Environmental Engineering Reference
In-Depth Information
IRRIGATION
The usual reason for developing the water resources of an area is to provide water for
irrigation. The need for irrigation may be to compensate for rainfall variability or it may
be to provide a regular water supply when and where rainfall is low. Whatever the reason
for the provision of water, irrigation allows crops to be produced whenever temperature
conditions allow.
The amount of water required by an irrigation system will depend upon many factors,
such as the type and stage of development of the crop, temperature and rainfall levels and
the nature of the soil. For the most efficient use of the water these factors have to be
taken into account; too much water or too little water does not produce maximum yields.
The methods of application of the irrigation water vary widely. In developing
countries surface methods are most common. These can range from simple traditional
'flow diversion' techniques to large and sophisticated 'centre pivot' schemes.
Unfortunately surface and sprinkler irrigation methods in the tropics lead to large
evaporation losses. To reduce them, attempts have been made to use underground water
transport in plastic tubing, but pumping is required to maintain flow. Similarly, trickle
irrigation can be utilized whereby water is released through small nozzles near the plants
or trees (Plate 27.8). The amounts of irrigation water needed are less than with the
traditional methods, as losses through seepage and evaporation are reduced. Weed
competition is minimized as most of the plots are dry. Enough water can be passed
through the root zone to prevent the build-up of salt. This method does have potential,
especially for tree crops, where the water can be directed straight to the roots.
Although irrigation holds great potential for food production by providing the
necessary water in dryland areas, there are many problems. Unless the soil and water
