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Chapter 2: Methodology and data used
Definition of irrigation potential
Definition of the basic units
Identification of the physical resources
Review of existing information on irrigation potential
Environmental considerations
Interpretation of the results
This chapter gives an overview of the methodology used for assessing the irrigation potential and of the different steps followed (Figure 1).
Definition of irrigation potential
This study refers to irrigation as the process by which water is diverted from a river or pumped from a well and used for the purpose of agricultural production. Areas under irrigation thus include areas equipped for full and partial control irrigation, spate irrigation areas, equipped wetland and inland valley bottoms (including fadamas), irrespective of their size or management type. It does not consider techniques related to on-farm water conservation like water harvesting.
The area which can potentially be irrigated depends on the physical resources 'soil' and 'water', combined with the irrigation water requirements as determined by the cropping patterns and climate. In this study it is called 'physical irrigation potential'. However, environmental and socioeconomic constraints also have to be taken into consideration in order to guarantee a sustainable use of the available physical resources. This means that in most cases the possibilities for irrigation development would be less than the physical irrigation potential.
Planning for water use can only be carried out on the basis of river basins. On the other hand, land use is usually computed or planned according to political boundaries. These two divisions of the continent were therefore combined to obtain the basic units used in this study.
First, the African continent was divided into 24 major hydrological units or basin groups, the ones defined in the previous study [20], and classified according to four main categories (Figure 2):
• 8 major river basins, draining to the sea: Senegal River, Niger River, Nile, Shebelli-Juba, Congo/Zaire River, Zambezi, Limpopo and Orange;
• 9 coastal regions grouping several small rivers, draining to the sea: Mediterranean, North-West, West, West Central, South-West, South Atlantic, Indian Ocean, East Central and North-East;
• 5 regions grouping several endorheic drainage basins: Lake Chad, Rift Valley, Okavango, South Interior and North Interior;
• 2 units grouping the islands: one unit is Madagascar and the other unit groups the islands of Cape Verde, Comoros, Mauritius, São Tome and Principe and Seychelles.
The last three categories group several small, independent drainage basins in order to limit the study to a workable number of units.
These 24 major hydrological units were combined with the 53 African countries (in GIS) to obtain 136 land units. These units form the basis of all computation and of the information gathered and analysed in this study and are referred to as 'basic units' (Tables 1 and 2).
Identification of the physical resources
Land resources
Water resources
Irrigation water requirements
Criteria were established to determine the soil and terrain suitability for irrigation on the basis of the information from the FAO-UNESCO soil map of the world.
The type of irrigation considered was surface irrigation. Introducing sprinkler irrigation or micro-irrigation on a large scale would require a revision of several of these criteria, probably leading to an increase in land suitable for irrigation.
In 1995 FAO has conducted a review of the annual renewable water resources of the African countries [21]. These figures were used as a basis for the present study and completed with more detailed information on the variation in water discharges in space and time. This information was compared with surface runoff estimates, calculated for each of the 136 basic units (in GIS) and based on the surface runoff map of Africa [28].
All calculations were based on renewable water resources, and mainly on surface water resources, except for arid countries where renewable groundwater already plays an important role in irrigation development. Non renewable groundwater resources (fossil water) were not taken into consideration. For arid countries, this may result in a relatively low irrigation potential, sometimes even lower than the area already under irrigation.
Assessment of the irrigation potential, based on soil and water resources, can only be done by simultaneously assessing the irrigation water requirements, which in turn depend on the cropping pattern and climate (rainfall and potential evapotranspiration). For this reason, irrigation cropping pattern zones were defined for current and potential scenarios and water requirements were computed using the FAO CROPWAT model [3]. The climate data used were the ones available on the FAOCLIM cd-rom [7]. From the resulting net water requirements, figures of gross irrigation water requirements (including water losses) were computed for each of the 136 basic units. These figures were then compared with those available from individual country studies.
Review of existing information on irrigation potential
The main component of the present study is the review of existing information on irrigation potential, mainly based on physical criteria, though other criteria are sometimes implicitly included as well, as explained in Chapter 6.
In the framework of the AQUASTAT programme, a library has been created containing all kinds of information related to irrigation and indexed by country: national water master plans, agricultural and/or irrigation sector reviews, project documents, country studies, statistics, etc. All these documents were reviewed for the present study.
In addition, more in-depth research on irrigation potential at country, basin and regional level was conducted in the various FAO libraries and connected information systems.
Two river basins, the Nile and the Niger, were subjected to detailed study, while in view of time constraints the other basins and regions were studied at a more global level. To the extent possible, information was collected for each of the 136 basic units. Where it was impossible to have exact figures at these levels, interpolations and/or estimates were made. All the information gathered from the literature was systematically cross-checked with the results of the studies of Chapters 3, 4 and 5.
This study concentrates on water use for agricultural purposes. Where national water master plans exist, water demand by other sectors (domestic, livestock, industrial, hydropower, navigation, etc.) was taken into consideration in assessing water availability for agricultural purposes. Especially in drier regions, competition for water may arise among the different sectors. In general, the quantity of water available for agriculture is the difference between the total quantity of water available and the water demands of other sectors.
The chief concern of the present study is the physical potential. It is impossible to integrate complex issues, like economic, political, social and environmental aspects into a purely quantitative assessment exercise. Nonetheless these issues are critical to a holistic vision of irrigation potential at continental level. The impact of the choice of one or the other land use, for instance, could radically modify the assessment of land which could be allocated for irrigation. A qualitative assessment of the environmental aspects is presented, highlighting the most relevant issues concerning irrigation development, though without presuming to be exhaustive.