The limitations of using 137Cs for estimating soil redistribution in semi-arid environments

The 137Cs technique for estimating net (ca. 30 years) soil flux has been used successfully in many environments. Its widespread use is probably because the 137Cs technique overcomes many of the problems of monitoring soil erosion and deposition (flux) over the medium-term (5 to 50 years) and at the hillslope scale. In this respect, the technique probably offers the greatest potential for measuring net soil flux in semi-arid environments where soil flux monitoring difficulties are compounded by considerable spatial and temporal variability of the controlling factors. However, there remain uncertainties in the underlying assumptions of the technique and difficulties in satisfying these assumptions, especially in semi-arid areas. Several key assumptions of the 137Cs technique for measuring net soil flux in semi-arid areas were investigated using data from southwest Niger. Samples were obtained along a toposequence typical of the region and at nested grid nodes stratified using geomorphological information for geostatistical estimation at unsampled locations providing complete coverage of the study area. The pervasive occurrence of dust made it difficult to identify an undisturbed site. A first approximation of the reference 137Cs inventory (2066±125 Bq m−2) was provided by modelling the 137Cs profile at an unvegetated site. Despite little evidence in the literature on problems of identifying a reference inventory, especially those suffering from wind erosion, it is likely that similar problems occur in other semi-arid areas. Problems with the preferential transport of 137Cs were identified by expressing the 137Cs concentration as a proportion of the weight in each grain-size fraction. However, it was partially accounted for in the calibration relationship by reducing (by a factor of 10) the 137Cs concentration of soil samples from the plateau. It is likely that workers in other semi-arid regions have similar problems of preferential transport of 137Cs and the method discussed here appears to be a valuable tool to indicate the potential for preferential 137Cs movement. A large disparity was found between the ‘undisturbed’ model and the structure of a 137Cs profile from a small alluvial fan. This was due to the removal of 137Cs-rich soil and its replacement with soil largely devoid of 137Cs, which probably originated from gully walls. Two separate models that related 137Cs movement to soil redistribution were used to calculate net soil flux. Additional modelling would be required to account for 137Cs dilution in order to measure net soil flux in gullied and badly eroded rangeland areas. Net soil flux was calculated at sites along the toposequence, at the nested grid sites and at unsampled locations across the study using geostatistics. The toposequence samples considerably underestimated the net soil loss relative to the nested grid samples and the geostatistical estimates of net soil flux. Toposequence sampling does not account for the spatial variation in net soil flux unless very well-defined geomorphological units that control the soil redistribution processes are evident. This form of sampling is of limited value for investigating the variation of 137Cs depth distribution at all but the smallest scale (largest areas) since it cannot easily be used to determine local variation in soil redistribution processes. A small improvement in net soil flux accuracy was provided by the geostatistical estimates, relative to the nested grid samples of net soil flux. This was thought to be due to the efficiency of the nested grid samples suggesting that geomorphological information can be used to limit the number of samples necessary to encompass several scales of net soil flux variation.