Delineation of irrigation cropping pattern zones
ü The criteria used for the delineation of the irrigation cropping pattern zones were, in order of decreasing importance: distribution of irrigated crops, average rainfall trends and patterns, topographic gradients, presence of large river valleys (Nile, Niger, Senegal), presence of extensive wetlands (the Sudd in Sudan), population pressure, technological differences and crop calendar above and below the equator (Zaire).
ü The starting point was the type of irrigated crops currently grown in Africa.
ü This resulted in 18 zones.
ü From these zones, sub-zones showing a different cropping intensity or a different crop calendar were defied.
ü This resulted in a total of 24 irrigation pattern zones which are considered to be homogeneous for:
• crops currently grown;
• crop calendar;
• cropping intensity.
ü Only the main crops currently grown, those occupying at least 85% of the irrigated area, were considered.
ü Land occupation of the remaining 15 % by secondary crops was assigned to the main crops.
ü An 'average' typical monthly crop calendar was assigned to each zone, based on work done by FAO's global information and early warning system, and on information from the reference library of FAO's agro-meteorology group, AQUASTAT and, for eastern Africa, from the IGADD crop production system zones inventory.
ü For each crop the actual cropping intensity was derived from national crop production and land use figures extracted from the FAO AGROSTAT  and AQUASTAT [21a] databases.
ü It ranges from 100 to 200%, according to the crop calendar.
ü The cropping intensity to be used in this study of irrigation potential ('potential' scenario) was generally estimated by increasing current values by 10 to 20%, but it was assumed that because of market limitations the current high intensity (in relative terms) of vegetables in certain parts of the continent would not be found in the potential scenario.
ü Therefore, intensities of cereal crops are higher in the potential scenario than in the actual situation.