Session 2 : Assessment of irrigation potential in Africa
126
available information, on the basis of which irrigation cropping pattern zones have been nevertheless
defined. While the definition of irrigation cropping pattern zones can be considered as satisfying at the
continental level, which is the purpose of the present study, it would not be precise enough at a country
level. Furthermore the cropping pattern which is defined is an average one, based on the actual
situation of irrigation development, and cannot presume what would be the cropping pattern in each
irrigated scheme for a larger irrigated area.
The influence of the cropping pattern zones on the quality of the output is of prime importance.
Extreme differences in irrigation water requirements can be observed in adjacent zones, as a
consequence of this approach. For instance, in Burkina Faso, areas located north of the 1 000 mm
isohyet line have a gross potential water requirement of 500 mm per year while areas located just south
of this line need more than 2 800 mm per year. This is mainly due to the definition of irrigated cropping
pattern zones, where it was decided that no rice was cultivated under 1 000 mm of rainfall per year.
Within the cropping pattern zones, the boundaries of irrigation water requirements zones follow rainfall
trends.
A second constraint is the low density of climate stations in several parts of the continent, notably
the desert areas. The Thiessen polygons method leads there to major approximations. This results in a
higher variation in net irrigation water requirements. The difference between two adjacent zones within
the same irrigated cropping pattern zone may be as high as 600 mm/yr (in the Sahara for example),
while the difference in other parts of Africa does not exceed 200 mm/yr. Resolution of climatic data
could also be improved by developing monthly maps of P and ET o on the basis of interpolations among
existing stations.
Other critical assumptions are those of the cropping patterns, actual and potential cropping
intensities, and irrigation efficiencies. Nevertheless, country studies on this specific matter generally
give figures for irrigation water requirements close to those assessed by this study.
The methodology has been developed on a large scale: the African continent. It could also be used
on a smaller scale, a river basin for example, in which case a better definition of irrigation cropping
pattern zones as well as a better estimation of irrigation efficiency and cropping intensity would give
better results. Development of monthly maps of P and ET o would also significantly improve the
resolution of the work.
BIBLIOGRAPHY
FAO. 1977. Crop water requirements. FAO Irrigation and Drainage Paper 24. Rome. 144 p.
FAO. 1987. Irrigation and water resources potential for Africa. FAO AGL/MISC/11/87. Rome. 127 p.
FAO. 1992. Expert consultation on revision of FAO methodologies for crop water requirements. Rome, 28-31
May 1990. 60 p.
FAO. 1995a. CROPWAT, a computer program for irrigation planning and management. FAO Irrigation and
Drainage Paper 46. Rome. 126 p.
FAO. 1995b. Irrigation in Africa in figures. FAO Water Report 7. Rome. 336 p.
FAO. 1995c. Study of the irrigation potential for Africa. Report on the computation of irrigation water
requirements at continental level. Internal report AGL/FAO. Rome. 36 p.
FAO (in preparation). Assessment of irrigation potential in Africa. Internal report AGL/FAO. Rome.
IFPRI. 1995. Water resources development in Africa: a review and synthesis of issues, potentials and
strategies for the future. Washington. 113 p.
126
available information, on the basis of which irrigation cropping pattern zones have been nevertheless
defined. While the definition of irrigation cropping pattern zones can be considered as satisfying at the
continental level, which is the purpose of the present study, it would not be precise enough at a country
level. Furthermore the cropping pattern which is defined is an average one, based on the actual
situation of irrigation development, and cannot presume what would be the cropping pattern in each
irrigated scheme for a larger irrigated area.
The influence of the cropping pattern zones on the quality of the output is of prime importance.
Extreme differences in irrigation water requirements can be observed in adjacent zones, as a
consequence of this approach. For instance, in Burkina Faso, areas located north of the 1 000 mm
isohyet line have a gross potential water requirement of 500 mm per year while areas located just south
of this line need more than 2 800 mm per year. This is mainly due to the definition of irrigated cropping
pattern zones, where it was decided that no rice was cultivated under 1 000 mm of rainfall per year.
Within the cropping pattern zones, the boundaries of irrigation water requirements zones follow rainfall
trends.
A second constraint is the low density of climate stations in several parts of the continent, notably
the desert areas. The Thiessen polygons method leads there to major approximations. This results in a
higher variation in net irrigation water requirements. The difference between two adjacent zones within
the same irrigated cropping pattern zone may be as high as 600 mm/yr (in the Sahara for example),
while the difference in other parts of Africa does not exceed 200 mm/yr. Resolution of climatic data
could also be improved by developing monthly maps of P and ET o on the basis of interpolations among
existing stations.
Other critical assumptions are those of the cropping patterns, actual and potential cropping
intensities, and irrigation efficiencies. Nevertheless, country studies on this specific matter generally
give figures for irrigation water requirements close to those assessed by this study.
The methodology has been developed on a large scale: the African continent. It could also be used
on a smaller scale, a river basin for example, in which case a better definition of irrigation cropping
pattern zones as well as a better estimation of irrigation efficiency and cropping intensity would give
better results. Development of monthly maps of P and ET o would also significantly improve the
resolution of the work.
BIBLIOGRAPHY
FAO. 1977. Crop water requirements. FAO Irrigation and Drainage Paper 24. Rome. 144 p.
FAO. 1987. Irrigation and water resources potential for Africa. FAO AGL/MISC/11/87. Rome. 127 p.
FAO. 1992. Expert consultation on revision of FAO methodologies for crop water requirements. Rome, 28-31
May 1990. 60 p.
FAO. 1995a. CROPWAT, a computer program for irrigation planning and management. FAO Irrigation and
Drainage Paper 46. Rome. 126 p.
FAO. 1995b. Irrigation in Africa in figures. FAO Water Report 7. Rome. 336 p.
FAO. 1995c. Study of the irrigation potential for Africa. Report on the computation of irrigation water
requirements at continental level. Internal report AGL/FAO. Rome. 36 p.
FAO (in preparation). Assessment of irrigation potential in Africa. Internal report AGL/FAO. Rome.
IFPRI. 1995. Water resources development in Africa: a review and synthesis of issues, potentials and
strategies for the future. Washington. 113 p.