ESTIMATING SURFACE EROSION USING CS-137 AT A SEMIARID SITE IN CENTRALOTAGO, NEW-ZEALAND

Many authors have indicated significant erosion in conjunction with gr azing of sheep and rabbits in the semi-arid lands of Central Otago. Ob servations, however, have generally been based on subjective assessmen ts influenced by vegetation depletion and proportions of bare ground. This paper quantifies net soil losses over the past 40 years using the radionuclide tracer caesium-137 (Cs-137). The study area was a ridge near Earnscleugh showing field evidence of surface erosion. Compared w ith a Cs-137 input of 275 Bq/m(2), analyses indicated a reduction in s oil depth by 3.4 cm over 40 years from the sunny upper sideslope site. This corresponds approximately to an annual loss of 10.2 t/ha. The lo ss rate indicates that past land use was unsustainable. Net losses up to this value have affected approximately 24% of the local landscape. Conversely, there was net soil deposition on the shady upper sideslope and the sunny and shady footslope sites. Estimated gains in soil dept h over 40 years range from 0.3 to 0.6 cm which correspond approximatel y to annual gains of 0.5 to 0.9 t/ha. A mean (N=10) contemporary topso il thickness of less than about 8 cm infers net soil loss from the stu dy area and this may apply to much of the Conroy land, system. Erosion and deposition is dominated by wind transport from north-westerly win ds.