SURFACE WATER RESOURCES BY BASIN
Almost all of the basins radiate from the central ridges that separate the Rift Valley from the highlands of Ethiopia to all directions out of the country. Basins drained by rivers originating from the mountains west of the Rift Valley flow West into Sudan , and those originating from the Eastern highland flow East into the Republic of Somalia . Rivers draining the Rift Valley Basins System originates from the adjoining highlands and flow North and South of the uplift in the Center of the Ethiopian Rift Valley North of Lake Ziway.
There is very big variation in the size of the Basins. The size of a basin in mainly the function of the geological formation. The Wabishebelle Basin is the largest basin in Ethiopian with an area of 202, 220 km 2 followed by the Abbay Basin covering an area of 199812km 2 . The smallest basin is the Aysha Basin with an area of 2223 km 2 followed by the Mereb that has an area with in Ethiopia of 5900km 2 .
Table: Important Physical Characteristics of the Ethiopian Basins
TOPOGRAPHY AND ALTITUDINAL GRADIENT
Ethiopia is often fabled as the country of high mountains, flat plateaus frequently cut by deep gorges and wide valleys in the highland and plains in the lowlands. The highlands with very rough terrain are impenetrable making accessibility as well as other development efforts a complex endeavor.
The Geophysical setting of the country is generally characterized by highland in the center circumscribed by the lowlands. High raising mountains with flat top and steep sides are common features of the Ethiopian Highland. The lowlands are flat with frequent incision by ravines and gullies. The transition from highland to lowlands is very abrupt with sharp falls and cataracts.
Except the Ogaden and Aysha Basins which are totally located in the lowlands, the other basins of Ethiopia exhibit a large altitudinal gradient the highest being in the Denakil Basin .
Table: Important Topographical Characteristics of the Ethiopian Basins
Topography and altitudinal settings of a basin are the main functions of geological formations and can clearly reflect the potential for and constraints to the development of the given basin.
Climate and Hydrology
Climate is the function of the location (latitudinal), altitude, angle of the sun, distance from oceans or other water bodies, terrain and the like. The different combination of these factors resulted in the prevalence of diverse climatic conditions in Ethiopia . The major climatic conditions in Ethiopia can be categorized as tropical in the south and southwest, climatic in the highlands and arid and semi-arid in the Northeastern and Southeastern lowlands. Climate has a direct implication on the development of the basin with particular influence on the availability and use of water and pattern of settlement.
Hydrology of an area is a direct reflection of the climate, the terrain and other physical characteristics such as porosity, permeability and vegetation cover. Water travels on the surface, underground and in the atmosphere in a well-known cycle. Here emphasis is given to part of the water that run on the surface in the form of streams and rivers or stored in the form of lakes.
The very high variability exhibited by the climatic components of the country over time and space is the main reason behind the spatial and temporal variability in the availability of water. The shape, size and other physical features of the River Basin does also contribute to the same. The nature of the river channel, which is a direct function of the relationship between the flow and formation of the channel, governs accessibility to and pattern of use of the waters flowing in the rivers or stored in natural depressions. Table indicates important climatic components and water resource potential of Basins in Ethiopia in terms of mean annual run-off, storages in major lakes and impoundments and underground.
Table: Important Climatic Features and Water Resource Potential of the Ethiopian Basins
SOCIO-ECONOMIC FEATURES OF THE BASIN OF ETHIOPIA
The socio-economic features of most of the basins of Ethiopia can be characterized by a simple but strategic terminology of Poverty and Isolation. Poverty expressed both in terms of absolute poverty or through some socio-economic indicators is one of the lowest in the world. The different parts of the basins are physically and psychologically isolated from each other and from other parts of the country. The physical isolation is mainly due to the terrain which is very rugged and impenetrable making accessibility and the development of infrastructure very challenging and the sever shortage of communication infrastructures and systems. The psychological isolation is the reflection of physical isolation in which case citizens have the perception of total isolation from the central government.
More than 90% of the inhabitants of the basin are rural and subsistent farmers. The ever increasing population of the basin coupled with absence of alternative source of livelihood resulted in high level of land fragmentation and total dependence on the natural resources of the basin. Land holding size per household barely exceeds 0.5 ha. Most peasants are subsistent farmers with little product to put on the market.
Except in some dry basins such as the Ogaden and Aysha, where the dominant mode of agriculture is totally pastoralism, mixed farming in the highland and dominantly livestock production in the lowlands is common feature. Urbanization is very low averaging 10% in most basins. Industrialization and the service sector are not well developed. Settlement pattern is very variable. The general trend is that settlement is very dense in the highlands and sparse in the lowland. Some important socio-economic features of the basins are detailed in the table below.
Table: Important Socio-economic features of the Ethiopian Basin
The most populous basin in Ethiopia is the Abbay Basin . The Abbay Basin is the most important Basin in Ethiopia by most criteria as it contributes about 45% of the countries surface water resources, 25% of the population, 20% of the landmass, 40 % of the nations agricultural product and most of the hydropower and irrigation potential of the country. Population density is highest in Rift Valley Lakes Basin indicating the immense pressure on the resource base. The basin with the lowest population size and density is the Aysha Dry Basin mainly due to its remoteness, inaccessibility, harsh environmental condition and low resource potential and shortage of socio-economic infrastructures and services. Despite the huge productive force in the basins (Age group 15-64), the high rate of illiteracy could be an obstacle to their development.
Administratively, most basins drain more than one Regional States. Awash Basin drains seven Regional States and / or City Councils where as the Oromia Regional State is drained by seven different basins. Three of the basins; Ogaden, Aysha and Mereb fall with in one Regional State , Somali and Tigray respectively and the Gambella Regional State falls within one basin ie the Baro-Akobo Basin . The discrepancy between the basin boundary which is considered as an appropriate unit for planning and management of water resources and the administrative boundary, within which most decisions of development are made, will be one of the challenges forthcoming with the progress of basin based water resources management approach under implementation.
POTENTIAL OF THE BASINS FOR FUTURE WATER RESOURCE DEVELOPMENT
Table: Simple Parameters Indicating the Potential of the Basin
By virtue of its high potential and less inhabitants, the Baro-Akobo basin has the highest value in most of these indicators. The efficiency of the basin to generate run-off is also the highest when compared to other basins. On the other side of the picture come the Aysha, Denakil, Mereb and Ogaden Basins with less prospect for future development of their water resources.