THE UTILITY OF CS-137 FOR MEASURING SOIL REDISTRIBUTION RATES IN SOUTHWEST NIGER

Measurable quantities of Cs-137 were found in soil samples from profil es on a toposequence in southwest Niger. Erosional and depositional si tes were identified by comparing Cs-137 inventories with a reference s ite, although, because of the addition of dust, the identification of such a site was difficult. A first approximation of the reference inve ntory (2066 +/- 125 Bq m(-2)) was achieved by modelling. Marked dispar ities between the Cs-137 profiles and the reference profile at some si tes may be due to the eluviation of Cs-137 fixed to clay, where soils have been exposed to cycles of wetting and drying. This hypothesis is supported by principal components analysis and non-hierarchical multiv ariate classification. Other divergences from the reference profile we re interpreted as the results of the redeposition of material with sma ll Cs-137 content, derived from subsoil and gully walls. These complic ations created the need for additional modelling to estimate the net s oil flux. At some sites the net soil flux was calculated using models that related Cs-137 movement to soil redistribution. It appears that a vegetation canopy protects accumulated dust from water erosion on ste ep slopes and from wind erosion on gentle slopes. The net soil flux wa s found to be -16 +/- 2 t ha(-1) yr(-1). (C) 1998 Elsevier Science B.V .