Drainage Systems And Water Resource Of Ethiopia And The Horn PDF

04/12/2024 Chapter Four Drainage Systems And Water Resource Of Ethiopia And The Horn Introduction 71% of the earth’s total surface is covered by water bodies (i.e. seas and oceans). Of the earth’s total water surface, – nearly 97.5% is alkaline accumulated...

04/12/2024 Chapter Four Drainage Systems And Water Resource Of Ethiopia And The Horn Introduction 71% of the earth’s total surface is covered by water bodies (i.e. seas and oceans). Of the earth’s total water surface, – nearly 97.5% is alkaline accumulated in seas and oceans. – 2.5% is fresh water, of which nearly; 68.7% is deposited in glaciers, 30.1% in ground water, 0.4% in surface waters and 0.8% in permafrost 1  04/12/2024 Cont. surface waters includes; water in lakes, rivers, atmosphere, soils and wetlands. Surface and ground waters are by far the most abundant and easily available fresh waters. Fresh waters are distributed unevenly throughout the world, which is affected by; – Latitudinal location, – climatic factors and – topographic factors Cont. Ethiopia has large volume of ground and surface water, which is because of; the topographical setup, relatively higher rainfall and nearness to the equator Around 0.7 % of the total land mass of Ethiopia is covered by water bodies. The country’s surface water potential is estimated to be 124.4 billion cubic meters (BCM). 2  04/12/2024 Major Drainage System of Ethiopia. Definition of terms – Drainage:- is the flow of water through well-defined channel. – A river system begins at a place called the source or headwater and ends at a point called mouth. – drainage system is made up of a principal river and its tributaries. – It is also defined as branched network of stream channels together with the adjacent land slopes they drain. Cont. The drainage pattern of an area is the outcome of; the geological processes, nature and structure of rocks, topography, slope, amount and the periodicity of the flow. A drainage basin is the topographic region from which a river and its tributaries collect both the surface runoff and subsurface flow. It is bounded by and separated from other river basins by a water divide or topographic divide. 3  04/12/2024 Drainage Basin Cont. The general patterns of major river basins in Ethiopia are determined by topographical structures which can be clarified as: a) The topography of the outward sloping of the Western and South eastern plateaus b) The structural formation of the Rift Valley with its in-ward- sloping escarpments resulting mainly in an inland drainage system. c) Faults and joints that structurally influence part of the courses of many rivers. 4  04/12/2024 Cont. Ethiopian drainage systems are broadly classified into three main categories, namely: a) Western b) Southeastern and c) Rift Valley Drainage Systems Western and the Southeastern drainage systems are separated by the Rift Valley system. The Western Drainage Systems;  Characteristics of western Drainage systems  the largest of all drainage systems  draining 40 percent of the total area of the country  carry 60 percent of the annual water flow.  Consist of four major river basins: Tekeze, flows westward Abay, flows westward Baro-Akobo, flows westward and Ghibe (Omo) flows southward  The Abay, Tekeze and Baro flow westward joining the Nile and finally ends at Mediterranean Sea. 5  04/12/2024 Cont. River Abay – It is the largest river both in volumetric discharge and coverage (199,812 km2) in the western drainage systems. – Abay river basin covers parts of; Amhara, Oromia and Benishangul-Gumuz regional states. – carries 65 percent of the annual water flow of the region – rises from Lake Tana (some sources indicate its origin from Sekela, Choke mountain) – flows about 1,450 kilometres and joins the White Nile in Khartoum, Sudan to form the Nile River. Cont. –> 60 streams drain the Abay with an elevation ranging between 500 - 4620 meters above sea level. –The largest of these is Ghilgel Abay (Little Abay). –Abay flows eastward, turns 180 degree to make a large bend and after cutting an impressive and deep gorge emerges out in the west. 6  04/12/2024 Cont. Tekeze River – It carries 12 percent of the annual water flow of the region – drains 87,733 Km2 of land surface. – The basin has two main tributaries; Angereb and Goang. – Tekeze River is termed Atbara in Sudan, which is a tributary of the Nile. – The total mean annual flow from the basin is estimated to be 8.2 billion metric cubes. Cont. The Baro-Akobo – Carries 17 percent of the annual water flow the region. – Baro Akobo river basin has an area of 75,912 km2, covering parts of the Benishangul-Gumuz, Gambella, Oromia, and SNNPR. – The total mean annual flow from the river basin is estimated to be 23.6 BMC. – The Baro together with Akobo forms the Sobat River in South Sudan. 