Télédétection et ressources en eau/Remote sensing and water resources
121
DETERMINING AREAS HOMOGENEOUS FOR CLIMATE
The second step consists of computing evapotranspiration; long-term average rainfall and potential
evapotranspiration data were used. Using the climatic database FAOCLIM, data for 1 025 stations
spread over Africa were used. Ideally, to obtain a spatial coverage of the continent, the best solution
would have been to interpolate rainfall and ET o data among the existing stations, considering the agroecological variations that exist among the stations. Another simpler solution is preferred: each station is
assigned an area of influence using Thiessen polygons (Figure 4). This method assumes linear variation
of precipitation among stations and assigns each segment of area to the nearest station. On average,
because of the low density of climate stations in several parts of Africa, a climate station is thus
representative for the climate over 29 330 km
2
, but it ranges from 18 000 to 485 000 km
2
. The method
leads to the result that the more isolated a station is, the higher its relative importance becomes.
As the quality of climatic data appears decisive for the quality of the results, it is decided to verify
them by plotting them against thematic maps of yearly average rainfall and ET o data. Quality of data
is obviously linked to the density of climate stations, which is much lower in the desert areas (Sahel,
Sahara, Kalahari). Rainfall data are compared with raster maps prepared by the Australian National
University (Figure 5). This results in some corrections to the climate database, but generally not more
than one month correction for only a few stations.
COMBINING THE TWO MAPS
The third step consists of combining the cropping pattern zones and the Thiessen polygons and results
in the definition of 1 437 basic map units. Each unit is thus characterized by one irrigation cropping
pattern and one climate.
COMPUTATION OF IRRIGATION WATER REQUIREMENTS
For all these units, the CROPWAT model - a computer program for crop water requirement
calculations developed by FAO (FAO, 1995a) - is used during the fourth step to compute net irrigation
water requirements. Inputs for the model are climatic parameters - rainfall and ET o - and crop
coefficients. Output from CROPWAT includes monthly net irrigation water requirements by crop.
Using the cropping pattern, and the actual and potential cropping intensity, net irrigation water
requirements per year are calculated for a theoretical hectare of irrigated land in each area, as explained
FIGURE 3
Example of cropping calendar for the Egyptian Nile and delta
121
DETERMINING AREAS HOMOGENEOUS FOR CLIMATE
The second step consists of computing evapotranspiration; long-term average rainfall and potential
evapotranspiration data were used. Using the climatic database FAOCLIM, data for 1 025 stations
spread over Africa were used. Ideally, to obtain a spatial coverage of the continent, the best solution
would have been to interpolate rainfall and ET o data among the existing stations, considering the agroecological variations that exist among the stations. Another simpler solution is preferred: each station is
assigned an area of influence using Thiessen polygons (Figure 4). This method assumes linear variation
of precipitation among stations and assigns each segment of area to the nearest station. On average,
because of the low density of climate stations in several parts of Africa, a climate station is thus
representative for the climate over 29 330 km
2
, but it ranges from 18 000 to 485 000 km
2
. The method
leads to the result that the more isolated a station is, the higher its relative importance becomes.
As the quality of climatic data appears decisive for the quality of the results, it is decided to verify
them by plotting them against thematic maps of yearly average rainfall and ET o data. Quality of data
is obviously linked to the density of climate stations, which is much lower in the desert areas (Sahel,
Sahara, Kalahari). Rainfall data are compared with raster maps prepared by the Australian National
University (Figure 5). This results in some corrections to the climate database, but generally not more
than one month correction for only a few stations.
COMBINING THE TWO MAPS
The third step consists of combining the cropping pattern zones and the Thiessen polygons and results
in the definition of 1 437 basic map units. Each unit is thus characterized by one irrigation cropping
pattern and one climate.
COMPUTATION OF IRRIGATION WATER REQUIREMENTS
For all these units, the CROPWAT model - a computer program for crop water requirement
calculations developed by FAO (FAO, 1995a) - is used during the fourth step to compute net irrigation
water requirements. Inputs for the model are climatic parameters - rainfall and ET o - and crop
coefficients. Output from CROPWAT includes monthly net irrigation water requirements by crop.
Using the cropping pattern, and the actual and potential cropping intensity, net irrigation water
requirements per year are calculated for a theoretical hectare of irrigated land in each area, as explained
FIGURE 3
Example of cropping calendar for the Egyptian Nile and delta