Ethiopia's Water Resources and Drainage Systems

• 71% of Earth’s surface is covered by water, mostly seas and oceans.
• 97.5% of Earth’s water is alkaline, accumulated in seas and oceans.
• The remaining 2.5% is fresh water.
• 68.7% of fresh water is in glaciers, 30.1% in groundwater, 0.8% in permafrost, and 0.4% in surface waters.
• Surface waters include lakes, rivers, the atmosphere, soils, and wetlands.
• Fresh water distribution is uneven due to latitudinal locations, climate, and topography.
• Ethiopia’s topography includes highlands, rugged terrain, and low plains.
• Ethiopia’s topography, rainfall, and location near the Equator contribute to larger volumes of ground and surface water.
• Water bodies cover about 0.7% of Ethiopia’s land mass.
• Ethiopia’s surface water potential is estimated at 124.4 billion cubic meters (BCM).
• Ethiopia is called the water tower of “Eastern Africa”.

Major Drainage Systems of Ethiopia

• Drainage is the flow of water through a well-defined channel.
• A drainage system includes a principal river and its tributaries.
• A river system starts at the source or headwater and ends at the mouth.
• A drainage system is a network of stream channels and adjacent land slopes.
• Drainage pattern results from geological processes, rock nature/structure, topography, slope, flow amount/periodicity.
• A drainage basin is a topographic region where a river and tributaries collect surface runoff and subsurface flow.
• A drainage basin is bounded and separated from other basins by a water divide or topographic divide.
• Major river basins in Ethiopia are determined by:
  • Outward sloping topography of the Western and Southeastern plateaus
  • Structural formation of the Rift Valley with inward-sloping escarpments (inland drainage)
  • Faults and joints influencing river courses
• Ethiopia has three drainage systems: Western, Southeastern, and Rift Valley.
• The Rift Valley system separates the Western and Southeastern drainage systems.

Western Drainage Systems

• The Western Drainage systems drain 40% of the country and carry 60% of the annual water flow.
• The catchment area is the westward sloping part of the western highlands and lowlands.
• The Western Drainage includes the river basins Tekeze, Abay, Baro-Akobo, and Ghibe (Omo),
• Ghibe (Omo) flows southward, unlike other rivers in the system.
• Abay, Tekeze, and Baro flow westward, joining the Nile, which ends at the Mediterranean Sea.
• The Abay is the largest river in the western drainage systems in both volumetric discharge and coverage.
• The Abay River basin covers an area of 199,812 km², including parts of Amhara, Oromia, and Benishangul-Gumuz.
• Abay rises from Lake Tana (some sources indicate its origin from Sekela, Choke mountain)
• Abay flows about 1,450 kilometers and joins the White Nile in Khartoum, Sudan to form the Nile River.
• Abay carries 65% of annual water flow for the region
• More than 60 streams drain the Abay within elevation ranging between 500 - 4261 meters above sea level
• The largest of these is Ghilgel Abay (Little Abay)
• It has left-bank tributaries.
• The Tekeze River and its tributaries carry 12% of the annual water flow
• Tekeze drains 82,350 km² of land surface with elevations between 536-4517 meters.
• Erosion in the Tekeze basin has resulted in large tablelands, plateau blocks, and mountain groups.
• The Tekeze basin has two main tributaries (Angereb and Goang) which rises in the central highlands of Ethiopia.
• Tekeze River is termed Atbara in Sudan.
• The total mean annual flow from the Tekeze basin is estimated to be 8.2 billion metric cubes (BMC).
• The Baro-Akobo and Ghibe / Omo Rivers drain the wettest highlands in the south and southwestern Ethiopia.
• They carry 17% and 6% 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.
• The 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 Baro Akobo river basin is estimated to be 23.6 BMC.
• The Baro joins with Akobo, forming the Sobat River in South Sudan.
• The Ghibe / Omo River empties into Chew-Bahir at the mouth of Lake Turkana, forming an inland drainage.

Southeastern Drainage Systems

• The Southeastern drainage systems drain the southeastern part of Ethiopia.
• This basin is mainly drained by Wabishebelle and Ghenale and slopes south-eastwards across plains deficient in water.
• Major highlands of this basin include plateaus of Arsi, Bale, Sidama, and Harerghe.
• Wabshebelle and Ghenale cross into Somalia, carrying 25% of Ethiopia's annual water flow.
• Ghenale River basin drains covers 171,042 km², including parts of Oromia, SNNPR, and Somali regions.
• Ghenale has fewer tributaries but carries more water than Wabishebelle, reaching the Indian Ocean.
• The Genale basin flows are estimated to be 5.8 BMC with elevations of 171-4385 meters above sea level.
• In Somalia, the Genale is called the Juba River.
• Wabishebelle has a total catchment area of 202,697 km² and is the largest in terms of catchment area.
• Wabishebelle drains parts of Oromia, Harari, and the Somali regions and is the longest river in Ethiopia.
• Its tributaries are mainly left bank and intermittent.
• The Wabishebelle fails to reach the Indian Ocean; it flows parallel to the coast and disappears in the sands near the Juba River.

