Irrigation is the watering of land by artificial methods. It provides water for plant growth in areas that have long periods of little or no rainfall. The water used for irrigation is taken from lakes, rivers, streams, and wells.
Irrigation is used chiefly in three types of climates. In desert regions, such as Egypt and the southwestern United States, farming would be impossible without irrigation. In regions with seasonal rainfall, such as California and Italy, irrigation makes farming possible even during dry months. In moist regions, such as the eastern United States and western Europe, irrigation maintains crops during periods of drought.
Throughout the world, over 550 million acres (220 million hectares) of land are under irrigation. This includes over 50 million acres (20 million hectares) in the United States and about 1 3/4 million acres (700,000 hectares) in Canada.
The amount of water needed for farming varies with the type of crop and the climate. For example, rice requires more water than does cotton. Wheat grown in a warm climate needs more water than wheat that grows in a cool climate. Any farmland must receive enough water to allow both for plant growth and for the evaporation of water from the soil.
In some countries, more water is used for irrigation than for any other purpose. Irrigation accounts for about one-third of the water used in the United States. As the world's population grows, the demand for water increases steadily. More and more people need water in their homes, and industry must have additional water as production rises. Steps have been taken to ration water in areas where the demand for water exceeds the supply. In the western United States, for example, the government rations water from the Colorado River to various states.
