CHAPTER FOUR - DRAINAGE SYSTEMS AND WATER RESOURCE OF ETHIOPIA AND THE HORN

4.1. Introduction
4.2. Major Drainage System of Ethiopia
4.3. Water Resources: Rivers, Lakes and Sub-Surface Water
      4.3.1. The Ethiopian Rivers
     4.3.2. The Ethiopian Lakes
     4.3.3. Subsurface (Ground) Water Resource of Ethiopia
4.4. Water Resources Potentials and Development in Ethiopia

Chapter's Objectives

At the end of this chapter you will be able to:
 Describe the major drainage systems in Ethiopia and the Horn,
 Examine the surface and ground water resource potentials of Ethiopia,
 Understand economic potentials of the water sector in Ethiopia.

4.1. Introduction

About 71% of the earth’s total surface is covered by water bodies majorly occupied by seas and oceans. Of the earth’s total water surface, nearly 97.5% is alkaline accumulated in seas and oceans. The remaining 2.5% is fresh water, of which nearly 68.7% is deposited in glaciers, 30.1% in ground water, 0.8% in permafrost and 0.4% in surface waters. Water in lakes, rivers, atmosphere, soils and wetlands are considered as surface waters. Surface and ground waters are by far the most abundant and easily available fresh waters. However, fresh water is distributed unevenly throughout the world following varied latitudinal locations, climatic and topographic setups.

As you have discussed in the third chapter, the topographic setup of Ethiopia is characterized by complex blend of massive highlands, rugged terrain, and low plains. The diverse topographical setup, relatively higher rainfall and its nearness to equator made the country to have larger volume of ground and surface water. Around 0.7 % of the total land mass of Ethiopia is covered by water bodies. Although it requires further detailed investigation, the country’s surface water potential as studied in different integrated river basin master plans is estimated to be 124.4 billion cubic meters (BCM). Consequently, many call Ethiopia, the water tower of “Eastern Africa”.

4.2. Major Drainage System of Ethiopia

The flow of water through well-defined channel is known as drainage. A drainage system is made up of a principal river and its tributaries (the rivers that flow into it). A river system begins at a place called the source or headwater and ends at a point called mouth. Therefore, a drainage system is branched network of stream channels together with the adjacent land slopes they drain. 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. The general patterns of major river basins in Ethiopia are determined by topographical structures which can be clarified as:

  1. The topography of the outward sloping of the Western and South eastern plateaus
  2. The structural formation of the Rift Valley with its in-ward-sloping escarpments resulting mainly in an inland drainage system.
  3. Faults and joints that structurally influence part of the courses of many rivers.

The Major Drainage Systems
Why do rivers rise from higher slope and flow towards the lower?
Following the complex physiographic setup and geological makeup, Ethiopia possesses three broadly classified drainage systems namely Western, South-eastern and Rift Valley Drainage Systems. Western and the South-eastern drainage systems are separated by the Rift Valley system.

The Western Drainage Systems are the largest of all drainage systems draining 40 percent of the total area of the country and carry 60 percent of the annual water flow. Most of the catchment area coextends with the westward sloping part of the western highlands and western lowlands. This drainage system comprises four major river basins namely the Tekeze, Abay, Baro-Akobo, Ghibe (Omo). Unlike other river basins in the system, the Ghibe (Omo) flows southward. The Abay, Tekezeand Baroflow westward ultimately joining the Nile which finally ends at Mediterranean Sea.

The largest river both in volumetric discharge and coverage in the western drainage systems is the Abay(Table 4.1). Abay river basin covers an area of 199,812 km2, covering parts of Amhara, Oromia and Benishangul-Gumuz regional states. Together with its tributaries, most of which are left-bank tributaries; it carries 65 percent of the annual water flow of the region. Abay which 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. More than 60 streams drain the Abay within elevation ranging between 500 – 4261 meters above sea level. The largest of these is GhilgelAbay(Little Abay). Abay flows eastward, turns 180o to make a large bend and after cutting an impressive and deep gorge emerges out in the west.

Similarly, the Tekezeand its tributaries, carrying 12 percent of the annual water flow of the region drains 82,350 Km2 of land surface within elevation ranging between 536-4517 meters above sea level. Erosion in the basin resulted in large tablelands, plateau blocks and isolated mountain groups. The basin has two main tributaries (Angereb and Goang) which rises in the central highlands of Ethiopia. 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 (BMC, here after).

