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Chapter 5: Irrigation water requirements
The assessment of the irrigation potential, based on soil and water resources, can only be done by simultaneously assessing the irrigation water requirements (IWR) (Figure 1).
Net irrigation water requirement (NIWR) is the quantity of water necessary for crop growth. It is expressed in millimeters per year or in m3/ha per year (1 mm = 10 m3/ha). It depends on the cropping pattern and the climate. Information on irrigation efficiency is necessary to be able to transform NIWR into gross irrigation water requirement (GIWR), which is the quantity of water to be applied in reality, taking into account water losses. Multiplying GIWR by the area that is suitable for irrigation gives the total water requirement for that area. In this study water requirements are expressed in km3/year.
Calculations of irrigation water requirements are done while preparing national water master plans or irrigation projects. Useful information was obtained from a number of country studies available from AQUASTAT [21a], but the information was based on many different approaches. For the purpose of this study the need was felt to develop a method of computing irrigation water requirements for the whole continent in a systematic way. In order to be able to do this at the scale of the continent, assumptions have to be made on the definition of areas to be considered homogeneous in terms of rainfall, potential evapotranspiration, cropping pattern, cropping intensity and irrigation efficiency.
Delineation of irrigation cropping pattern zones
Definition of the climate stations' area of influence
Combination of cropping pattern zones with the climate stations
Calculation of irrigation water requirements
For the calculation of irrigation water requirements the following steps have been followed:
• Delineation of major irrigation cropping pattern zones. These zones are considered homogeneous in terms of types of irrigated crops grown, crop calendar, cropping intensity and gross irrigation efficiency. Represented on the map of Africa, they should be viewed as regions where some homogeneity can be found in terms of irrigated crops. The cropping pattern proposed for the zone should be viewed as representative of an 'average' rather than a 'typical' irrigation scheme.
• Definition of the area of influence of the climate stations (in GIS) and quality check on the climate data.
• Combination of the irrigation cropping pattern zones with the climate stations' zones (in GIS) to obtain basic mapping units.
• Calculation of net and gross irrigation water requirements for different scenarios.
• Comparison with existing data and final adjustment.
Delineation of irrigation cropping pattern zones
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).
The starting point was the type of irrigated crops currently grown in Africa. This resulted in 18 zones. From these zones, sub-zones showing a different cropping intensity or a different crop calendar were defied. This resulted in a total of 24 irrigation pattern zones (Figure 8), which are considered to be homogeneous for:
• crops currently grown;
• crop calendar;
• cropping intensity.
Only the main crops currently grown, those occupying at least 85% of the irrigated area, were considered. Land occupation of the remaining 15 % by secondary crops was assigned to the main crops.
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.
For each crop the actual cropping intensity was derived from national crop production and land use figures extracted from the FAO AGROSTAT [6] and AQUASTAT [21a] databases. It ranges from 100 to 200%, according to the crop calendar. 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. Therefore, intensities of cereal crops are higher in the potential scenario than in the actual situation.
Table 7 summarizes the cropping pattern, crop calendar and cropping intensities for the 24 zones used in this study.
Definition of the climate stations' area of influence
The climate data from the FAOCLIM cd-rom were used, as this was the most up to date climate database available [7]. This data set includes long term average rainfall and reference potential evapotranspiration (ET0) data for 1025 stations throughout Africa. ET0 was calculated by the Penman-Monteith method [4].
To obtain a spatial coverage of climate data (P. ET0) over the continent, each station was assigned an area of influence using the Thiessen polygons method. This method assigns an area of 'nearest vicinity' to each climate station. Figure 9 gives an indication of the density of the stations over the continent. As expected, the desert areas in northern and southern Africa are much less well covered than the rest of the continent. The rainfall data were compared with raster maps prepared by the Australian National University [23] and corrected where necessary.
Combination of cropping pattern zones with the climate stations
In ArcInfo, the 24 cropping pattern zones and the 1025 climate station data were merged. This resulted in 1437 basic map features, homogeneous in irrigation cropping characteristics and climate. All further calculations were carried out on these 1437 basic mapping units.
Calculation of irrigation water requirements
Crop water requirements (CWR) for a given crop, i, are given by:
unit: mm
where 
is the crop
coefficient of the given crop i during the growth stage t and
where T is the final growth stage.
Each crop has its own water requirements. Net irrigation water requirements (NIWR) in a specific scheme for a given year are thus the sum of individual crop water requirements (CWRi) calculated for each irrigated crop i. Multiple cropping (several cropping periods per year) is thus automatically taken into account by separately computing crop water requirements for each cropping period. By dividing by the area of the scheme (S. in ha), a value for irrigation water requirements is obtained and can be expressed in mm or in m3/ha (1 mm = 10 m3/ha).
unit: mm
where Si is the area cultivated with the crop i in ha.
The cropping intensity of the scheme can be defined as:
FAO's CROPWAT software (version 5.7) was used to compute NIWR for each of the 137 basic units described in chapter 2 [3]. The model was run for three different scenarios:
• actual cropping intensity, effective rainfall1;
• potential cropping intensity, effective rainfall;
• potential cropping intensity, dependable rainfall2.1Effective rainfall was computed according to the "USDA Soil Conservation Service Method" formula in [3], page 21.
2 Dependable rainfall, the combined effect of dependable rainfall (80% probability of exceedance) and estimated 'losses, due to runoff and percolation, was calculated according to the formula in [3], page 21.
Gross irrigation water requirement (GIWR) is the amount of water to be extracted (by diversion, pumping) and applied to the irrigation scheme. It includes NIWR plus water losses:
unit. mm
where E is the global efficiency of the irrigation system.
Limited objective information on irrigation efficiency was available and estimates were based on several criteria:
• figures found in literature;
• type of crops irrigated;
• the level of intensification of the irrigation techniques.
In this study the irrigation efficiencies for the 'potential' scenario range from 45 to 80% (Table 8).
Point observations were further generalized by hand to obtain zones of homogeneous irrigation water requirements (HIWR). A total of 84 HIWR zones were defined.
The methodology was tested and calibrated using a case study on the Egyptian Nile Valley and Delta where water requirements and availability could be computed with relative precision.