General summary Asia
Drainage, flood control and environmental and health issues
In most of Asia, drainage is closely linked to irrigation. In traditional terraced paddy cultivation, water flows from one plot to another and no distinction can be made between irrigation and drainage. Bhutan, Japan, China, the Philippines and Viet Nam have specifically mentioned this type of drainage but it can apply to most of the area where paddy rice is cultivated.
In several humid countries of the region, large segments of lowland or wetland are used for paddy cultivation. In such cases, while these areas are generally or usually accounted for as irrigated land, the main purpose of water control is to ensure appropriate control of water level and drainage. Typologies differ from one country to another to indicate very similar situations. Bangladesh and Cambodia use the terms controlled flooding or inundation, which are typical of paddy cultivation in the major deltas (Brahmaputra, Mekong). Lao PDR reports on lowland flooded rice.
In these areas, drainage and flood control are also very much related. In Bangladesh, on average, 22 percent of the country is flooded every year and 50 percent of water development expenditures are spent on flood control and drainage. In Myanmar, in the Ayeyarwady Delta, drainage and flood control structures are also linked: in 1995 a total of 193 000 ha were reported to be equipped for surface drainage which is considered as a form of flood protection. Drainage covers 1 million ha in north and central Viet Nam, mostly in the Red River Delta. Flood protected areas in China represent 32.69 million ha. The extreme case of agriculture under flood conditions is floating rice, which is reported in Cambodia, but can probably be found in other countries of the sub-region.
Data on drainage infrastructure associated to irrigation in arid and semi-arid areas concern mostly northern China, India and Mongolia. In China as a whole (no distinction can be made between arid and humid areas), it was estimated in 1996 that 24.58 million ha were subject to waterlogging, of which 20.28 million ha were equipped with drainage. In India, drainage works have been undertaken on about 5.8 million ha (12 percent of the irrigated area), but investment in drainage works associated with irrigation schemes has been widely neglected and drainage systems are usually in very poor maintenance condition. No data were available from Mongolia.
A specific case of drainage is reported for Malaysia when 940 000 ha are drained, of which 600 000 ha for oil palm cultivation.
In arid and semi-arid areas of northern China, waterlogging, salinity and alkalinization are considered serious constraints on agricultural development in irrigated land. Saline/alkaline cultivated land in China covers 7.73 million ha (5.51 million ha of which have been improved). In India, waterlogging due to irrigation covers an area of about 2.46 million ha; 3.06 million ha are affected by salinity and 0.24 million ha by alkalinity problems. Salinization is also reported in the central dry zones of Myanmar, where major groundwater pumping irrigation schemes are located.
Although total water withdrawal remains limited compared to water resources in Southeast Asia (about 5 percent), the large amounts of water diverted, mostly for agriculture, in those countries, have an environmental impact which may assume important proportions locally. Intrusion of saltwater in deltas is a concern in Myanmar, Viet Nam and parts of India. Excessive groundwater exploitation around Bangkok, in Thailand, creates land subsidence and exacerbates already existing flood problems. In several countries, competition for water is becoming increasingly important, with direct implications for agriculture. This point is discussed in the next section.
Information on water-borne diseases is difficult to obtain and probably very incomplete. Cambodia, Philippines and Thailand have detailed statistics on the subject, which are summarized in Table 10.