SOIL-EROSION ON YAK-GRAZING STEPS IN THE LANGTANG-HIMAL, NEPAL

The distribution of shallow soil-erosion scars (up to 1.2 m in depth) was mapped on a scale of 1:19,200 in the upper valley of the Langtang Himal, central Nepal. It coincides with the yak-grazing grassland along transhumance routes. Since soil erosion occurs on the stepped slopes, slope origin was examined using the grazing model of Howard and Higgins (1987). The result shows that all stepped slopes were formed by yak grazing, except those near Tangshap which were caused by sheep. The denudation rates by soil erosion on the yak-grazing steps are estimated at 0.02-0.16 mm/yr, when the period of intensive grazing is assumed to be 50 years. Soil erosion on yak-grazing steps is the most conspicuous phenomenon on the valley slopes. In terms of overall soil transportation from the High Himalaya to the lower plains, however, soil erosion on yak-grazing steps in the Langtang Valley appears to be a negligible contributor. It is concluded, therefore, that sediment transport to the lower elevations during the 50-year period since the introduction of transhumance is not significant.