SURFACE EROSION ASSESSMENT IN THE SOUTH-CANTERBURY DOWNLANDS, NEW-ZEALAND USING CS-137 DISTRIBUTION

Redistribution of the radionuclide tracer Cs-137 was used to examine t he pattern of erosion and deposition at two sites with contrasting lon g-term land uses (pasture and cropping) in the South Canterbury downla nds, New Zealand. There were clear differences between the two land us e types in variation in Cs-137 concentrations and areal activity, eros ion rates and topsoil depth variability. Erosion and deposition have r esulted in greater variability and lower mean levels of Cs-137 areal a ctivity under cropping (46.3 mBq cm(-2)) than pasture (55.0 mBq cm(-2) ). At the cropping site, erosion and deposition roughly balanced with the mean value over all sampling sites, suggesting no net soil loss, b ut considerable redistribution of soil within paddocks. At the pasture site results suggested slight net deposition. There was evidence for both sheet/rill and wind erosion being important in soil redistributio n. While there was no difference in mean topsoil depth between pasture and cropping, there were significant differences with slope position. At the pasture site, there was little variation of topsoil depth with slope position, except for swales which tended to be deeper, whereas at the cropping site there was considerable variation in topsoil depth with slope position. Topsoil depth was a poor indicator of erosion st atus.