Water is a finite resource essential for all forms of life; without water, life as currently known could not exist. Most of the hydrosphere is salt water (97.20%); freshwater represents only 0.65% (8.5 x 106 km3). This quantity of water is not uniformly distributed, in either space or time. In many countries and continents the freshwater supply is becoming scarce or has become an actual problem. Drought has aggravated the situation. During the past 20 years, the population has increased significantly. As a result, water requirements also have increased.

Over the next 40 years, the world's population will increase from 5.3 x 109 to about 9 x 109 inhabitants. Thus, the agricultural sector will be encouraged to duplicate the world's food production and to improve average yields and the proportion of irrigated lands. More than 82% of the world's crop land is not irrigated, depending only on rain. In many potentially arable lands of semiarid areas, water is a limiting factor and high costs often make irrigation schemes unattractive. A large part of rainfed agriculture requires soil and water conservation measures and techniques aimed at preventing degradation and maintaining yield potentials. Sustainable rainfed agriculture requires management of the land so that production and productivity are enhanced while a healthy ecological balance is sustained within the agricultural ecosystem.

Irrigated lands include 18% of cultivated land, but produce 33% of the food that is necessary for the world population. Technologies and management techniques are available to improve the use of irrigation water, including off-farm and on-farm measures for improving efficiency and conservation. Irrigation scheduling for example, is a management technique that helps to determine the timing and the amounts of water to be applied.

In summary, water resources play a major role in agriculture in general, and in irrigated agriculture in particular. Unfortunately, our ability to substantially increase crop production in dryland farming has not been given as much attention as it has been in irrigated agriculture. Efficient water use in irrigated agriculture can achieve several goals, such as conserving freshwater, maintaining high standards of public safety, and improving environmental quality. Water availability can be controlled through irrigation and also through a range of other management techniques. To maximize water-use efficiency when water supplies are limited, the farm manager has a variety of options available. This chapter discusses water management in rainfed and irrigated agriculture.

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