1. 1. Hyper-arid (PR/PET < 0.02) 3. Semi-arid (0.20  RR/PET < 0.5) 5. Humid (R/PET> 0.65) Figure 1. Dry land areas in Ethiopia delineated on the bases of PR/PET ratio. (Source: Tamirie Hawando. 2001) Figure 2. Relationship between elevation, rainfall and evaporation. Aridity index Land area (km 2 x 1000) Humid (moist) 1.25 350 Dry sub-humid 0.60 300 Semi-arid 0.38 207 Arid 0.12 300 Hyper-arid 0.03 53 DESERTIFICATION A CHALLENGE TO THE HORN OF AFRICA: AN EXAMPLE OF THE AFAR RGION Girma Taddese, g.taddesse@cgiar.org and Don Peden,d.peden@cgiar.org International Livestock Research Institute P. O. Box 5689 Addis Ababa EthiopiaINTORODUCTION Manifestations of desertification include accelerated soil erosion by wind and water, increasing salinization of soils and near-surface groundwater supplies, a reduction in soil moisture retention, an increase in surface runoff and stream flow variability, a reduction in species diversity and plant biomass, and a reduction in the overall productivity in dry land ecosystems with an attendant impoverishment of the human communities dependent on these ecosystems. Additional impacts include an increase in particulate and trace gas emissions from biomass burning in dry lands and an increase in atmospheric dust loads. Desertification can generally be defined as land degradation in arid, semi- arid and dry sub-humid areas resulting mainly from adverse human impact and climate variation. This phenomenon overlaps with a number of related environmental issues such as climate change, population growth, trade and biodiversity. There is also a complex cause-effect relationship between desertification, population growth and poverty. Observation Site Lower Awash Valley (Afar Region) Figure 3. Soil moisture regime zones in Ethiopia (Source Hawando 1989). 1. Udic SMR = Ppt > 1000 mm, AWC = 300-392 mm 2. Ustic SMR = Ppt = 700-1000 mm, AWC = 192-342 mm 3. Xeric SMR = Ppt = 300-700 mm, AWC = 50-198 mm 4. Aridic SMR = Ppt < 300 mm, (SMR = Soil moisture regime, AWC = Available water holding capacity, Ppt = Precipitation) Uses 1. Perennial and annual crops & forestry 2. Annual crops, perennial crops grazing forestry j 3. Rangelands, annual crops, irrigated farms 4. Rangelands, farms, sporadic annual crops Constraints Poor farming systems, deforestation, over grazing & population pressure Poor farming system, deforestation, over grazing, population pressure & land degradation Deforestation, over grazing, erosion, salinity, draught, erratic & low rainfall Moisture deficit, wind erosion, drought, over grazing erratic and low rainfall, salinity Figure 8. Aggressive Prosopis Juliflora species is salt and draught tolerant, due to tough throne and alkaloid content it is not preferred by livestock (Girma Taddese and Shimelis Taddese. 1996. Julifolora Reclaims wasteland in Middle Awash Valley. IAR News of Agricultural Research. Vol. 11 No. 2). Figure 4. Salt patches and and salt tolerant grass and shrub species. Figure 5. Salt and draught tolerant shrub species staggering in extreme climatic conditions. Figure 6. Draught and over grazing persistent shrub and grass species in plain areas can survive with little shower of rainfall. Figure 7. Exposition of the slope type and erosion-draught tolerant plant spices may some times help in sand stabilization. Figure 9. Dead land with out vegetation of higher order. Figure 10. Wherever Awash River crosses there is life. Figure 11. The last chance for extinction for this plant species and complete desertification. Figure 12. We can stop desertification if we really understand the nature of arid land and allow the community to care of their land. Figure 13. Research and Development is needed to understand this draught- salt tolerant and wind persistent multi purpose plant species. 2. Arid (0.2  RR/PET  0.20) 4. Dry sub-humid (0.5  RR/PET< 0.65)