GULLY EROSION IN MANGATU FOREST, NEW-ZEALAND, ESTIMATED FROM DIGITAL ELEVATION MODELS

The methodology and errors involved in determining the amount of sedim ent produced during two (19.5 and 33.2 year) periods by 11 (c. 0.01 - >0.20 km(2)) gullies within a 4 km(2) area in the headwaters of the Wa ipaoa River basin, New Zealand, using sequential digital elevation mod els are described. Sediment production from all gullies within the stu dy area was 0.99 +/- 0.03 x 10(6) t a(-1) (2480 +/- 80 t ha(-1) a(-1)) during the period from 1939 to 1958. It declined to 0.62 +/- 0.02 x 1 0(6) t a(-1) (1550 +/- 50 t ha(-1) a(-1)) during the period from 1958 to 1992, when many of the smaller gullies were stabilized by a program me of afforestation, which commenced in 1960. Both figures are very hi gh by global standards. The two largest (the Tarndale and Mangatu) gul ly complexes together generated 73 and 95 per cent of the sediment in the specified time periods, but the latter amount is equivalent to onl y c. 5 per cent of the total annual sediment load of the Waipaoa River . (C) 1998 John Wiley & Sons, Ltd.