The share of irrigated land ranges widely, from 4 percent of the total area cropped in Africa to 42 percent in South Asia. The leading countries are India and China with about 58 million and 55 million irrigated land, respectively (amounting to 29 percent and 52 percent of all cropland).
Irrigation and drainage will continue to be an important source of productivity growth, especially in Sub-Saharan Africa and parts of Latin America that still have large untapped water resources for agriculture. In other regions where the scope for further expanding irrigated agriculture is limited, more efforts are needed to enhance the policy, technical, and governance aspects of agricultural water use.
Many areas in dry climates have natural brackish or salty lakes, or even dry salt pans. Soils are often laden with lime (calcium carbonate) or salt (sodium chloride), and many sedimentary rocks contain natural salts. Promising irrigation areas thus may have natural salts in rocks or soils that will easily be transferred into fields as soon as irrigation water is applied, and even that water may come from rivers that have become saltier from evaporation along their courses.
Investment in irrigation projects pays best in dry areas where evaporation is high. Water is never pure, but has mineral salts dissolved in it. Evaporation will therefore make it saltier still. Rivers flowing through dry or desert areas lose water by evaporation, and become salty.
Ancient people must have had strong backs from having to haul buckets full of water to pour on their first plants. Pouring water on fields is still a common irrigation method today—but other, more efficient and mechanized methods are also used.
Almost 60 percent of all the world's freshwater withdrawals go towards irrigation uses. Large-scale farming could not provide food for the world's large populations without the irrigation of crop fields by water gotten from rivers, lakes, reservoirs, and wells
If drainage is inadequate, salts may accumulate, a process called salination. Salination occurs because the irrigation water carries salts to the fields, but the plants absorb only the water and leave the salt behind. Due to the lack of drainage, these excess salts remain on the fields, where they can cause serious harm to the plants and even prevent the plants from growing.
Irrigation is the application of water to farmland by means other than rainfall. It is usually required for growing crops in regions that receive less than 10 inches of precipitation per year and highly advisable in regions that receive only slightly more precipitation or have long dry seasons or droughts during the growing season. More than half of all the farmers in the world practice some form of irrigation.
More sophisticated methods of water application are used when larger areas require irrigation. There are three commonly used methods: surface irrigation, sprinkler irrigation and drip irrigation
An adequate water supply is important for plant growth. When rainfall is not sufficient, the plants must receive additional water from irrigation. Various methods can be used to supply irrigation water to the plants. Each method has its advantages and disadvantages. These should be taken into account when choosing the method which is best suited to the local circumstances.