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Chapter 6: Review of existing information on irrigation potential

Methodology and limitations
Results per major basin group
Data quality assessment

As explained in Chapter 2, the African continent has been divided into 24 major hydrological units, each unit corresponding either to a large river basin or to a group of smaller river basins (Figure 2). The term 'basin', indicating the major hydrological unit, is used in the text and tables of this report.

Table 9 presents some general characteristics of the 24 major basin groups are presented.

TABLE 9: Major basin groups: areas and rainfall (Hutchinson et al. 1995 [23])

The North Interior, which corresponds to the Saharan desert, occupies nearly 20% of the African continent. Rainfall is extremely low in this region, with an annual average of only 40 mm (Figure 7), and the irrigation potential is less than 0.2% of the irrigation potential of the whole continent.

The Congo/Zaire River basin, the West Coast, the West Central Coast and Madagascar are the four wettest regions, with an average annual rainfall of over 1400 mm, and they also occupy about 20% of the African continent. The sum of the irrigation potentials of these four regions is more than 40% of the irrigation potential of the whole continent. This is 200 times the irrigation potential of the North Interior for same area.

Methodology and limitations

For each basic unit located within the major basin (see Table 1) all available information was systematically reviewed and cross-checked with the results of the studies of the previous chapters. When discrepancies were found, country based information was generally given precedence over the continental figures.

One difficulty doing a literature review at continental scale, involving 53 countries and over 1000 references, is that of inconsistency of information. Although the focus has been on the physical irrigation potential, some figures found in the literature might already have also taken into consideration other aspects, such as economic or environmental ones (without, however, explicitly mentioning them). Country studies may implicitly include some assumptions on a reasonable level of investment and demand, and allow for other constraints, like environmental and social factors.

In terms of discharge there is no unique period of reference. This can have an important impact on average discharges and thus water availability over different periods. For example, the average annual discharge of the White Nile entering Sudan from Uganda during the period 1961-1980 (50 km³/year) was nearly twice the average annual discharge during the period 1905-1960 (27 km³/year). The recent drought years in southern Africa also lead to different averages depending on the period of reference considered. Furthermore, progressive development of agriculture and other water uses reduces discharge and prevents correct assessment of natural flow. All information available on discharges has been reported on the Maps 1 to 22 at the end of this chapter. Discharges are average figures and all figures have a reference. However, as they refer to different periods of reference, they should not be considered as giving a consistent overview of the river discharges of the African continent, but rather as indicative figures.

This review gives no details on seasonal variations of flows, which necessitate the construction of storage reservoirs, except for the Nile and the Niger basins which have been studied more in detail. In general such information is available where national water master plans have been drawn up.

Nor does this review give details on the distance and elevation between suitable land and available water, though irrigation potential figures given by country often take this factor into consideration.

The literature reviewed did not always clearly indicate whether the irrigation potential figure refers to total potential or to identified potential.

This review concentrates on surface water resources, except for arid regions, where the use of groundwater for irrigation purposes already plays an important role, and for the cases where information is readily available. Only renewable groundwater was taken into consideration and not the fossil water resources. In general, the priority use of groundwater was considered to be for other purposes (domestic, livestock, etc.), but this report gives no details of other water requirements. Such information is available where national water master plans exist. Oases were not studied in detail, although they may sometimes use renewable water (Mauritania).

For the sake of simplicity, this review considers that if a certain quantity of water is abstracted upstream, the same quantity is subtracted from the resource downstream, except for those basins where a detailed description of the relation between upstream and downstream abstractions is available.

Unsurprisingly, more information is available for arid countries, where water is a limiting factor to agricultural production, than for humid countries, where water is abundant. For those humid countries for which no information was available, estimates or interpolations, based on figures in other similar regions, were combined with results from the GIS study (see chapter 4) to assess the irrigation potential. Where only global figures were available for a country as a whole, the distribution over the different basic units was estimated on the basis of information on land, water and population. Every time the estimate had to be made for one of the two reasons cited above, it is indicated by an asterisk '[*]'.

Results per major basin group

The Senegal River basin
The Niger River basin
The Lake Chad basin
The Nile Basin
The Rift Valley
The Shebelli - Juba basin
The Congo/Zaire River basin
The Zambezi basin
The Okavango basin
The Limpopo basin
The Orange basin
The South Interior
The North Interior
The Mediterranean coast
The North West Coast
The West Coast
The Gambia River Basin
The Volta Basin
The West Coast, excluding the Gambia River and Volta basins
The west central coast
The south west coast
The South Atlantic coast
The Indian Ocean coast
The East Central coast
The North East coast
Madagascar
Islands

For each of the 24 major basin groups, a description is given of the main river system and discharges, the irrigation potential, irrigation water requirements and the areas already under irrigation at present. An evaluation of the irrigation potential for each of the 136 basic units as well as for the 53 countries and 24 major basin groups as a whole is given in chapter 8.

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