7  04/12/2024 Cont. The Ghibe/ Omo river – Carries 6% of the annual water flow. – drains an area of 79,000 km2 with an estimated mean annual flow of 16.6 BMC. – The Ghibe / Omo River finally empties in to the Chew- Bahir at the mouth of Lake Turkana (an elongated Rift Valley lake) thereby forming an inland drainage. The Southeastern Drainage Systems Nearly the entire physiographic region of southeastern part of Ethiopia is drained by the southeastern drainage systems. The basin drained by Wabishebelle and Ghenale, slopes south-eastwards across large water deficient plains. both rivers cross the border into Somalia, carrying 25 percent of the annual water flow of Ethiopia. 8  04/12/2024 Cont. Wabishebelle River basin – Wabishebelle with a total catchment area of 202,697 km2, is the largest river in terms catchment area. – It drains parts of Oromia, Harari and the Somali regions. – It is the longest river in Ethiopia. – Its tributaries are mainly left bank and, most of them, are intermittent (Ramis Erer, Daketa Fafan) – It fails to reach the Indian Ocean where at the end of its journey it flows parallel to the coast before its water disappears in the sands, just near the Juba River. Cont. Ghenale River basin – has an area of 171,042 km2, covering parts of Oromia, SNNPR, and Somali regions. – It has fewer tributaries (Dawa, Weyb, Welmel, Mena) but carries more water than Wabishebelle, reaches the Indian Ocean. – The basin flows estimated to be 5.8 BMC within elevation ranging between 171-4385 meters above sea level. – In Somalia it is named the Juba River. 9  04/12/2024 The Rift Valley Drainage System; This drainage system is an area characterized by; small amount of rainfall, high evaporation and small catchment area. Awash river basin (the most utilized river in Ethiopia) is the only major river. The total catchment area is 114,123 km2 The average annual discharge is about 4.9 billion cubic meters. Annual Length of the river Catchment Terminus/ Major River Volume Area (km2) Mouth Tributaries BM Total Inside Outside Inland Akaki, Kesem, Awash 114, 123 4.9 (within 1250 1250 - Borena, Mile Ethiopia) Cont. Its source is the Shewan plateau (central Ethiopia) Covers some parts of the Amhara, Oromia, Afar, Somali, Dire Dawa, and Addis Ababa City Administration. Has a length of 1250 kms flows in a northeast direction and ends up in Lake Abe along the Ethio-Djibouti border. In this system, there is no one general flow direction as the streams flow in all directions. 10  04/12/2024 Cont. The Afar drainage sub-basin has practically no stream flow and It is an area of; little rain, very high temperature and very high evaporation. the main surface waters in the basin are Lake Afrera and Asale, which were formed through tectonic activities. Cont. The Southern part of the Rift Valley sub-basin is characterized by a number of lakes and small streams and hence, described as ‘lakes region’. The lakes occupy fault depression. There are small streams that drain down from the nearby mountain slopes which supply water to the lakes. For example; – Meki and Katar Rivers flow into Ziway; – Bilate into Abaya; and Segen into Chew Bahir. 11  04/12/2024 Cont. Likewise, some of these lakes are interconnected. – Lake Ziway drain into Lake Abijiata through the streams of Bulbula – Lake Langano drain into Lake Abijiata through the streams of Horocolo. Water Resources: Rivers, Lakes and Sub-Surface Water The Ethiopian Rivers Altogether, Ethiopian rivers form 12 major watersheds separating the Mediterranean Sea from the Indian Ocean drainage systems. 12  04/12/2024 Drainage Basins of Ethiopia The General Characteristics of Ethiopian Rivers Ethiopian rivers have the following characteristics:  Almost all major rivers originate from the highlands with an elevation of >1500 meters a.s.l.  Majority of Ethiopian rivers are trans-boundary,  extreme seasonal fluctuation (i.e. high volume during wet season and low volume during the dry season)  Have rapids and waterfalls along their course as they flow from highland to low land.  They have cuts, steep-sided river valleys and deep gorges along their courses,  Rivers in Ethiopia flow on steep slopes having steep profiles.  Some of them serve as boundaries (both international and domestic administrative units). 13  04/12/2024 Cont. These characteristics of Ethiopian Rivers attributed by; – the highland nature of the Ethiopian landmass, – surface ruggedness, – the outward inclination of the highlands, and – the climatic conditions. The Ethiopian Lakes Most of the lakes in Ethiopia are located within the Rift Valley System. All Ethiopian lakes are the result of tectonic process that took place during Quaternary period of Cenozoic era. The lakes in the Rift valley drainage are mainly formed on faulted depressions and are clustered along the system forming linear pattern. 