Estimating surface erosion using Cs-137 at a semi-arid site in Central Otago, New Zealand

Many authors have indicated significant erosion in conjunction with grazing of sheep and rabbits in the semi-arid lands of Central Otago. Observations, however, have generally been based on subjective assessments 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 with a Cs-137 input of 275 Bq/m(2), analyses indicated a reduction in soil 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 loss 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 depth over 40 years range from 0.3 to 0.6 cm which correspond approximately to annual gains of 0.5 to 0.9 t/ha. A mean (N=10) contemporary topsoil thickness of less than about 8 cm infers net soil loss from the study area and this may apply to much of the Conroy land, system. Erosion and deposition is dominated by wind transport from north-westerly winds.