Télédétection et ressources en eau/Remote sensing and water resources
119
allowing the development of irrigation. It is therefore rather theoretical, and explains why an irrigated
cropping pattern is also assigned to some parts of the Sahara desert where irrigation is obviously not
feasible everywhere. It should also be mentioned that this irrigation cropping pattern is different from
the actual rainfed cropping pattern. The available literature (country studies, masterplans, sectorial
studies, etc) gives generally figures for this cropping pattern at the scheme level or at the country level,
but rarely at the regional level and almost never at the river basin level. The available information
(FAO, 1987, 1995b) is nevertheless used to define an average irrigated cropping pattern, based on the
cropping pattern in irrigated schemes spread over the continent.
In order to improve the definition of the homogeneous areas for irrigated cropping patterns, other
factors are also considered: topographic slopes which may prevent development of specific crops;
rainfall trends and rainfall patterns which have a direct influence on cropping patterns; presence of
wetland such as the Sudd marshes land in Sudan or rivers such as the Nile River in Egypt or the Niger
River in Mali which affect the kind of crops which are cultivated under irrigation; population pressure
which may affect both actual and potential cropping intensity; technological differences which may
affect the kind of crop cultivated.
Furthermore, the cropping pattern computed on the basis of actual irrigated agriculture shows a
diversity of crops which would not exist if irrigation was developed on a larger scale. When drawing
the map of cropping pattern, it was decided to retain only the main crops, which represent together at
least 85% of the total irrigated area. Land occupation by the remaining 15% or less of secondary crops
is assigned to the main crops.
For the specific case of irrigated vegetables, their cultivated area may represent a large percentage
of total irrigated crops because of the low development of irrigation, but would not be much more
developed in real terms if irrigation development covered the whole potential, due to market limitations.
Two other variables significantly affect the water requirements: (1) the cropping intensity, and (2)
the water distribution efficiency. Little information exists on global irrigation efficiency (FAO, 1977).
It was decided to assess the efficiency on the base of the irrigated cropping pattern and of regions since
irrigation efficiency may vary critically according to the level of intensification of irrigation techniques,
traditional cultivation, management practices. One figure for both variables was obtained for each
cropping pattern zone.
Because the study should serve the purpose of assessing both current and future water use, two
scenarios are developed: one with the actual situation, meaning the actual cropping intensity and the
actual irrigation efficiency, and the other with potential values for those two variables. Potential
cropping intensity is generally estimated by increasing current values by 10 to 20%, while potential
efficiencies are estimated to be only 5 to 10% higher than the actual figures.
This process results in the definition of 24 zones for the African continent, subsequently digitized
and stored in Arc-Info (Figure 2). For each of them, a typical crop calendar is developed for all major
crops (see example in Figure 3).
119
allowing the development of irrigation. It is therefore rather theoretical, and explains why an irrigated
cropping pattern is also assigned to some parts of the Sahara desert where irrigation is obviously not
feasible everywhere. It should also be mentioned that this irrigation cropping pattern is different from
the actual rainfed cropping pattern. The available literature (country studies, masterplans, sectorial
studies, etc) gives generally figures for this cropping pattern at the scheme level or at the country level,
but rarely at the regional level and almost never at the river basin level. The available information
(FAO, 1987, 1995b) is nevertheless used to define an average irrigated cropping pattern, based on the
cropping pattern in irrigated schemes spread over the continent.
In order to improve the definition of the homogeneous areas for irrigated cropping patterns, other
factors are also considered: topographic slopes which may prevent development of specific crops;
rainfall trends and rainfall patterns which have a direct influence on cropping patterns; presence of
wetland such as the Sudd marshes land in Sudan or rivers such as the Nile River in Egypt or the Niger
River in Mali which affect the kind of crops which are cultivated under irrigation; population pressure
which may affect both actual and potential cropping intensity; technological differences which may
affect the kind of crop cultivated.
Furthermore, the cropping pattern computed on the basis of actual irrigated agriculture shows a
diversity of crops which would not exist if irrigation was developed on a larger scale. When drawing
the map of cropping pattern, it was decided to retain only the main crops, which represent together at
least 85% of the total irrigated area. Land occupation by the remaining 15% or less of secondary crops
is assigned to the main crops.
For the specific case of irrigated vegetables, their cultivated area may represent a large percentage
of total irrigated crops because of the low development of irrigation, but would not be much more
developed in real terms if irrigation development covered the whole potential, due to market limitations.
Two other variables significantly affect the water requirements: (1) the cropping intensity, and (2)
the water distribution efficiency. Little information exists on global irrigation efficiency (FAO, 1977).
It was decided to assess the efficiency on the base of the irrigated cropping pattern and of regions since
irrigation efficiency may vary critically according to the level of intensification of irrigation techniques,
traditional cultivation, management practices. One figure for both variables was obtained for each
cropping pattern zone.
Because the study should serve the purpose of assessing both current and future water use, two
scenarios are developed: one with the actual situation, meaning the actual cropping intensity and the
actual irrigation efficiency, and the other with potential values for those two variables. Potential
cropping intensity is generally estimated by increasing current values by 10 to 20%, while potential
efficiencies are estimated to be only 5 to 10% higher than the actual figures.
This process results in the definition of 24 zones for the African continent, subsequently digitized
and stored in Arc-Info (Figure 2). For each of them, a typical crop calendar is developed for all major
crops (see example in Figure 3).