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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

Data used

Precipitation data

The precipitation data used for this study are based on data prepared and published by the International Institute for Applied Systems Analysis (IIASA) (Leemans and Cramer, 1991). The IIASA data correspond to an imaginary "net" covering the earth's surface with a mesh size of 0.5 degrees. This is equivalent to about 60 km - an area of about 3 600 km² - at the equator. Monthly climatic data for each cell in the net are provided by weather stations and consist of normal values of monthly climate elements. For each of the stations used in the gridding exercise, data have been averaged over a 30 year period from 1961 to 1990. Using spatial extrapolation, a value is computed for each cell based on the values for the closest stations. More information on the subject, as well as dowloadable climatic datasets, can be found on the website of the Agrometeorology Group of FAO's Sustainable Development Department.

A map with the average annual precipitation can be displayed by clicking on Average annual precipitation map.

Evapotranspiration data

Data on reference evapotranspiration used for this study have also been prepared by IIASA for FAO (FAO, 2000). The resolution of this dataset is equal to the resolution of the precipitation dataset; 0.5║ latitude by 0.5║ longitude, with mean monthly values for global land areas, excluding Antarctica, for the period 1961-1990. The dataset has been prepared according to the FAO Penman - Monteith method with limited climatic data as described in FAO Irrigation and Drainage Paper 56 (FAO, 1998c). The input data used to calculate ETo are part of the "CRU Global Climate Dataset" prepared by the Climate Research Unit of the University of East Anglia, UK, and distributed through the website of the International Panel on Climate Change (IPCC).

A map with the annual average reference evapotranspiration data over Africa can be displayed by clicking on Annual average evapotranspiration map. A digital map with average monthly distribution of reference evapotranspiration for the whole world can be downloaded by clicking on Average monthly distribution of evapotranspiration.

Topographical data

The topographical data used for this study to describe the horizontal hydrological characteristics of the land surface are derived from the HYDRO1k geographic database developed by the US Geological Survey's EROS Data Center in cooperation with UNEP/GRID Sioux Falls.
The core dataset of this database is a hydrologically corrected Digital Elevation Model (DEM) Digital Elevation Model of Africa in raster format with a spatial resolution of 1 × 1 kilometre cells. From this DEM several datasets have been derived of which the following have been used for this study:

  • a raster dataset (1 × 1 km) with the flow direction of the DEM (for each cell of the DEM it is assumed that the water flows towards the lowest cell of the eight neigbouring cells);
  • a raster dataset (1 × 1 km) with the flow accumulation of the DEM (each cell has a value equal to the amount of cells upstream of the cell; this dataset has been derived from the dataset with the previous one);
  • a vector dataset with delineated drainage basins classified in six levels (this dataset is derived from the flow accumulation in combination with the flow direction data sets).

Hydrological network and water bodies

The digital data layer with the drainage pattern used for this project is a 1:5 000 000 line coverage with the rivers of Africa that was digitized in 1994 for the UNEP/FAO Desertification Assessment and Mapping Project.

The coverage with waterbody originates from the Digital Chart of the World 1:1 000 000. The waterbodies for Africa have been characterized (as lake, lagoon, reservoir, etc.), named (if the names were easily available) and provided with fish-catch data (if available) by FAO (FAO, 1998b). The data layer as used in this project contains all the waterbodies that had a name and were not characterized as rivers. The data on fish catch have not been included because this was beyond the scope of the project. Digital maps are available for both the drainage patternDrainage pattern for Africa and the water bodiesWater bodies in Africa.

The dataset with information on dams was developed by FAO (1996). The basic reference for the database was the World Register of Large Dams, prepared by the International Commission On Large Dams (ICOLD, 1989). For several African countries, additional information was available from national reports which were used to check or to complete the information obtained from the ICOLD publication. The georeferenced database on African dams has been described by FAO and can be viewed and downloaded by clicking African dams.

Soil moisture

Information with regard to the "maximum soil moisture storage capacity" and "easily available soil moisture" was derived from the Digital Soil Map of the World (FAO, 1998a).

This data layer consists of raster information with a resolution of 5 × 5 arc-minutes. The data on soil moisture are presented in terms of relative importance of different classes. In this study and for the purpose of model calculations an average maximum soil moisture storage capacity was computed and assigned to each class.

The spatial distribution of the maximum soil moisture storage capacity for the whole world can be viewed and downloaded by clicking Soil moisture capacity .

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