 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 advantages. However, one important uncertainty associated with the use of 137Cs measurements to estimate soil erosion rates is the need to employ a calibration relationship to convert the measured 137Cs inventory to an estimate of the erosion rate. Existing calibration procedures are commonly subdivided into empirical relationships, based on independent measurements of soil loss, and theoretical models, that make use of existing understanding of the fate and behaviour of fallout radionuclides in eroding soils to derive a relationship between erosion rate and the reduction in the 137Cs inventory relative to the local reference value. There have been few attempts to validate these theoretical calibration models and there is an important need for such validation if the 137Cs approach is to be more widely applied. This paper reports the results of a study aimed at validating the use of a simple exponential pro®le distribution model to convert measurements of 137Cs inventories on uncultivated soils to estimates of soil erosion rates. It is based on a small (1.38 ha) catchment in Calabria, southern Italy, for which measurements of sediment output are available for the catchment outlet. Because there is no evidence of signi®cant deposition within the catchment, a sediment delivery ratio close to 1.0 can be assumed. It is therefore possible to make a direct comparison between the estimate of the mean annual erosion rate within the catchment derived from 137Cs 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 erosion rate estimated using 137Cs measure- ments. The results of the comparison show close agreement between the estimated and the measured erosion rates and therefore provide an effective validation of the use of the 137Cs approach and, more particularly, a pro®le distribution calibration model, to estimate soil erosion rates in this small catchment. Further studies are required to extend such independent validation to other environments, including cultivated soils, and to different calibration procedures.