The Baro-Akoboand Ghibe / Omorivers drain the wettest highlands in the south and southwestern Ethiopia. They carry 17 percent and 6 percent of the annual water flow respectively. The Ghibe/ Omo river basin drains an area of 79,000 km2 with an estimated mean annual flow of 16.6 BMC. In the lower course, the Baro River flows across an extensive marshy land. BaroAkobo 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 Barotogether with Akobo forms the Sobat River in South Sudan. The Ghibe / OmoRiver finally empties in to the Chew-Bahir at the mouth of Lake Turkana (an elongated Rift Valley lake) thereby forming an inland drainage.

Nearly the entire physiographic region of southeastern part of Ethiopia is drained by the southeastern drainage systems. The basin which is mainly drained by Wabishebelleand Ghenale, slopes south-eastwards across large water deficient plains. Major highlands of this basin include plateaus of Arsi, Bale, Sidamaand Harerghe. Wabshebelle and Ghenale rivers cross the border into Somalia, carrying 25 percent of the annual water flow of Ethiopia.

Ghenale River basin has an area of 171,042 km2, covering parts of Oromia, SNNPR, and Somali regions. Ghenale, which has fewer tributaries 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.

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. Despite its size, the Wabishebelle 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.

The Rift Valley drainage system is an area of small amount of rainfall, high evaporation and small catchment area. The size of the drainage area is restricted by the outward sloping highlands, which starts right from the edge of the escarpment. The Rift Valley drainage system is therefore left with the slopes of the escarpment and the Rift Valley floor itself as the catchment area.

The only major river basin is that of the Awash. Awash river basin has a catchment area of 114,123 km2 and has an average annual discharge of 4.9 billion cubic meters. The Awash River originates from Shewan plateau in central highlands of Ethiopia, and flows 1250 kms. It covers parts of the Amhara, Oromia, Afar, Somali, Dire Dawa, and Addis Ababa City Administration. Awash is the most utilized river in the country.

In the Rift Valley drainage systems, there is no one general flow direction, as the streams flow in all directions. Following the Rift Valley orientation, the Awash flows in a northeast direction. It finally ends in a maze of small lakes and marshy area; the largest of which is Lake Abe on the Ethio-Djibouti border.

The Afar drainage sub-basin has practically no stream flow. It is an area of little rain, very high temperature and very high evaporation. Lake Afreraand Asaleare the only main surface waters in the basin which are not the result of any meaningful surface flow. Their formation is related to tectonic activities.

The Southern part of the Rift Valley sub-basin is characterized by a number of lakes and small streams. It is also 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, Mekiand KatarRivers flow into Ziway; Bilateinto Abaya; and Segeninto Chew Bahir. Likewise, some of these lakes are interconnected. Lakes Ziwayand Langanodrain into Lake Abijiatathrough the small streams of Bulbulaand Horocolorespectively.

Ethiopia River Basins

4.3. Water Resources: Rivers, Lakes and Sub-Surface Water

Why do majority of Ethiopian rivers end at seas and oceans?

4.3.1. The Ethiopian Rivers

Unlike many other African countries, Ethiopia is endowed with many rivers. Majority of the rivers originate from highland areas and cross the Ethiopian boundary. Altogether, Ethiopian rivers form 12 major watersheds (see fig. 4.1) separating the Mediterranean Sea from the Indian Ocean drainage systems.

Table 4.1: Data on major Ethiopian rivers

River

Catchment Area(km2)

Annual Volume BMC

Terminus/Mouth

Major tributaries

Abay

199,812

4.5

Mediterranean

Dabus, Dedessa, Fincha, Guder, Muger, Jema, Beshilo

Wabishebelle

202,697

3.4

Coast of Indian Ocean

RamisErer, DaketaFafan

GenaleDawa

171,042

6

Indian-Ocean

Dawa, Weyb, Welmel, Mena

Awash

114,123

4.9

Inland (within Ethiopia)

Akaki, Kesem, Borkena, Mile

Tekeze

87,733

8.2

Mediterranean

Goang, Angereb

Gibe (Omo)

79,000

16.6

Lake Turkana

Gojeb

BaroAkobo

75,912

23.23

Mediterranean

Akobo

Source: Compiled from different basin development master plans

General Characteristics of Ethiopian Rivers

Owing to the highland nature of the Ethiopian landmass, surface ruggedness, the outward inclination of the highlands, and the climatic conditions, Ethiopian rivers have the following characteristics.

