HYDROLOGIC RESPONSE TO CATTLE GRAZING IN THE ETHIOPIAN HIGHLANDS

The effect of grazing pressure on infiltration, runoff, and soil loss was studied on a natural pasture during the 1995 rainy season in the E thiopian highlands. The study was conducted on 0.01 ha plots establish ed on sites with 0-4% and 4-8% slopes at the International Livestock R esearch Institute (ILRI) Debre Zeit research station, 50km south of Ad dis Ababa. The grazing regimes were: light grazing stocked at 0.6 anim al-unit-months (AUM) ha(-1); moderate grazing stocked at 1.8 AUM ha(-1 ); heavy grazing stocked at 3.0 AUM ha(-1); very heavy grazing stocked at 4.2 AUM ha(-1); very heavy grazing on ploughed soil stocked at 4.2 AUM ha(-1); and a control with no grazing. Heavy to very heavy grazin g pressure significantly increased surface runoff and soil loss and re duced infiltrability of the soil. It was observed that fine textured s oils were more susceptible to trampling effects than coarse textured s oils, and that reduction in infiltration rates was greater on soils wh ich had been tilled and exposed to very heavy trampling. The problems of high runoff and erosion rates on the upper slopes is likely to be e xacerbated by the fact that during the rainy season higher grazing pre ssure is exerted on the upper than lower slopes. Sediments produced fr om the highlands, which form headwaters of major rivers in the region, are likely to pollute streams and lakes and pile up on bottom-lands, in stream channels, and in reservoirs. With some modifications, the pl ot design presented here can be used for assessing livestock impacts o n natural resources on different landforms at large scales such as wat ersheds. How the same amount of livestock mass dispersed by different livestock species impacts on the grazing lands needs to be studied fur ther. (C) 1997 Elsevier Science B.V.