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Water control is a term often used by irrigation engineers when referring to large-scale operations, where irrigation canals, bunds or weirs and lateral channels or canals can be used, often with the assistance of large pumps, to regulate the flow of water to any particular area.
The quality of irrigation water is important, since water which contains a high level of dissolved salts such as chlorides and suspended solids is harmful to some crops. Large-scale irrigation schemes have been installed in many parts of West Africa and have been used for crops such as rice, sugar cane, bananas, tobacco and some tree crops such as citrus. The following are some of the methods which can be used on a small or commercial scale for irrigating a wide range of farm crops.
Watering Cans:
On a garden scale, water can be applied to crops by watering cans, fitted with either a fine or coarse nozzle, depending on the stage of growth of the plants.
Furrow Irrigation;
Where a perennial stream is available, this can be dammed so that water can be directed along channels or furrows running between the rows or beds of crops.
The water penetrates the soil on either side of the furrow, supplying the roots with adequate amounts of water. This method is suitable for vegetables and other intensively planted crops grown on a small scale.
Basin Irrigation;
For fruit trees, where the trees are widely spaced, a single furrow is excavated from the main water supply channel, which generally runs between the rows of a trees, so that the water can be directed around the base of individual trees. This method is particularly useful for recently planted trees.
Siphon Tubes;
These are curved tubes, made of plastic material, which can be dipped into the main water supply channel. They are filled with water and then the lower end is quickly directed to the furrow in which the water level is slightly lower than it is in the main channel. The water will then continue to flow into the furrow for as long as the siphon tube remains filled with water.
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Hand Operated Pumps;
These are suitable for use on a garden scale and for supplying water to livestock. Many of these are of the reciprocating piston type which can be used from a well or borehole, others are termed diaphragm and semirotary pumps. These are suitable for lifting water relatively short distances of up to 4 m since the weight of water in the suction pipe limits operation at any greater height.
Hydraulic Ram;
used where the source of water is a perennial stream which has a fall of more than 1m over a relatively short distance. They are capable of lifting water to a height of about 30 m, using the force of the stream to obtain this lift.
Motor Driven Pumps And Sprinklers;
A wide range of pumps which are driven by small petrol or diesel motors is available. Most of these Pumps are of the centrifugal type and are effective for pumping water for relatively long distances through irrigation pipes, usually to sprinklers which distribute the water through rotating nozzles. These systems are most suitable for medium-scale vegetable growers who have a basic understanding of pump and machine operation and can carry out maintenance.
For larger scale operations, tractor mounted pumps can be used; these are also of the centrifugal type and several designs of pump are available, some with additional capacity which are referred to as multistage pumps. These installations are most suitable for plantation crops or high-value annual crops.
Drip Irrigation;
In recent years, the development of the drip irrigation system has shown that tree-crops, in particular, can benefit from the use of this system of irrigation. The water is supplied from a header tank raised a few metres above soil level and runs through plastic tubes to nozzles which are placed at the base of the tree. One nozzle is normally sufficient to provide an adequate supply of water to each tree, the water drips from the nozzle at a rate which is determined by the type of nozzle used. This system has not yet been widely used in West Africa but it is likely that, in drier savanna areas, and particularly on well-drained soils, it could become an efficient method of supplementing the soil water reserves during the dry season.