14  04/12/2024 Lakes of Ethiopia Cont. Lake Tana, the largest lake in Ethiopia occupies a shallow depression in the highlands. The Tana depression is believed to be formed following slower sinking and reservoir by lava flow between Gojjam and Gonder massifs. Ethiopia is also gifted with crater lakes. These include; – the lakes at and around Bishoftu, – Wonchi (near Ambo), – Hayk (near Dessie) and – on top of Mount Zikwala. 15  04/12/2024 Cont. Lake Ashenge (Tigray) is formed on a tectonic basin. Other types of lakes in Ethiopia are man-made such as; – Lakes Koka, – Fincha and – Melka Wakena, within main Ethiopian rift, – Lake Abaya is the largest of all the lakes in the system. – The deepest is Shalla lake and – The shallowest is Ziway lake in the central Ethiopian Rift. The southern tip of the Rift Valley, Chew Bahir is drained by Segan and Woito. Cont. Area and depth of some of Ethiopian Lakes 16  04/12/2024 Subsurface (Ground) Water Resource of Ethiopia As compared to surface water resources, Ethiopia has lower ground water potential. The availability of groundwater resource is determined by – Climatic and – geophysical conditions. The nation groundwater potential is believed to range between 12-30 BMC Water Resources Potentials and Dev’t uses in Ethiopia Some of the social and economic ‘potential development uses’ of water resource are:  Hydro-electric Potential  Irrigation and Transportation  Fishing and Recreation 17  04/12/2024 Hydro-electric Potential Ethiopian rivers have a very high potential for generating electricity. The exploitable potential of hydroelectric power is estimated at about 45,000 megawatts. Grand Ethiopian Renaissance Dam (GERD) is the country’s largest dam under construction aiming to generate 6400 megawatts. Gilgel Gibe III hydropower project has gone operational generating 1870 megawatts. Cont. Currently Ethiopia is administering 16 hydroelectric power plants constructed on; No. hydroelectric Installed Capacity (MW) power plants 1 Gibe III: 1,870 MW 2 Beles: 460 MW The first hydroelectric power 3 Gilgel Gibe II: 420 MW 4 Tekeze: 300 MW generation plant was installed on 5 Gilgel Gibe I: 184 MW 6 Melka Wakena: 153 MW 7 Fincha: 134 MW Akaki River (Aba Samuel) in 8 Amarti Neshe: 95 MW 9 Tis Abay II: 73 MW 1932 , 10 Koka: 43.2 MW 11 Awash II: 32 MW 12 Awash III: 32 MW 13 Tis Abay I: 11.4 MW 14 Aba Samuel : 6.6 MW All of them generating close to 15 Gilgel Gibe IV: 2,160 MW 16 GERD: 6,000 MW 6978 megawatts of energy. Total 11,974 MW Total operational 6,978 MW 18  04/12/2024 Cont. Besides the domestic use of generated electricity, the country is exporting electricity to the neighboring countries. The major problem related to the use of Ethiopian rivers for the generation of hydroelectric power is; – the seasonal flow fluctuations and impact of climate change and variabilities. – The severe erosion from the highlands and sedimentation in the reservoirs. Irrigation and Transportation The terrain in Ethiopia is so rugged that it limits the uses of Ethiopian rivers both for irrigation and transportation. Obstacles In the highlands, – steep slopes, – rapids, – waterfalls, – narrow and deep valleys and gorges But on the lowlands, their demand for irrigation is high. 19  04/12/2024 Cont. Ethiopia’s potential of irrigation is estimated to be 5.3 million hectares. The Baro-Akobo and Genale Dawa river systems have large irrigation potential compared to other basins. Despite the untapped irrigation practice, more than 60% of the area under irrigation so far is located in Rift Valley Drainage System. Except few, majority of hydro-electric reservoirs are multi-purpose and are expected to contribute for irrigation. Cont. Majority of Ethiopian rivers are not suitable for transportation. – The Baro at its lower course is the only navigable river. – Lake Tana and Abaya are relatively the most used for transportation. 20  04/12/2024 Fishing and Recreation Ethiopian lakes are rich in fish and The present annual production of fish is estimated to be 31.5 thousand tons. The exploitable potential is > the current production. Lake Tana has the leading potential of fish exploitation estimated to be between 8,000-10,000 tons per year and Lake Chamo having Fish production estimated at 4,500 tons per year. Cont. More than 60% of fish supplies are coming from Ethiopian Rift Valley lakes. However, some of the lakes are currently threatened by; sedimentation, invasive species (water hyacinth), over exploitation and expansion of investments around lakes. 21  04/12/2024 Cont. Ethiopian rivers and lakes are also important for recreational and tourist attractions. This is because of; – The presence of a variety of fish, birds and other aquatic life forms in the lakes, – the scenic beauty of the lakes, – the hot springs around them, – the spectacular river gorges and – the most impressive waterfalls As they are the natural habitat of a variety of wild life, some of which are only endemic to Ethiopia, their value for scientific purposes is immense 22