Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
Climate and rainfall features
The climate is classifiable as tropical humid in the highlands that include the areas surrounding Jima and
around the headwaters of the Gojeb River. For the rest, and greatest part, of the watershed the climate is
classifiable as a tropical sub-humid, intermediate between the tropical humid and the hot arid climate
characteristic of the southernmost part of the floodplain toward Lake Turkana. The seasonal variation in
climate is associated with the oscillation of the Inter-Tropical Convergence Zone (ITCZ), a low pressure area
of convergence. The ITCZ shifts during the year northwards across southern Ethiopia from September to
November and southwards from March to May, giving origin to the alternation of a wet (from June to
September) and a dry (from December to April) season. During the wet season the area is under the influence
of Atlantic equatorial westerlies and southerly winds from the Indian Ocean, producing strong precipitation,
mainly due to the Atlantic moist component. During the dry season the moist air comes from the Gulf of
Aden and the Indian Ocean, causing little rains.
As the main source of moist air is from the Atlantic Ocean, from the SW, the eastern parts of the highlands
are more or less rain shadowed. This effect can be seen on the isohyetal map of the area, reported in the Omo
River Master Plan (Figure 5.4). The area of greatest rainfall is to the NW of Jima (outside the Gibe Basin).
Rainfall declines sharply in the lower lying southern parts of the basin. According to the isohyetal map of the
OGMP, the rainfall ranges from over 1900 mm/year in the northern and western part of the basin, to about
1200 mm/year in the lower part of the project area. Rainfall rates decrease strongly southwards, to less than
300 mm/year near Lake Turkana.
Hydro-meteorological data
The hydrological elaborations reported in the following sections are mainly based on rainfall and runoff data
provided mainly by gauges of the Ethiopian National Meteorological Service Agency and Ethiopian Water
Resources Authority. In addition external data were used coming only from officially approved documents
related to other project in the same area. These documents are:
1) the already mentioned “Omo-Gibe River Basin Integrated Development Master Plan Study”, Vol. VI -
Water Resources Surveys and Inventories, Dec 1996, carried out by Richard Woodroofe & Associates
for the Ministry of Water Resources of the Federal Democratic Republic of Ethiopia,
2) the Gilgel Gibe Hydroelectric Project, Hydrological Report, Nov 1995, carried out by ELC
Elettroconsult and ENEL SpA for the Ministry of Mines and Energy - Ethiopian Electric Light and
Power Authority,
3) the Gojeb Medium Hydropower Project, Pre-Feasibility Study, Vol.1 - Executive Summary, Vol. 3 -
Annex A, Hydrology, Oct 1996, carried out by Howard Humphreys, Coyne et Bellier, Rust Kennedy &
Donkin,
4) the Feasibility Study of Halele-Werabesa Stage II Hydropower Project, Main Report, Dec 2004, carried
out by Lahmeyer International GmbH for the Ethiopian Electric Power Corporation.
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