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Chapter 8: General results and conclusions
Physical resources
Review of existing formation on irrigation potential
Conclusions
This report describes the different steps leading to the assessment of the irrigation potential for Africa, as presented in a schematic way in Figure 1. It concentrates mainly on the physical factors controlling irrigation potential - land and water - and only deals with renewable water resources. However, the country studies used in the assessment may implicitly include some assumptions on a reasonable level of investment and allow for other constraints such as environmental and social factors, or the use of non-renewable water resources.
The African continent was divided into 24 major hydrologic units, or major basin groups, as presented in Figure 2 and Table 9. These 24 units were combined with the 53 African countries to obtain 136 basic land units, which were the basis for all calculations and for the information gathered and analysed in this study. These 136 basic units are presented in Tables 1 and 2.
The soil and terrain suitability for surface irrigation is presented in the Figures 3 to 5 and Tables 4 and 5.
Figure 3 shows the soil and terrain suitability per type of crop (rice and upland crops, rice being given priority for land suitable in both cases). Table 4 and Figure 4 present the total area of land suitable for surface irrigation as a percentage of the area of the basin for each of the 24 major basin groups. Table 5 and Figure 5 present it as a percentage of the area of the country for each of the 53 countries.
The approach used to compute soil suitability for irrigation has its limitations in the fact that it is based on the information obtained from the 1:5000000 soil map of the world [1]. In particular, the results have proved sensitive to several selection criteria (see Chapter 3), like terrain slope, and no account is taken of distance and elevation of suitable land in relation to water sources. Nonetheless, the results give a fair idea of the distribution of land for irrigation over the continent. Both figures show that the Sahara Desert in northern Africa has the smallest percentage of land suitable for surface irrigation (< 10%). The suitable land of Egypt is concentrated in the Nile Valley and Delta. The southern African region also has relatively little suitable land. The most suitable areas are located in central Africa (part of the Zambezi basin, the Congo/Zaire and upper Nile basins) and in the Shebelli-Juba basin.
The Sahara Desert is the driest region with an average annual rainfall of less than 40 mm (Table 9 and Figure 7). The West Central Coast is the most humid region with an average annual rainfall of almost 1800 mm, followed by Madagascar (1700 mm), Congo/Zaire (1470 mm) and the West Coast (1435 mm). For all the other major basin groups, average annual rainfall is less than 1000 mm.
Figure 6 and Table 6 show the internal renewable water resources for the 53 African countries. The differences between arid and humid regions are clearly demonstrated. Total annual internal renewable water resources are less than 50 km³ for the whole northern African region and almost 2000 };m³, or 40 times as much, for the central African region, while the surface areas of both regions are more or less the same.
Irrigation water requirements (IWR)
The assessment of irrigation potential, based on land and water resources, can only be done through the assessment of the irrigation water requirements, which is a function of cropping pattern and climate (rainfall and evapotranspiration). For the purposes of the present study a methodology was developed to assess irrigation water requirements in Africa. Twenty-four irrigation cropping pattern zones were defined, being considered homogeneous in terms of crop calendar, cropping intensity and irrigation efficiency (Table 7 and Figure 8). Irrigation water requirements were computed for different scenarios using the climate data from the FAOCLIM cd-rom and the FAO CROPWAT model in combination with GIS. The results are presented in the Figures 10 and 11. A total of 84 homogeneous irrigation water requirement zones were defined. Table 8 summarizes the data obtained for each of the 84 zones.
Comparing the results with figures available from country studies shows that the methodology manages to assess regional estimates of IWR relatively well. Discrepancies with country studies find their origin mostly in the cropping pattern, cropping intensity and irrigation efficiency scenarios.
Review of existing formation on irrigation potential
The review and compilation of existing information on irrigation potential is the main component of this study. Most of the irrigation potential studies are based on physical criteria, but implicitly account for technical and economic considerations by concentrating on areas where irrigation is economically feasible (market, demand) and does not present technical difficulties (access to land and water). All the information was cross-checked with the results of the studies on soil and terrain suitability, water resources and irrigation water requirements and completed where necessary.
Maps I to 22 show the information collected on annual discharges for each major basin group. It is important to stress that the review concentrated mainly on surface water resources. In arid regions, where the use of groundwater for irrigation purposes already plays an important role, groundwater was considered in the study. Only renewable groundwater was taken into consideration and not the fossil water resources. This choice can lead to considerable discrepancies for countries which include fossil water in their computation of irrigation potential. Libya, for instance, estimates its irrigation potential at 750000 ha, while this study mentions only 40000 ha, based on renewable water resources. It was beyond the scope of this study to discuss the use of fossil water and it was removed for the sake of homogeneity in computation.
In most country studies the need for sharing water between agriculture, industries, communities and other uses is taken into account in the assessment of irrigation potential. The fact remains that the part allocated to agriculture depends on assumptions on the rate of development of the other sectors.
Tables 88 and 89 show the degree of availability of information on water resources and irrigation potential by country and by basic unit. Reasonably detailed information was available for about 30% of the countries. In general, little information was available for humid countries. This may be linked to the fact that countries with limited water resources need to plan more carefully the use of these resources and their distribution over the different sectors than the more humid countries. This results in a larger availability of Water Master Plans and irrigation development in the drier countries.
Table 91 shows the irrigation potential estimates resulting from the country studies, amounting to a total of 46.7 million hectares. The study by basin, using the same data as at country level, gives a total of 42.5 million hectares. The difference of 4.2 million hectares is explained by three main factors:
• different countries within a river basin considered the same water as being available for their use (double counting);
• several arid countries included the use of fossil water in their estimates (e.g. Algeria, Libya, Tunisia) while this study considers only renewable water resources;
• several countries considered lower irrigation water requirements in the computation of irrigation potential than recommended in this study (e.g. Algeria, Tunisia).