Irrigation avoids the constraints that inadequate water supply places on crop production in most parts of the world. The most spectacular irrigation schemes are those in arid regions of mid-latitudes where fully irrigated crops can achieve high yields made possible by high insolation. But irrigation practice covers a wide range of locations, environments, methods of water application, and production targets. Increasingly, as competition for water resources intensifies, so is the practice of deficit irrigation that is designed to increase the efficiency of limited water supply by removing only the most serious limitations of water shortage to crop growth. At the same time it is evident that excess irrigation in some places is not only wasting water but also reducing productivity and causing environmental damage, especially through salinization. This chapter deals with the principles of environmentally sound and productive use of water in irrigation as a key issue in crop ecology and natural resource management.

Irrigation and world food supply

Irrigated agriculture produces an estimated 40% of world crop production from 280 Mha, or just 18% of the total cropped area (FAOSTAT). For this it uses 69% of all withdrawals from streams and ground water compared with 21% for industry and 10% for domestic use. As population increases, irrigated agriculture is facing increasing competition for water from other uses at a time when the expansion, not restriction, of irrigated area is an evident way to increase food production (Wallace 2000).