Irrigation development may have both positive and negative impacts on the environment. To be sustainable, irrigation must avoid the negative impacts.
The positive aspect of irrigation is that, by intensifying food and
forage production in the most favourable lands, it can allow a country to reduce excessive
pressure on marginal lands now under rain-fed cultivation or grazing. Such lands are
already undergoing a process of degra-dation (known, in semi-arid areas, as desertification).
The transition of people who have subsisted for generations on rain-fed lands to irrigated
farming may be a difficult social change. However, change will in any case be unavoidable
in areas where land degradation becomes acute. Where the opportunity exists for irrigation
devel-opment, it can serve as a constructive alternative to either famine or mass
The potentially negative environmental impacts of irrigation development may occur off-site as well as on-site. The off-site effects may take place upstream of the land to be developed, as where a river is to be dammed for the purpose of supplying irrigation water. Another set of problems may be generated downstream of the irrigated area by the disposal of excess water that may contain harmful concentrations of salts, organic wastes, pathogenic organisms and agrochemical residues.
Of most direct concern are the potential on-site impacts. Irrigated lands, especially in river valleys prone to high water-table conditions, typically require drainage. Otherwise, they are subject to the twin scourges of waterlogging and salinization. Because groundwater drainage is a complex, exacting and expensive operation (often more expensive than the initial development of irrigation itself), there is a temptation to start new irrigation projects while ignoring the need for drainage or delaying its installation until it is actually needed. The trouble is that, by the time the need for drainage becomes inescapable, the cost of implementing it may be prohibitive.
A thorough discussion of drainage is beyond the scope of this publication. Suffice it to say here that irrigation developers should be mindful of the potential need for drainage and make provision for it in their plans. At the very least, irrigators in each area should monitor the position of the water-table as it tends to rise, by means of observation wells (piezometers). Sampling the water in such wells will allow monitoring of the quality of the groundwater towards which the leaching fraction of irrigation water percolates. Such monitoring can provide an early warning of the eventual danger of salinization, and can guide irrigation practice towards minimizing that danger. Although small-scale irrigation development is less likely to cause waterlogging and salinization than large-scale development, the danger of land degradation should never be ignored.