Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
5.4.4 Hydrology
Downstream of the dam the Omo river flows “entrenched” for about 200 km and through an “incision”,
generally more than 10 m deep, for about 150 km.
Along the reach heading W several small though perennial effluents (Sana, Deme, Zage, Irgene) join the
river draining the highlands from E/S. Average rainfall in these sub-basins ranges from about 1500 mm/year
(south) to 1000 mm/year (east).
Along the reach heading SW the Zinga and Mansa tributaries (smaller) and the Denchiya and Toshi/Sherma
rivers (larger) join the Omo after draining the upper highlands (mainly covered with a vegetation of wooden
shrubland). The average rainfall ranges from about 1600 mm (north) to 900 mm (south).
About 80 km downstream the longitudinal slope of the riverbed flattens and gorge opens as the river crosses
a hilly and semi-arid landscape.
The river changes in characters with a meandering river channel ending at the river delta area. In this stretch
the river is clearly in an evolutionary process, probably related also to the “shrinking” of the Turkana lake,
where abandoned meanders and upcoming ones are clearly visible.
Along this stretch the Aku and particularly the Maiki, tapering streams with perennial tributaries, drain the
highlands along the Omo basin boundaries.
Average rainfall in this area ranges from about 1500 mm/year (north-west) to 500 mm/year (south).
The Omo River then flows south across the arid plains and the Delta for some 100 kms.
The total catchment of the Omo at the Lake Turkana is about 73500 km 2 .
The average rainfall within the whole Omo River watershed varies from 1900 mm/year (north-west) to less
than 300 mm/year (south).
The mean annual flows of the Omo River at the Lake Turkana have been evaluated 20 by means of an
estimate of the mean flows in the residual basin Gibe III-Turkana.
Since no robust flow measurements series are available downstream Abelti/Gojeb the analysis has been
carried out by elaborating rainfall data of the residual basin.
The table below shows the rainfall rates, the runoff coefficient and the mean flows for the main subcatchments
of the Omo basin: the mean annual rainfall for the residual basin downstream Gibe III site
(denoted as “Residual 2”) is 870 mm/year.
This value is far below the rainfall rate of the upper part of the basin, characterised by a wetter climate.
20 300 ENV RAG003A Additional Study on Downstream Impact – April 2008
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