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Three main drainage systems can be distinguished:
- The Mereb-Gash and Tekeze-Setit River systems, draining into the Nile River;
- The eastern escarpment and the Barka-Anseba River systems, draining into the Red Sea;
- The river systems of a narrow strip of land along the south-eastern border with Ethiopia, draining into the closed Danakil Basin.
Although no measurement of runoff is available, the internally produced renewable water resources are estimated at around 2.8 km│/yr, most of which are located in the western part of the country (Table 3).
There is only one perennial river, the Setit River, which also forms the border with Ethiopia. All other rivers are seasonal and contain water only after rainfall and are dry for the rest of the year. There are no natural fresh surface water bodies in the country. Artificially dammed water bodies are found here and there in the highland parts of the country.
Groundwater can be tapped in all parts of the country but not in the quantities and of the qualities desired. Four hydro-geological units, based on the different geological units, recharge conditions and hydraulic characteristics, can be detailed:
- Granular aquifers, which cover large areas in the western and eastern lowlands and along river valleys and flood plains. Unconsolidated aquifers consisting of the alluvial and colluvial sediments are also found in the Asmara area, Red Sea coastal plains and at the foot of fault scraps and mountains;
- Fissured and jointed volcanic aquifers, which are found in the central highland plateau southeast of Asmara and west of Assab, the Alid hot spring and in the southern part of the country;
- Fissured and karstic aquifers of consolidated sedimentary rocks, limestone, coral reefs, evaporate deposits and the marbles of metamorphic assemblages;
- Fissured aquifers of the basement rocks of crystalline metamorphic rocks and associated intrusive rocks, which are localized along weathered and fractured zones, with limited groundwater resources,
The recent water point inventory counts 5 365 water points. About 3 374 are unprotected dug wells and 1 233 are contaminated surface water points. Typical borehole depths are in the range of 20 to 70 m. Deep aquifers are not known. Problems of groundwater depletion have been reported in various parts of the country. Apparently there are a few natural springs, but an inventory is not available.
Currently there are about 187 dams with a capacity of over 50 000 m3 each. About 42 percent are for municipal use and irrigation, 40 percent for municipalities only, 13 percent for irrigation, and 5 percent are not used. The total capacity reaches 94 million m3.