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-wheat systems. Irrigated rice receives about 40% of the world’s irrigation water and 30% of the world’s developed freshwater resources. The productivity of rice is typically higher when irrigated, with average irrigated yields about 5.4 t/ha. In temperate climatic regions, a single irrigated rice crop is grown per year, with high yield that can reach 8–10 t/ha or more.
Rainfed lowland environments
Rainfed lowland rice is grown in river deltas and coastal areas, using bunded fields that are flooded with rainwater for at least part of the cropping season. About 60 million hectares of rainfed lowlands supply about 20% of the world’s rice production. Rainfed rice environments experience many abiotic stresses, such as salinity, and high uncertainty in timing, duration, and intensity of rainfall. Some 27 million hectares of rainfed rice are frequently affected by drought. Up to 20 million hectares may experience uncontrolled flooding, ranging from flash floods of relatively short duration to deepwater areas that may be submerged under more than 100 cm of water for a few months. Salinity is widespread in coastal areas.
Rainfed lowland rice predominates in areas of greatest poverty: South Asia, parts of Southeast Asia, and essentially all of Africa. Because the environments are so difficult and yields so unreliable, farmers rarely apply fertilizer and tend to not grow improved varieties. Productivity in rainfed lowland areas is typically very low, with yields of 1–2.5 t/ha. Rice farming families in these areas often remain trapped in poverty.
Rainfed upland environments
Nearly 100 million people depend on the production of rice from rainfed upland regions to provide them with rice to eat as their daily staple food. Almost two-thirds of the world’s total upland rice area is in Asia. Bangladesh, Cambodia, China, India, Indonesia, Myanmar, Thailand, and Vietnam are important producers. Rainfed upland rice covers about 14 million hectares but, because of many constraints, the productivity from these areas is typically low with yields of about 1 t/ha. Upland rice contributes only about 4% of the world’s total rice production.
Rainfed upland rice is grown much like wheat or maize, in mixed farming systems without irrigation and without puddling. The ecosystem is extremely diverse, including fields that are level, gently rolling or steep, at altitudes up to 2,000 meters and with rainfall ranging from 1,000 to 4,500 mm annually. Soils range from highly fertile to highly weathered, infertile and acidic, but only 15 percent of total upland rice grows where soils are fertile and the growing season is long.
Upland environments are highly heterogeneous, with climates ranging from humid to subhumid, soils from relatively fertile to highly infertile, and topography from flat to steeply sloping. With low population density and limited market access, shifting cultivation with long (more than 15 years) fallow periods was historically the dominant land-use system. Some 70% of Asia’s upland rice areas have made the transition to permanent systems where rice is grown every year and is closely integrated with other crops and livestock.
Many upland farmers plant local rice varieties that do not respond well to improved management practices – but these are well adapted to their environments and produce grains that meet local needs.
Although the rice technology of the 1960s and 70s focused on irrigated rice, farmers in the uplands were not forgotten. Researchers produced cultivars adapted to poor soils, and with improved blast resistance and drought tolerance. Some have outyielded traditional varieties by more than 100 percent in evaluations. Scientists at national agricultural research systems have crossed these improved rices with local cultivars and farmers are now beginning to grow the progeny. But more improvements are needed to meet the new challenges.
In Central and West Africa, the rice belt of Africa, upland areas represent about 40% of the area under rice cultivation and employ about 70% of the region’s rice farmers. As market access remains limited, most of the world’s upland rice farmers tend to be self-sufficient by producing a range of agricultural outputs. Poverty is widespread in these upland areas.