Wet rice

A paddy field is a flooded parcel of arable land used for growing semiaquatic rice. Paddy cultivation should not be confused with cultivation of deep water rice, which is grown in flooded conditions with water more than 50 cm (20 in) deep for at least a month. Genetic evidence shows that all forms of paddy rice, both indica and japonica, spring from a domestication of the wild rice Oryza rufipogon that first occurred 8,200–13,500 years ago South of the Yangtze River in present-day China. However, the domesticated indica subspecies currently appears to be a product of the introgression of favorable alleles from japonica at a later date, so that there are possibly several events of cultivation and domestication. Paddy fields are the typical feature of rice farming in east, south and southeast Asia. Fields can be built into steep hillsides as terraces and adjacent to depressed or steeply sloped features such as rivers or marshes. They can require a great deal of labor and materials to create, and need large quantities of water for irrigation. Oxen and water buffalo, adapted for life in wetlands, are important working animals used extensively in paddy field farming.

During the 20th century, paddy-field farming became the dominant form of growing rice. Hill tribes of Thailand still cultivate dry-soil varieties called upland rice. Paddy field farming is practiced in Asia, namely in Cambodia, Bangladesh, China, Taiwan, India, Indonesia, Iran, Japan, North Korea, South Korea, Malaysia, Myanmar, Nepal, Pakistan, the Philippines, Sri Lanka, Thailand, Vietnam, and Laos, and in Europe, Northern Italy, the Camargue in France, and in Spain, particularly in the Albufera de València wetlands in the Valencian Land, the Ebre Delta in Catalonia and the Guadalquivir wetlands in Andalusia, as well as along the eastern coast of Brazil, the Artibonite Valley in Haiti, and Sacramento Valley in California, among other places. Paddy fields are a major source of atmospheric methane and have been estimated to contribute in the range of 50 to 100 million tonnes of the gas per annum. Studies have shown that this can be significantly reduced while also boosting crop yield by draining the paddies to allow the soil to aerate to interrupt methane production. Studies have also shown the variability in assessment of methane emission using local, regional and global factors and calling for better inventorisation based on micro level data.

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