Irrigation is an agricultural technique to supply crops with water. The importance of an irrigation system is that it makes available the one and only resource that limits agricultural development in the area: water. So significant is irrigation to agriculture that the world's irrigated croplands account for 73% of annual global water withdrawals, the bulk of most countries' water supplies. It is invaluable in helping to feed the growing numbers of people around the world. However, the UN Food and Agriculture Organization (FAO) estimates that half of the total area of irrigated croplands may be in danger from salinization, alkalization and waterlogging, typically as a result of over-irrigation and poor drainage conditions. It is not uncommon that 70-80% of the irrigated water does not reach the intended crops. In most cases, irrigation management needs to be improved.
Increasing the efficiency of irrigation would call for improvement in technical infrastructure and adoption of more efficient management methods. For instance, lining irrigation canals can save water by minimizing seepage. Even more effective would be avoiding the use of more water than is necessary, through the assessment of water needs for crops in various places, education of farmers on optimal use of water, and adoption of more efficient irrigation technology. It appears that drip irrigation is the most efficient method. Given the wide range of efficiency for the various other systems – some 40-80% for gravity flow, 75-85% for a centre pivot sprinkler, and 60-92% for a drip system – use of more efficient methods needs to be combined with sound management in order to ensure the best test result.
Farmers could also reduce water withdrawal by scheduling their irrigation according to the actual weather conditions, evapo-transpiration rates, soil moisture and water requirement for particular crops. Co-ordination of the use and management of ground water and surface water can significantly increase the total efficiency of irrigation water in particular agricultural regions. Other available options to reduce the pressure of the demand for fresh water are the use of brackish water and the use of treated waste water for irrigation of salt-tolerant crops and for supplying certain industrial users. Brackish water for irrigation already plays a certain role in some countries, particularly in western Asia.
Recently, the cost/benefit issue has led to analysing irrigation investments in terms of grain prices needed to cover costs. In developing countries, capital costs for large-scale irrigation projects typically amount to some US$5,000/hectare, whereas those for small-scale projects such as tubewells and line lift pumps amount to only $1,000/hectare. In view of the cost differential, large incremental grain yields would be required to justify large-scale systems. In the past, large projects have contributed only 1/3-1 ton/hectare in yield increase as against a requirement of 2.5-15 tonnes/hectare to justify costs. It is therefore preferable both from the ecological and economic points of view to concentrate more efforts on irrigation systems that involve low capital costs.