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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

Model results

This section presents the results of the model for the major basins and sub-basins. The data and model results of the major basins are presented in the following Excel file. A map of the major basins can be viewed by clicking onBasins map.

The model results per sub-basin are presented per major basin in an Excel file. The spatial distribution of the sub-basins can be viewed and downloaded by clicking River Basins of Africa. The tables in the Excel file show the characteristics and components of the water balance for each sub-basin. Some of the indicators call for the following comments:

Identification number of the sub-basin immediately downstream: this parameter makes it possible to understand the flowpath across the sub-basins, under the assumption that a sub-basin has only one single outlet.

Natural inflow: this parameter represents the annual flow at the entrance of the sub-basin resulting from the model simulation, without taking into account withdrawals for irrigation in the upstream sub-basins.

Natural outflow: this parameter is equal to the flow accumulation at the outlet of the sub-basin, as resulting from the model simulation, without taking into account the withdrawal for irrigation.

Internal renewable water resources (IRWR) is the difference between the natural outflow and the natural inflow calculated by the model and represents the sub-basin contribution to the overall runoff of the major basin. In cases where the natural outflow is less than the natural inflow, the IRWR is zero. This is the case, for example, of the inner delta of the Niger River in Mali (sub-basin 891- Niger 17), due to the intense evapotranspiration occurring in the swamps. IRWR could also be calculated by multiplying the difference Precipitation - Actual evapotranspiration by the sub-basin area. However, because of the rounding of the figures expressed in mm, the result might be slightly different from the one indicated in the table.

Actual inflow and Actual outflow are the natural flows corrected with the net irrigation water use in each sub-basin. In the case of closed basins ending in a lake or any evaporative area (Rift Valley, Okavango, Lake Chad), the natural flow balance is nil. If water is withdrawn for irrigation, the actual outflow calculated by the model is negative which is obviously impossible. Actually, that withdrawal is resulting in evapotranspiration which is already accounted for as actual evapotranspiration and it could have been simply deleted from the actual outflow since the balance of the closed basin will automatically be adjusted by a decrease of the evaporative area to offset the artificial evapotranspiration due to irrigation. Eventually the total actual evapotranspiration will remain the same.

Percentage of irrigation water use over total water resources is an indicator of the stress generated by the irrigation water use on the water resources (total natural inflow + internal water resources). It should obviously be lower than 100 percent; however there are a number of cases where, in a sub-basin, water is imported from a nearby basin (water transfer), consequently increasing the water resources of the sub-basin artificially. Using non renewable groundwater resources also has a similar effect of generating an apparent high stress on the water resources. Hereafter are a few examples of water transfer or use of non renewable groundwater resources producing a high indicator of stress:

Major basin Sub-basin Percent of irrigation water use over total water resources Comments
Lake Chad Basin Yedseram 2 205.88 Transfer of water from the Lake Chad to the South Chad Irrigation Scheme in Nigeria.
Limpopo Basin Mogalakwna 201.92 Transfer of water from Limpopo river.
Mediterranean Coast Jeffara 2 164.00 Non-renewable groundwater resources from the Sahara, mostly in Tunisia.
Mediterranean Coast Sawf al Jin 418.60 Use of non-renewable groundwater resources.
Mediterranean Coast Tunisian East Coast 263.64 Use of non-renewable groundwater resources.
Nile Basin Damanhur 4 860.34 The three sub-basins belong to the Nile Delta but the automatic basin delineation did not include them in the Nile river basin but practically all the irrigated area in these three sub-basins receives water from the Nile.

Nile Basin Nile Delta 1 659.19
North East Coast Bitter Lake 10 187.50
North West Coast Souss Massa 331.22 This area is known for the intensive overexploitation of the groundwater resources.
North West Coast Tensift 105.90 This is perhaps due to an underestimation of the water resources.
Orange Basin Hartbees 200.00 Transfer from the Orange river.
Zambezi Basin Sengwa 213.33 Transfer from the Lake Kariba on the Zambezi river.

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