 Almost all major rivers originate from the highlands elevating more than 1500 meters above sea level,

 Majority of Ethiopian rivers are trans-boundary,

 Due to the marked seasonality of rainfall, Ethiopian rivers are characterized by extreme seasonal fluctuation. In the wet season, runoff is higher and rivers are full bursting their banks, destroying small bridges, damage roads and flooding low lands; during the dry seasons they became mere trickles of water or even dry up,

 Due to surface ruggedness they have rapids and waterfalls along their course,

 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 the rivers serve as boundaries, both international and domestic administrative units

4.3.2. The Ethiopian Lakes

Why majority of lakes in Ethiopia do are clustered in rift valley system? We have lakes already dried and drying up. How do you think that happened?

Relatively Ethiopia is rich in lakes. Almost all Ethiopian lakes are result of tectonic process that took place during Quaternary period of Cenozoic era. Except few Ethiopian lakes, majority of lakes are located within the Rift Valley System. The lakes in the drainage are mainly formed on faulted depressions and are clustered along the system forming linear pattern.

Lake Tana, the largest lake in Ethiopia (Table 4.2) 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 the Crater Lake on top of Mount Zikwala. Lake Ashenge(Tigray) is formed on a tectonic basin. Other types of lakes in Ethiopia are man-made such as Lakes Koka, Finchaand MelkaWakena, and many other lakes dammed following hydroelectric power generation projects.

Cluster of lakes are lined up within main Ethiopian rift. Lake Abaya is the largest of all the lakes in the system. The southern tip of the Rift Valley forms the marshy land called the Chew Bahirwhich is drained by Seganand Woito. Shala and Ziway are the shallowest and the deepest lakes in the central Ethiopian Rift (Table 4.2).

Table 4.2: Area and depth of some of Ethiopian Lakes

Lake

Area (km2)

Max. Depth(m)

Lake

Area (km2)

Max. Depth(m)

Tana

3600

9

Abijata

205

14

Abaya

1162

13.1

Awassa

129

10

Chamo

551

13

Ashenge

20

25

Ziway

442

8.95

Hayk

5

23

Shala

409

266*

Beseka

48.5

11

Koka

205

9

 

Source: Compiled from different sources

4.3.3. Subsurface (Ground) Water Resource of Ethiopia

As compared to surface water resources, Ethiopia has lower ground water potential. However, there exists higher total exploitable groundwater potential. Climatic and geophysical conditions determine the availability of groundwater resource. Based on existing scanty knowledge, the groundwater potential of Ethiopia is estimated to be 2.6 – 6.5 BMC. However, this estimate is now considered underestimated. Considering various separate studies, Ethiopian potential of groundwater is believed to range between 12-30 BMC.

Ethiopia Lakes

4.4. Water Resources Potentials and Development in Ethiopia

The potential of exploitable water in Ethiopia is huge, so why is our agriculture still rainfed?  What do you think are reasons that diminish the irrelevance of Ethiopian rivers for transportation? The enormous water resource potential of Ethiopia is underutilized due to so many factors. However, there are plenteous of opportunities that can transform the resource into our collective social and economic needs. The followings are some of potential development uses of water resource of Ethiopia.

  1. a) Hydro-electric Potential

Ethiopian rivers have a very high potential for generating electricity. The exploitable potential of hydroelectric power is estimated at about 45000 megawatts. The first hydroelectric power generation plant was installed on Akaki River (Aba Samuel) in 1932. Currently many hydroelectric power dams are operating and many others are under construction to realize Ethiopia‟s ambitious energy goals. 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. Currently Ethiopia is administering 14 hydroelectric power plants constructed on Lake Aba Samuel, Koka, Tis Abay, Awash, MelkaWakena, Sor, Fincha, Gibe/Omo, TanaBeles and Tekeze, generating close to 4000 megawatts of energy.

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 is also a critical problem for hydroelectric power generation.

  1. b) Irrigation and Transportation

The lowlands demand for irrigation is high. Regardless of existing physiographic setups, Ethiopia‟s potential of irrigation is estimated to be 5.3 million hectares.

The Baro-Akobo and GenaleDawariver 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.

Majority of Ethiopian rivers are not suitable for transportation. The Baroat its lower course is the only navigable river. Comparatively, Ethiopian lakes are much suitable for transportation than rivers. Lake Tana and Abaya are relatively the most used for transportation.

  1. c) Fishing and Recreation

The majority of Ethiopian lakes are rich in fish. Currently the annual production of fish is estimated to be 31.5 thousand tons. The exploitable potential is however, by far greater than the current production. Exploitable fish potential in lakes varies. Currently Lake Tana leads the potential by estimated 8,000-10,000 tons per year. Fish production from Lake Chamo is estimated at 4,500 tons per year. However, more than 60% of fish supplies are coming from Ethiopian main 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.

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