Worldwide, around 79 million ha of rice is grown under irrigated conditions. While this is only half of the total rice area, it accounts for about 75% of the world's annual rice production. In Asia, nearly 60% of the 138 million hectares devoted to rice production annually is irrigated, where rice is often grown in monoculture with two to three crops a year depending upon water availability. Other rice ecosystems include the rainfed lowland (35% of total rice area), characterized by a lack of water control, with floods and drought being potential problems, and the upland and deepwater ecosystems (5% of total rice area), where yields are low and extremely variable.