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 alon g transhumance routes. Since soil erosion occurs on the stepped slopes , slope origin was examined using the grazing model of Howard and Higg ins (1987). The result shows that all stepped slopes were formed by ya k grazing, except those near Tangshap which were caused by sheep. The denudation rates by soil erosion on the yak-grazing steps are estimate d 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 conspicu ous phenomenon on the valley slopes. In terms of overall soil transpor tation from the High Himalaya to the lower plains, however, soil erosi on on yak-grazing steps in the Langtang Valley appears to be a negligi ble contributor. It is concluded, therefore, that sediment transport t o the lower elevations during the 50-year period since the introductio n of transhumance is not significant.