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
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.