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Ethiopia is endowed with a substantial amount of water resources but very high hydrological variability. The surface water resource potential is impressive, but little developed. The country possesses twelve major river basins, which form four major drainage systems (Table 2):
- The Nile basin (including Abbay or Blue Nile, Baro-Akobo, Setit-Tekeze/Atbara and Mereb) covers 33 percent of the country and drains the northern and central parts westwards;
- The Rift Valley (including Awash, Denakil, Omo-Gibe and Central Lakes) covers 28 percent of the country and consists of a group of independent interior basins extending from Djibouti in the north to the United Republic of Tanzania in the south, with nearly half of its total area being located in Ethiopia;
- The Shebelli-Juba basin (including Wabi-Shebelle and Genale-Dawa) covers 33 percent of the country and drains the southeastern mountains towards Somalia and the Indian Ocean;
- The North-East Coast (including the Ogaden and Gulf of Aden basins) covers 6 percent of the country.
All river basins except the Nile basin face water shortages (EU, 2011). Most of the rivers in Ethiopia are seasonal and there are almost no perennial rivers below 1 500 m altitude. About 70 percent of the total runoff takes place during the period June-September. Dry season flow originates from springs which provide base flows for small-scale irrigation.
Table 2 presents the internal runoff in each basin. Due to evaporative losses, not all flow reaches adjoining countries. For example, out of the 17 960 million m³/year of the Omo-Gibe in the Rift valley only 10 000 million m³/year arrive at the border with Kenya. Also, out of the 23 600 million m³/year of the Baro-Akobo in the Nile basin, only 13 000 million m³/year are seen at the border with South Sudan.
Intense rainfall sometimes causes flooding particularly along the Awash river and in the lower Baro-Akobo and Wabe-Shebelle river basins, causing damage to standing crops and infrastructures. The construction of dykes mitigated the problem but has not provided a long-lasting solution.
Ethiopia has 11 freshwater lakes and 9 saline lakes, 4 crater lakes as well as over 12 major wetland areas. Most of the largest lakes are found in the Rift Valley, except Lake Tana which is the source of Abbay River in the Nile Basin. Most Rift Valley lakes have no surface water outlets, i.e. they are endorheic, hence extremely saline. Lakes Langano, Abbaya and Chamo are freshwater lakes and not endorheic, but because of diminishing outflow, they are becoming increasingly saline. Most of the lakes are rich in fish. Lakes Shala and Abiyata have naturally high concentrations of chemicals and a soda ash operation is located on the shore of the latter for the production of sodium carbonate (IWMI, 2007). The total area of wetlands in Ethiopia is estimated between 1.4 and 1.8 million ha (EPA, 2003; IUCN, 2010). Floodplains are mostly found in the north-western and western highlands, the Rift Valley and the eastern highlands but some are also located in lowlands.
The groundwater potential of the country is not known with any certainty, but so far only a small fraction of the groundwater has been developed. It is however more easily available than surface water in the arid areas and supplies about 80 percent of the existing drinking water sources (EPA and UNEP, 2008). Traditional wells are widely used by nomads.
Internal renewable surface water resources are estimated at 120 000 million m³/year and renewable groundwater resources at around 20 000 million m³/year, but 18 000 million m³/year is considered to be overlap between surface water and groundwater, which gives a value of total internal renewable water resources (IRWR) of 122 000 million m³/year (Table 2). External water resources are null and the surface water leaving the country is estimated at 96 500 million m³/year, of which:
- 64 600 million m³/year flow into Sudan through the Blue Nile and its tributaries (52 600 million m³/year), the Atbara rive (4 370 million m³) and the Setit-Tekeze river (7 630 million m³/year);
- 13 000 million m³/year flow into South Sudan through the Baro and Akobo rivers forming the Sobat river;
- 8 200 million m³/year flow into Somalia through the Genale and Dawa rivers forming the Juba river (5 900 million m³/year) and the Shebelle river (2 300 million m³/year);
- 10 000 million m³/year flow into Kenya through the Omo river into Lake Turkana; and
- 700 million m³/year flow into Eritrea.
Ethiopia has many small, medium and large reservoir dams constructed for hydropower generation, irrigation and drinking water supply. There are currently 12 hydropower plants cumulating a total installed capacity of 1 945 MW (ODI, 2015). Micro-dams, i.e. dams with a water storage capacity of less than 0.15 million m³, have been constructed for small-scale irrigation, especially around 1999 and 2000 in the Amhara and Tigray regional states. Total dam capacity was estimated at 6 540 million m³ in 2008, and increased tremendously in recent years to reach about 31 484 million m³ in 2015. The new Tekeze dam, built on the eponym river in the Nile basin and completed in 2009, has a storage capacity of 9 000 million m³ and thus largely exceeds the previous largest Koka dam (1 900 million m³). The new Gilgel Gibe III dam, built on the Omo river, has a storage capacity of 14 000 million m³ and started to generate electricity after its completion in 2015. It will soon be supplanted by the controversial Grand Ethiopian Renaissance Dam (GERD) started in 2011 on the Abbay (Blue Nile) river close to the Sudanese border mostly for hydropower generation. The GERD dam, expected to create a huge reservoir of 79 000 million m³, is as of May 2016 almost 70 percent completed.
Desalinization is not practiced in Ethiopia, being a landlocked country, and treatment of wastewater is still marginal. The lack of wastewater treatment is partly explained by the way wastewater is collected since the major wastewater disposal system in Addis Ababa, but also in most large and medium cities of the country, is by vacuum trucks. Only 7.5 percent of the collected wastewater is collected by the sewerage system. The Kalitiy and Kotebe wastewater treatment plants are the only two in the capital. The produced municipal wastewater in the 9 main cities–Addis Ababa, Dire Dawa, Hawassa, Mekelle, Bahir dar, Adama,Gondar, Jimna ad Hara–is estimated in 2014 at 226 million m³, out of which only 0.8 million m³ is collected, mostly by trucks (MoWIE, 2015).