Irrigation

Irrigation is the artificial application of water to land for the purpose of supplying moisture essential for plant growth. Although irrigation is an old art, its importance has increased in recent times with the increased demand for food to meet the needs of a growing population. Today, irrigation accounts for 70 percent of all water used by humans. Although irrigation works are found almost everywhere, they are most important in arid and semiarid regions, where the quantity and timing of rainfall are not inadequate for crops. For instance, in those parts of Asia where rainfall is seasonal, and most precipitation occurs in three to four months of the year, irrigation is highly critical to agriculture.

The importance of irrigation is evident from the fact that, historically, the development of human civilizations has followed the development of irrigation. For instance, Egypt claims to have had the world's oldest dam, 355 feet long and 40 feet high, built some 5,000 years ago to store water for drinking and irrigation. In fact, basin irrigation introduced in the Nile valley around 3000 b.c.e. still plays an important part in Egyptian agriculture. Similarly, in the valley of the Euphrates and the Tigris (the two rivers which pass through Turkey, Syria, and Iraq, and which defined what was once called Mesopotamia), there are remains of the largest irrigation canals built around 2200 b.c. Historical references to the practice of irrigation from wells, tanks, and canals are also found in countries such as China, India, Iraq, the former U.S.S.R., Mexico, and Peru.

The importance of irrigation has grown further since the 19th century, particularly with the application of modern engineering technology. The 19th century saw the world's irrigated area increase from 8 million hectares to about 40 million hectares, of which the single largest share was that of India (17 million hectares). Considerable irrigation development also took place in the western United States and Italy in the latter half of the 19th century. Irrigation development continued in the 20th century, particularly after World War II, when the use of irrigation technology, along with other inputs, led to the so-called Green Revolution in many parts of Asia. But since the late 1970s, irrigation expansion has slowed markedly due to a number of reasons such as low commodity prices, high energy costs, and economic conditions that discourage agricultural production.

Currently, the Asian continent accounts for nearly 70 percent of the irrigated area in the world, and the Americas for 15 percent. The three countries with the largest areas under irrigation are India, China, and United States, in that order.

According to estimates by the Food and Agriculture Organization, the share of irrigation in world crop production is expected to increase in the next decades, although the rate of increase will slow down. Irrigated land will expand by 45 million hectares to reach a total of about 242 million hectares by 2030, at a projected annual growth rate of 0.6 percent compared with the 1.9 percent observed in [Page 957]the period from 1963–99. The expansion of irrigation is projected to be strongest in south Asia, East Asia, and Near East/North Africa. Rising economic and environmental costs, along with declining public investments in irrigation, means that a significant increase in the rate of addition of irrigated areas is unlikely in the future; therefore, irrigation's contribution to food production will have to come from improving existing systems rather than from expanding them to new lands.

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