Rice (Oryza sativa L.) is one of the most important cereals that supplies 20% of the total calorie consumption of the world in 2000 (IRRI 2007). Rice is cultivated in many countries throughout the world, but 88.4% of rice area is concentrated in Asia and its production reaches 90.4% of the total of the world (Table 14.1). The current rice production is more than twice of that in the 1960s, and will further increase to feed the growing human population, especially in Asian countries.
According to the water availability, rice area is distributed to the following four environments: irrigated, rainfed lowland, upland, and deepwater. In Asia, the rice area distributed to irrigated, rainfed lowland, upland, and deepwater is 57, 33,7 and 3%, respectively (Fig. 14.1). Irrigated rice is grown in lowland fields with ensured irrigation water for one or more crops a year. Now-a-days, some kinds of irrigation systems are adapted to all the producing countries, but are not entirely spread over because of the socio-economic and geographical reasons. Rice yield in each country almost corresponds to the spreading rate of irrigation system, though other inputs, such as fertilizer and machinery, also contribute to the yield (Table 14.1 and Fig. 14.1). Therefore, it is expected that irrigation systems will spread to paddy fields in currently non-irrigated lowland area with the socio-economic advancement in the near future.
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