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Water Resources and Irrigation in Africa


Introduction Background and rationale Data used Water balance model
Current water use for agriculture Model calibration Results References

Current water use in agriculture

The water balance model has been applied for two situations, a natural situation and a situation in which water use for irrigation has been taken into account and compared to the calculated runoff. Water use for rainfed agriculture has been included in the general water balance of the model. Water use for sectors other than agriculture has not been considered in the water balance because it is mostly a non-consumptive use and generally negligible when compared to agriculture water use.


Methodology for the computation of irrigation water requirement

The computation of the irrigation water requirements is based on the following formula:

IRc(m) = max( 0 , ( (Kc(m) × ETo(m)) - ETa(m) ) ) × A × CI(m)

where:  
   
Irc Irrigation water requirements to cover crop evapotranspiration
Kc Crop coefficient
ETo Reference evapotranspiration in mm
ETa Actual evapotranspiration
A Area under irrigation as percentage of the total area
CI Cropping intensity
m Monthly time step of the calculation.

The formula indicates that the crop water requirement is equal to Kc × ETo, part of which (ETa) was supplied by the precipitation. When ETa is higher than the water requirement, the irrigation water requirement is considered nil.

The following procedure has been followed for the calculation of irrigation water requirements:

  • 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.
  • Elaboration of a grid of the irrigated areas (in percentage of land area) and computation of the percentage of area under irrigation per sub-basin.
  • Calculation of crops water requirements on the basis of the reference evapotranspiration (ETo) and actual Evapotranspiration (ETa) already computed from the soil-water balance model.
  • Calculation of irrigation water use per sub-basin, assuming that use is equal to requirement.

The procedure is represented schematically in the following flowchart. 


Delineation of irrigation cropping pattern zones

The delineation of the irrigation cropping pattern zones was done by compiling information of various types: 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. The map with irrigation cropping pattern zones is based on the FAO publication on Irrigation potential in Africa (FAO, 1997). This resulted in a total of 23 irrigation pattern zones which are considered to be homogeneous with respect to crop calendar and cropping intensity Irrigation zones.

An ‘average’ typical monthly crop calendar was assigned to each zone, mainly based on work done by FAO's global information and early warning system, and on information from the reference library of AQUASTAT.

For each crop the actual cropping intensity was derived from national crop production and land use figures extracted from the FAOSTAT and AQUASTAT databases.

Table 1 summarises the cropping pattern, crop calendar and monthly and yearly cropping intensities for the 23 zones used in this study. The figures shown on Table 1 relate to the different crops and, in correspondence to the growing period of the crop, represent the percentage of the irrigated area in that particular zone, assigned to that crop category.


Calculation of crop water requirements

Crop water requirement were calculated using the modified Penman-Monteith method as described in FAO Irrigation and Drainage Paper 56; Crop evapotranspiration, guidelines for computing crop water requirements.

The crop evapotranspiration coefficients shown in Table 2 have been used in the model for the four different stages of crop growth (initial, development, middle and late stage). For ease of calculation it is assumed that the initial stage, the development stage and the late stage take one month each. The rest of the growing period (always one or more months) is assumed to be the middle stage.

Crop water requirement ETo(m) × Kc(m) were calculated for each grid cell month by month and then compared to the actual evapotranspiration ETa(m) resulting from the soil water balance model. Their difference was then multiplied by the cropping intensity to obtain a monthly grid of irrigation water requirement.


Map of irrigated areas

The irrigated areas considered here correspond to that part of the water managed areas equipped with hydraulic structures: full or partial control irrigation, equipped wetland or valley bottoms and areas equipped for spate irrigation, i.e. the areas which have modified the natural flow regime of the rivers or groundwater. The data used in this study concern only the equipped irrigated areas which do not necessarily correspond to the actually irrigated area. The data used to determine the spatial distribution of the irrigated areas are the following:

  • data on irrigated areas by country, collected during the survey organized by FAO in 1987 over the whole continent except South Africa. The study identified a total of 9 544 000 ha irrigated area, out of which 3 132 000 were documented with location, type of irrigation, source of water and cropping pattern.
  • data collected for AQUASTAT in 1994-95 through country-based reviews of literature and existing information, and through detailed questionnaire addressed to the countries.

The quality and reliability of the available data are not homogeneous: for some countries, data on irrigated area, including the localization, are available at province level: for other countries, the data are available at sub-basin level, and for some countries the only available figure corresponds to the total irrigated area of the whole country. Table 3 shows the various categories of information available on irrigated area.

In GIS, the information was spatially distributed over the continent. The irrigated area was assigned to the polygons referring to where the data were collected (country, provinces, sub-basins, etc.). From this the percentage of irrigated area over the polygon area was calculated. The data obtained were then converted into a grid layer. The distribution of the percentage of area under irrigation by sub-basin can be viewed by clicking on Irrigated areas. Multiplying the irrigation water requirement per unit irrigated area by the percentage of area under irrigation led to a grid of irrigation water use (assuming that the requirements are satisfied). Combining this grid with the map of the sub-basins made it possible to assign a value of the irrigation water use to each sub-basin.

The map of irrigated areas has been updated after the project and is being maintained by FAO in co-operation with the Center for Environmental Systems Research of the University of Kassel. The website with the latest information on the global map of irrigated areas can be found here.



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