Rice grows in a wide range of environments and is productive in many situations where other crops would fail. Most classifications of rice environments are based on altitude (upland vs. lowland) and water source (irrigated or rainfed).

Rice grows in a wide range of environments and is productive in many situations where other crops would fail. Most classifications of rice environments are based on altitude (upland vs. lowland) and water source (irrigated or rainfed).
Irrigated rice environments
Worldwide, about 80 million hectares of irrigated lowland rice provide 75% of the world’s rice production. These systems remain the most important rice production systems for food security, particularly in Asian countries. Irrigated rice is grown in bunded fields or paddies, which are surrounded by a small embankment that keeps the water in. Water supply is more assured and one or more crops a year can be grown. Farmers generally try to maintain 5–10 centimeters (cm) of water (‘floodwater’) on the field. By and large, irrigated rice farms are small, with the majority in the 0.5 to 2 hectare range. In many humid tropical and subtropical areas, irrigated rice is grown continuously with two or even three crops a year. This practice of growing rice after rice is centuries old and has been shown to be one of the most sustainable agricultural systems in the world because of the unique nutrient cycling that occurs in flooded rice fields.
Significant areas of irrigated rice are also grown in rotation with a range of other crops, including about 20 million hectares of rice