INFILTRATION RATES, SURFACE RUNOFF, AND SOIL LOSS AS INFLUENCED BY GRAZING PRESSURE IN THE ETHIOPIAN HIGHLANDS

The effect of grazing pressure on infiltration, runoff, and soil loss was studied on a natural pasture during the rainy season of 1995 in th e Ethiopian highlands. The study was conducted at two sites with 0-4% and 4-8% slopes at the International Livestock Research Institute (ILR I) Debre Zeit research station, 50 km south of Addis Ababa. The grazin g regimes were: light grazing stocked at 0.6 animal-unit-months (AUM)/ ha; moderate grazing stocked at 1.8 AUM/ha; heavy grazing stocked at 3 .0 AUM/ha; very heavy grazing stocked at 4.2 AUM/ ha; very heavy grazi ng on ploughed soil stocked at 4.2 AUM/ha; and a control with no grazi ng. Heavy to very heavy grazing pressure significantly reduced biomass amounts, ground vegetative cover, increased surface runoff and soil l oss, and reduced infiltrability of the soil. Reduction in infiltration rates was greater on soils which had been ploughed and exposed to ver y heavy trampling. It was observed that, for the same % vegetative cov er, more soil loss occurred from plots on steep than gentle slopes, an d that gentle slopes could withstand more grazing pressure without ser iously affecting the ground biomass regeneration compared to steeper s lopes. Thus, there is a need for developing 'slope-specific' grazing m anagement schedules particularly in the highland ecozones rather than making blanket recommendations for all slopes. More research is needed to quantify annual biophysical changes in order to assess cumulative long-term effects of grazing and trampling on vegetation, soil, and hy drology of grazing lands. Modelling such effects is essential for land use planning in this fragile highland environment.