Journal of the Royal Society of New Zealand abstracts

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

A. E. Hewitt*

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 (¹³⁷Cs). The study area was a ridge near Earnscleugh showing field evidence of surface erosion. Compared with a ¹³⁷Cs input of 275 Bq/m², 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.

Keywords: soil erosion, sustainability, caesium, tussock grasslands

(c) Journal of The Royal Society of New Zealand,

Volume 26, Number 1, March 1996, pp 107-118

PDF file of entire paper: medium quality (726K); (scanned from paper original: notes about this process)

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