Because plants require water in order to grow, agriculture represents the largest single use of global water. About 73 percent of all fresh water withdrawn from groundwater supplies, rivers, and lakes is used in the irrigation of crops. Almost 15 percent of the world's croplands are irrigated. Water use varies among countries. Some countries have abundant water supplies and irrigate liberally, while water is very scarce in other countries and must be used very carefully. Because as much as 80 percent of the water intended for irrigation is lost to evaporation before reaching the plants, the efficiency of water use in some countries can be very low.
There is no doubt that irrigation has dramatically increased crop production in many areas, but some irrigation practices have been detrimental. Overwatering can lead to a water logging of the soil. Water logging cuts off the supply of oxygen to the roots, and the plants die. Irrigation of crops in dry climates can often result in salinization of the soil. In these climates, the irrigation water rapidly evaporates from the soil, leaving behind mineral salts that were dissolved in the water. As the salts accumulate, they become lethal to most plants. Some experts estimate that as much as one-third of the world's agricultural soil has been damaged by salinization. There is also an argument as to whether the increased usage of water for agriculture has decreased the supply of potable water fit for other uses.
See also: Agricultural Diversity, Impact on Soil Resources, Fertilizers, Commercial Impact
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