SOIL SLIP EROSION AS A CONSTRAINT TO HILL COUNTRY PASTURE PRODUCTION

The results and implications of applying remote sensing techniques together with pasture measurements to quantify the influence of soil slip erosion oh hill country pasture productivity in the Wairarapa are reviewed. Sequential aerial photographs were used to identify, date, and measure the area of slip scars of different age. The reduction in potential productivity of hillslopes due to erosion was then determined by measuring the pasture growth ratesof the different aged slips (and uneroded ground) and integrating these with the proportions of the hillslopes of each age class. Three years of pasture measurements in the Wairarapa hill country showed that although slips revegetated rapidly over the first 20 years to within 7080% of the uneroded productivity, further recovery was slow. Our evidence suggests that these man-modified forest soils, once eroded, may never regain the same potential for agricultural production under a pasture regime. Reduction in pasture production associated with erosion was most severe in summer and mid-winter when farmers have the highest risk of major feed deficits. Since native forest removal, the reduction due to erosion in potential pastoral productivity, integrated over whole hillslopes, has reached 18% on these hillslopes. An estimation of the extent to which improved farming technology has probably masked this reduction in potential productivity is discussed. On hillslopes where severe erosion occurs continually, farmers will be faced with increasing difficulty in maintaining animal production levels unless soil conservation and farm management strategies are designed to further mask the impacts of erosion and thereby prolong the viability of pastoral agriculture on such hill country. Keywords: soil slip erosion; pasture production; hillslope; potential pasture production; hill country pasture; erosion rate; farm management technology; physiographic areas; fertiliser level.