Rift Valley Drainage System

• The Rift Valley drainage system has little rainfall, high evaporation, and a small catchment area.
• The size of the drainage area is restricted by the outward sloping highlands.
• The slopes of the escarpment and the Rift Valley floor serve as the catchment area.
• The only major river basin here is the Awash.
• The Awash river basin has a catchment area of 114,123 km² and 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 Ethiopia.
• Streams in the Rift Valley drainage systems flow in all directions.
• The Awash flows in a northeast direction and ends in a maze of small lakes and marshy areas.
• Lake Abe on the Ethio-Djibouti border is the largest of these lakes and mashy area.
• The Afar drainage sub-basin has practically no stream flow due to little rain, high temperature, and high evaporation.
• Lake Afrera and Asale are the only main surface waters in the Afar basin, not resulting from surface flow, but from tectonic activities.
• The southern part of the Rift Valley sub-basin has many lakes and small streams and is described as a lakes region.
• The lakes occupy fault depressions.
• Small streams drain from nearby mountain slopes, supplying water to the lakes.
• For example, Meki and Katar Rivers flow into Ziway; Bilate into Abaya; and Segen into Chew Bahir.
• Some lakes are interconnected.
• Lakes Ziway and Langano drain into Lake Abijiata through the small streams of Bulbula and Horocolo, respectively.

Ethiopian Rivers

• Ethiopia has many rivers, unlike other African countries.
• Most of the rivers originate from highlands and cross the Ethiopian boundary.
• Ethiopian rivers form 12 major watersheds, separating the Mediterranean Sea from the Indian Ocean drainage systems.
• Almost all major rivers originate from highlands above 1500 meters.
• Majority of Ethiopian rivers are trans-boundary.
• Ethiopian rivers have extreme seasonal fluctuations due to rainfall patterns.
• During the wet season, runoff is higher, causing rivers to burst their banks, damaging infrastructure and flooding lowlands.
• During the dry seasons, rivers become mere trickles or dry up.
• They have cuts, steep-sided river valleys, and deep gorges along their courses.
• Owing to the rugged Ethiopian landscape and outward inclination of the highlands, Ethiopian rivers have rapids and waterfalls.
• Rivers in Ethiopia flow on steep slopes, having steep profiles.
• Some rivers serve as international and domestic boundaries.

Ethiopian Lakes

• Most Ethiopian lakes are the result of tectonic processes from the Quaternary period of the Cenozoic era.
• Most lakes are in the Rift Valley System.
• The lakes are mainly formed on faulted depressions and clustered along the system, forming a linear pattern.
• Lake Tana, Ethiopia’s largest lake, is in a shallow depression in the highlands.
• The Tana depression is believed to have formed after sinking and reservoir by lava flow between Gojjam and Gonder massifs.
• Ethiopia has crater lakes around Bishoftu, Wonchi (near Ambo), Hayk (near Dessie), and on top of Mount Zikwala.
• Lake Ashenge (Tigray) formed in a tectonic basin.
• Man-made lakes in Ethiopia include Koka, Fincha, and Melka Wakena, created by damming for hydroelectric power generation.
• Clusters of lakes are aligned within the main Ethiopian rift.
• Lake Abaya is largest lake in the system.
• The southern tip of the Rift Valley is marshy land called the Chew Bahir drained by Segan and Woito.
• Shala and Ziway are the shallowest and the deepest lakes in the central Ethiopian Rift.

Subsurface (Ground) Water Resource of Ethiopia

• Compared to surface water, Ethiopia has lower groundwater potential.
• Higher total exploitable groundwater potential exists.
• Climatic and geophysical conditions determine groundwater availability.
• Groundwater potential is estimated to be 2.6 - 6.5 BMC, but is now considered underestimated.
• Ethiopian potential groundwater is believed to range between 12-30 BMC based on different studies.

Water Resources Potentials and Development in Ethiopia

• Ethiopia has a high hydroelectric potential in its rivers, estimated at about 45000 megawatts.
• The first hydroelectric power plant was installed on the Akaki River (Aba Samuel) in 1932.
• Grand Ethiopian Renaissance Dam (GERD) is the largest dam under construction, aiming to generate 6400 megawatts.
• Gilgel Gibe III hydropower project generates 1870 megawatts.
• Ethiopia manages 14 hydroelectric plants on Lake Aba Samuel, Koka, Tis Abay, Awash, Melka Wakena, Sor, Fincha, Gibe/Omo, Tana Beles and Tekeze, generating close to 4000 megawatts.
• Besides domestic use, Ethiopia exports electricity.
• Major problems include seasonal flow fluctuations, climate change impacts, erosion, and sedimentation in reservoirs.
• Ethiopia irrigation potential is estimated to be 5.3 million hectares.
• The Baro-Akobo and Genale Dawa river systems have high irrigation potential, though untapped.
• Over 60% of the area under irrigation is in the Rift Valley Drainage System.
• Most hydro-electric reservoirs are multi-purpose and help with irrigation.
• Most Ethiopian rivers are not suitable for transportation.
• The Baro is only navigable river at its lower course .
• Lake Tana and Abaya are used for transportation.
• Most Ethiopian lakes are rich in fish, with an annual production of 31.5 thousand tons.
• Greater exploitable potential exists.
• Lake Tana leads the potential at 8,000-10,000 tons per year.
• Lake Chamo produces 4,500 tons per year.
• Over 60% of fish comes from Rift Valley lakes.
• Some lakes are threatened by sedimentation, invasive species (water hyacinth), overexploitation, and expansion of investments.
• Ethiopian rivers and lakes are tourist spots because of scenic beauty and natural habitat for wildlife including endemic species.