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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa>

1.1.15 Tidal Irrigation

Technical Description

Tidal heights of 1.3 m and 1.0 m exist in the Gambia River at Jahally and Pacharr, respectively. Special intake structures with gates were constructed, which, when opened at high tide, allow brackish tidal water to enter irrigation channels leading to the farms to be irrigated. The gates are opened for a period of between 3 and 24 hours depending on the size of the area to be irrigated. Tidal irrigation is coordinated with pumped irrigation, and is used to irrigate low lying areas along the Gambia River. Pumped irrigation systems are used for areas at higher elevations.

Extent of Use

This technology is used in an on going project in The Gambia, West Africa.

Operation and Maintenance

Once installed, this technology can be maintained by locally-trained artisans with general farm experience.

Level of Involvement

The technology is locally- or community-controlled, although some expertise is required for the design, siting, construction and maintenance of the civil structures.

Costs

The capital costs of the project, at 1983/84 prices, was $7.5 million. The current operation and maintenance cost is about $220/ha/year.

Effectiveness of the Technology

The technology has been very successful in paddy rice cultivation. Using tidal irrigation, double cropping of 167 ha and 850 ha is achieved annually at Jahally and Pacharr, respectively. Similarly, irrigation of 440 ha and 125 ha is achieved annually at Jahally and Pacharr, respectively, with pumped irrigation. The annual yield is 9 t/ ha/year at an annual cost of $70/t/year.

Suitability

This technology is appropriate in areas where tidal and river conditions are similar to those at Jahally and Pacharr. Generally, the river is in a relatively flat basin with an high degree of tidal intrusion. This technology should only be used with salt-tolerant crops.

Environmental Benefits

There are no known environmental benefits. Drawbacks include a risk of salination and creation of a barrier to migratory fishes.

Advantages

The technology is very successful because, once the intake structures and irrigation channels have been constructed, the operation and maintenance costs are very low. Maintenance work on the irrigation channels, and clearing of weeds and bush from the channels and arable area, can be done by the local farmers.

Disadvantages

The major difficulty experienced is the lack of availability of spare parts.

Cultural Acceptability

It is accepted by the local community and is seen as an income generating project.

Further Development of the Technology

No further development of the technology is necessary.

Information Sources

International Fund for Agricultural Development (IFAD), African Development Bank (ADB), and the German Government: Fulladu West District Project, The Gambia.

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