Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
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 May 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 5.1). 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.
In the project area, the average annual air temperature is 20.4 °C.
Table 5.1, Table 5.2 and Table 5.3 show monthly maximum, minimum and mean values of temperatures,
recorded at Sekoru, Wolayita Sodo, Gojeb and Agaro stations. These stations, referring to dam location, are
sited respectively upstream the reservoir, in the middle between the dam and end of the reservoir,
downstream the dam.
Table 5.1: Maximum monthly temperatures (°C)
Jan. Feb Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
Agaro 29.3 30.2 30.2 29.9 29.4 27.8 25.9 26.1 27.0 27.8 28.2 28.5
Gojeb 31.6 32.0 32.1 31.2 30.5 28.8 28.0 28.0 28.5 29.6 30.7 31.1
Sekorru 27.3 28.2 28.5 27.5 26.9 24.9 23.1 23.1 24.0 25.8 27.0 27.2
Walyita Sodo 25.9 26.8 26.7 25.5 24.2 22.6 21.5 21.8 23.1 24.2 25.4 25.9
Table 5.2: Minimum monthly temperatures (°C)
Jan. Feb Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
Agaro 11.0 12.0 13.0 13.5 13.2 13.2 13.0 13.0 12.6 12.1 11.3 10.8
Gojeb 12.7 14.1 15.4 15.7 16.4 16.1 15.6 15.4 15.4 14.4 13.1 12.7
Sekorru 12.4 13.4 13.9 14.3 14.2 13.4 13.0 13.1 12.8 12.3 12.0 12.1
Walyita Sodo 13.2 14.0 14.5 14.1 13.8 13.2 12.6 13.0 12.9 13.0 13.4 13.2
Table 5.3: Mean monthly temperatures (°C)
Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.
Agaro 20.1 21.1 21.6 21.7 21.3 20.5 19.5 19.6 19.8 20.0 19.8 19.7
Gojeb 22.2 23.6 23.8 23.5 23.5 22.5 21.8 21.7 22.0 22.0 21.9 21.9
Sekorru 19.9 20.8 21.2 20.9 20.6 19.2 18.1 18.1 18.4 19.1 18.0 19.7
Walyita Sodo 19.6 20.4 20.6 19.8 19.0 17.9 17.1 17.4 18.0 18.6 19.4 19.6
The annual rainfall of the Gibe catchments area, measured in the same stations, varies from a minimum of
1,230 mm in the Wolayita Sodo station, to a maximum of about 1,640 mm in the Agaro station. Rainfall
decreases throughout the catchments with a decrease in elevation. The average annual rainfall calculated
over the whole Gibe III basin where the site 5 dam is located is 1,426 mm. Table 5.4 shows monthly mean
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