EROSION, IN THE EYE OF THE BEHOLDER

J.B.J. HARRISON

First International Congress

of the European Society of Soil Conservation

Gilsoe England, 1994

The major concern of soil conservators and scientists studying erosion is the identification and amelioration of erosion, that can be attributed to human activities. Distinguishing between 'natural' erosion and 'accelerated erosion is much less of a science than we would like to believe. To some extent, our approach to assessing the degree of erosion and the measures we advocate to control it, are determined by our experience, background and culture. The history of soil conservation in the eastern Southern Alps of New Zealand provides an example of how cultural attitudes had a strong influence on the evaluation of 'accelerated' erosion and on the methods advocated for controlling it. The eastern foothills of the Southern Alps are characterized by large areas of talus slopes and erosion scars exposing bare soil, whereas further to the west the slopes are covered by a dense rain forest. The visual appearance of the eastern foothills was sufficient to convince soil conservators that this was an area with the major soil erosion problems. The degree of soil erosion on the eastern foothills was determined by measuring the aerial extent of bare ground and depth of soil lost. Over a period of 40 years much of this area was 'retired' from active grazing and burning, and considerable effort was spent in trying to revegetate both the bare soil and the talus slopes. Over the last 15 years research has provided a geomorphic framework for evaluating the soil erosion hazard of the southern Alps. The rates of erosion in the eastern foothills as measured by the sediment yield of drainage basins was found to be within the range expected for an area with 1 m/year rainfall. Rates of erosion are much higher in the densely vegetated slopes to the west where rainfall often exceeds 10 m/year. The background of the early researchers led them to believe that vegetation cover was all that was necessary to control erosion and the lack of vegetation results in high rates of erosion. Controlling the erosion on the eastern foothills now appears to be unnecessary and uneconomic.