Water management in irrigated rice - Rice Knowledge Bank ...

Rice and water11.1 Rice environmentsWorldwide, there are about 150 million hectares ofrice land, which provide around 550–600 milliontons of rough rice annually (Maclean et al 2002).Rice is unique among the major food crops in itsability to grow in a wide range of hydrologicalsituations, soil types, and climates. Rice is the onlycereal that can grow in wetland conditions.Depending on the hydrology of where rice isgrown, the rice environment can be classified intoirrigated lowland rice (79 million ha), rainfed lowlandrice (54 million ha), flood-prone rice (11 millionha), and upland rice (14 million ha). Lowlandrice is also called “paddy rice.” Lowland rice fieldshave saturated (anaerobic) soil conditions with pondedwater for at least 20% of the crop’s duration.In irrigated lowlands, the availability of irrigationassures that ponded water is maintained for at least80% of the crop’s duration. In rainfed lowlands,rainfall is the only source of water to the field andno certain duration of ponded water can be assured(depending on vagaries of rainfall). In flood-proneenvironments, the fields suffer periodically fromexcess water and uncontrolled, deep flooding (morethan 25 cm for 10 days or more). Deepwater riceand floating rice are found in these environments.Upland rice fields have well-drained, nonsaturated(aerobic) soil conditions without ponded water formore than 80% of the crop’s duration.1.2 Irrigated lowlandsThe 79 million ha of irrigated lowlands provide 75%of the world’s rice production (Maclean et al 2002;Fig. 1.1). At the turn of the Millennium, countryaverageirrigated rice yields in Asia ranged from 3to 9 t ha –1 , with an overall average of about 5 t ha –1(Maclean et al 2002). Irrigated rice is mostly grownwith supplementary irrigation in the wet season, andis entirely reliant on irrigation in the dry season.Significant areas of rice are grown in rotation witha range of other crops, such as the 15–20 million haof rice-wheat systems (Timsina and Connor 2001,Dawe et al 2004). Irrigation systems vary widely,and include Individual pump irrigation from shallowtubewells (down to about 15-m depth). Small- to medium-scale community-basedpump irrigation from deep wells (down to200–300-m depth). Small- to medium-scale community-basedsurface irrigation where water is divertedfrom ponds or reservoirs (for example,the tank system in southern India and SriLanka). Small- to medium-scale community-basedsurface irrigation where water is directlydiverted from a river (run-off-the-river irrigation). Large-scale surface irrigation where water isdiverted from reservoirs or lakes. Conjunctive groundwater-surface-waterirrigation schemes (can be small to largescale).In each type of system, the ownership andcontrol of water may vary widely.1.3 The rice field and its water balanceIrrigated lowland rice is grown under floodedconditions. Mostly, rice is first raised in a separateseedbed and subsequently transplanted into therice field when the seedlings are 2–3 weeks old.