Water Resources and Irrigation in Africa
Background and rationale
Assessment of freshwater is becoming of increasing importance with regard to the growing pressure on this scarce resource. The current interest in issues related to global water resources assessment has been emphasised after the UN-CSD called for a freshwater assessment exercise and more recently when the World Water Council, the Global Water Partnership and the World Commission on Water in the 21st Century decided to promote a World Water Vision for the year 2025.
For the African continent as a whole, 85 % of water withdrawals are directed towards agriculture and this percentage is even higher in the arid and semi-arid part of Africa. In those areas the water withdrawn for agriculture from the hydrologic system may represent a significant part of the water resources. Assessing the impact of irrigation on water resources, the potential for future development and the overall impact of human activities on the water cycle requires an approach by river basin. In the framework of the AQUASTAT survey, data on water use for irrigation were collected and presented on a country basis (FAO, 1995a and FAO, 1995b). Water resources assessment on country level was based on hydrological information on the main rivers extrapolated to areas where direct measurement were not available. However, the information obtained was not spatially documented in a satisfactory way and it was difficult to compare water resources to water use over the same spatial units. Another problem resulted from the variability of the water resources assessment methodologies from one country to another which made assessments of large river basins difficult and uncertain.
In the present survey, available information was processed through a continental GIS-based model to provide, at the scale of the African continent, a comprehensive picture of the different elements of the water balance. This approach makes the best use of scattered information and makes it possible to extrapolate point data or data available at country level to develop a credible picture of the situation of the continent’s water use and its impact on water resources. It also has the advantage of presenting a homogenous methodology to compute water balance across the continent.
The model used in the study is simple and is entirely performed within the GIS environment. It makes the best possible use of available information, be it regional coverages of the main climatic elements of the water balance (precipitation, potential evapotranspiration), soil properties or irrigation. The model is divided in two parts: a vertical soil - water balance model, performed monthly for every grid cell (10 km × 10 km), computes the part of precipitation which does not return to the atmosphere through evapotranspiration. This water, called "surplus" in the study, is then routed through the landscape in the rivers by the horizontal part of the model. In GIS, this is performed by generating a grid-based hydrological network based on an available digital elevation model (DEM). A detailed description of the model is given further in the section describing the water balance model.
As mentioned above, agriculture represents the bulk of water withdrawal in Africa. In addition, agriculture is the sector representing the highest rate of consumptive use of water (about 45%), while the domestic and industrial sectors typically use 5 to 10% of the water they withdraw from the rivers or aquifers. In terms of water balance, an accurate assessment of agricultural water withdrawal and consumptive use of water in irrigation is therefore critical to the overall quality of results. Country figures of irrigated land areas are not sufficient: the exact location of the areas under irrigation and a good knowledge of their cropping calendar are necessary to estimate irrigation water requirements. The information collected through the AQUASTAT survey for Africa and additional mapped information were therefore used to prepare a map of irrigation in Africa and identify major irrigation cropping pattern zones.
The following sections present in details the data and the model used in assessing the water balance for Africa.
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