Geography: Water and drainage

• About 71% of the earth's total surface is covered by water bodies, majorly occupied by seas and oceans
• Composition of the Earth's total water surface

  • Nearly 97.5% is alkaline accumulated in seas and oceans
  • The remaining 2.5% is fresh water
• Composition of the Earth's total water

  • Nearly 68.7% in glaciers
  • 30.1% in ground water
  • 0.8% in permafrost
  • 0.4% in surface water
• Surface waters


  Water in lakes, rivers, atmosphere, soils and wetlands
• Surface and ground waters


  The most abundant and easily available fresh waters
• Fresh water is unevenly distributed throughout the world due to varied latitudinal locations, varied climatic conditions and varied topographic setups
• Ethiopia

  Has larger volume of ground and surface water due to diverse topographical setup, relatively higher rainfall and its nearness to equator
• Drainage


  The flow of water through well-defined channel
• Drainage system


  A principal river and its tributaries
• Drainage basin


  The topographic region from which a river and its tributaries collect both the surface runoff and subsurface flow
• Water divide/Topographic divide


  Boundary that separates one drainage basin from another
• Major drainage systems in Ethiopia

  • Western Drainage Systems
  • Southeastern Drainage Systems
  • Rift Valley Drainage Systems
• Western Drainage Systems

  • Largest of all drainage systems, draining 40% of the total area of the country and carrying 60% of the annual water flow
  • Comprises four major river basins: Tekeze, Abay, Baro-Akobo, Ghibe (Omo)
• Abay River


  Largest river in the western drainage systems, covering an area of 199,812 km2 and carrying 65% of the annual water flow of the region
• Tekeze River


  Carries 12% of the annual water flow of the region, draining 82,350 km2 of land surface
• Baro-Akobo River


  Drains the wettest highlands in the south and southwestern Ethiopia, carrying 17% of the annual water flow
• Ghibe/Omo River

  Drains an area of 79,000 km2 with an estimated mean annual flow of 16.6 BMC, carrying 6% of the annual water flow
• Southeastern Drainage Systems

  • Mainly drained by Wabishebelle and Ghenale Rivers, carrying 25% of the annual water flow of Ethiopia
• Ghenale River Basin


  Has an area of 171,042 km2, with an estimated basin flow of 5.8 BMC
• Wabishebelle River Basin

  Largest river basin in terms of catchment area, with a total catchment area of 202,697 km2
• Rift Valley Drainage System

  • Small amount of rainfall, high evaporation and small catchment area, with the only major river basin being the Awash
• Awash River Basin


  Has a catchment area of 114,123 km2 and an average annual discharge of 4.9 billion cubic meters, covering parts of Amhara, Oromia, Afar, Somali, Dire Dawa, and Addis Ababa City Administration
• At the end of Wabishebelle's journey, it sinks or disappears in the sands near the Juba River
• Rift Valley Drainage System

  • Small amount of rainfall
  • High evaporation
  • Small catchment area
• The only major river basin in the Rift Valley drainage system is the Awash River basin
• Awash River basin

  • Catchment area of 114,123 km2
  • Average annual discharge of 4.9 billion cubic meters
• Awash River

  1. Originates from Shewan plateau in central highlands of Ethiopia

  2. Flows 1250 kms
• Regions covered by the Awash River basin

  • Amhara
  • Oromia
  • Afar
  • Somali
  • Dire Dawa
  • Addis Ababa City Administration
• Awash is the most utilized river in Ethiopia
• In the Rift Valley drainage systems, there is no one general flow direction as the streams flow in all directions
• Awash River


  1. Flows in a northeast direction

  2. Ends in a maze/network of small lakes and marshy area
• The largest lake in the Rift Valley drainage system is Lake Abe on the Ethio-Djibouti border
• Afar drainage sub-basin


  • Practically no stream flow
  • Little rain
  • Very high temperature
  • Very high evaporation
• Lake Afrera and Asale are the only main surface waters in the Afar drainage sub-basin, which are not the result of any meaningful surface flow
• Formation of Lake Afrera and Asale


  Related to tectonic activities
• Southern part of the Rift Valley sub-basin

  • Characterized by a number of lakes and a number of small streams
  • Lakes occupy fault depressions
  • Small streams drain down from nearby mountain slopes and supply water to the lakes
• Rivers flowing into lakes in the Southern part of the Rift Valley sub-basin


  • Meki and Katar Rivers flow into Lake Ziway
  • Bilate into Lake Abaya
  • Segen into Chew Bahir
• Some of the Rift Valley lakes are interconnected, e.g. Lake Ziway drains into Lake Abijiata through the small stream of Bulbula, and Lake Langano drains into Lake Abijiata through the small stream Horocolo
• Ethiopia is endowed with many rivers
• Majority of the rivers in Ethiopia originate from highland areas and cross the Ethiopian boundary