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The Rift Valley

The Rift Valley, located in Eastern Africa, covers just over 2% of the continent and spreads over seven countries (Map 5 and Table 29).

Table 29: The Rift Valley: areas and rainfall by country

The Rift Valley consists of a group of independent interior basins, extending from Djibouti in the north to Tanzania in the south, nearly half being located in Ethiopia.

Rivers and discharges

The Danakil basin is a very dry basin and only rainfall of more than 10 mm results in rapid floods lasting not more than a few hours. Annual runoff is less than 1 km³.

Lake Abbé, a salt lake on the border between Djibouti and Ethiopia, is in the Awash basin. The main part of the Awash basin is in Ethiopia, with annual rainfall ranging from 200 mm in the north to over 1900 mm in the south. The annual runoff in this basin is estimated at 4.6 km³ [108].

Table 30: The different basins within the Rift Valley

The Central lakes basin, which groups several lakes, is also mainly located in Ethiopia, with a small part continuing into Kenya. Total annual runoff is estimated at 5.64 km³ [108].

The Omo-Gibe basin, with rivers flowing into Lake Turkana (also called Lake Rudolph) is mainly located in Ethiopia and Kenya, with small parts in Sudan and Uganda. From Ethiopia the Omo and Gibe Rivers flow into the lake, while from Kenya the Turkwel and Kerio Rivers flow into the lake. Annual runoff in this basin is estimated at 16.1 km³ [108].

In the southern part of Kenya and the northern part of Tanzania the Southern Lakes basins are grouped, of which Lake Natron and Lake Eyasi are the most important ones.

Irrigation potential and water requirements

Agriculture in Djibouti is only possible with irrigation. The cultivable land is estimated at 6000 ha, but only 674 ha are equipped for irrigation, of which 300 ha are in the Rift Valley [93]. No detailed information is available on irrigation potential, but with the available water resources it has been estimated at 1000 ha, of which 450 ha are estimated to be in the Rift Valley [*].

A narrow strip along the south-eastern border of Eritrea drains into the Danakil depression. Due to its closed topography and arid climate, it is characterized by highly saline soils and groundwater and has little agricultural potential. The potential for irrigation is estimated to be negligeable [*].

Most of the irrigation developed to date in Ethiopia is located in the Awash basin. The irrigation potential in the Rift Valley region in Ethiopia is estimated at 790000 ha, distributed over the different basins as follows [106]:

Table 31: Water resources, irrigation potential and water requirements in the different Rift Valley basins in Ethiopia [106]

While the total water requirement is only one-fourth of the annual runoff, the development of the irrigation potential would require important storage works.

Less than 1% of Sudan lies in the Rift Valley. It is a swampy area, with no information available on water resources and irrigation potential.

The border between Uganda and Kenya coincides more or less with the watershed line of the Rift Valley basin. In Uganda the water resources are rather limited. The irrigation potential is estimated to be negligible [*].

The difference in climate in the Rift Valley in Kenya is quite distinct. The rainfall is considerable, more than 1500 mm/year at the edges of the Rift Valley and decreasing rapidly to under 200 mm in the valley bottom. In this study, irrigation water requirements are estimated at 10500 m³/ha per year in the north and at 12000 m /ha per year in the south. The irrigation potential, identified in the national water master plan and based on 80% dependable flow, is estimated at about 35900 ha in the northern Omo-Gibe basin and 16600 ha in the southern Lower Ewaso Ng'iro basin [125]. For the Kerio Valley, located in the Omo-Gibe basin, a special development authority has been established by the Kenyan Parliament to study the possibility of transferring water from the Lake Victoria basin to this basin (as explained in the section The Nile basin).

For Tanzania an irrigation potential of only 1060 ha has been identified in this area [199]. This basin is also the location of the Vembere Plateau of the Manonga River basin, an area for which plans to transfer water from the Lake Victoria basin have existed since the beginning of the century, as explained in the section The Nile basin.

Table 32 summarizes the irrigation potential and the water requirements for the whole Rift Valley region.

Table 32: Rift Valley: irrigation potential water requirements and areas under irrigation

No water problems are expected for the development of this potential, though a lot of storage works will be necessary.

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