Thai farmers traditionally relied on rain and flood water for crops, but the amount needed for rice cultivation was not always received. By the mid-1800s, a number of canals had been constructed in the central plain to carry floodwaters from the Chao Phraya, and in the latter half of the century other canals ÍÍÍÍwere dug. The canals did not form a controlled irrigation system, however, but simply a distribution net, and whether additional water could be made available depended on the level of the rivers. Records covering almost a hundred years to 1930 showed that in about one-third of the years water from the rivers was insufficient, resulting in considerable crop losses. In 1902 the government contracted with a Dutch expert to develop a controlled irrigation plan for the entire country but failed to take further action. Droughts in 1910 and 1911 led to renewed interest and the hiring of a British irrigation specialist. Nevertheless, the first irrigation project was not completed until 1922.

By 1938 about 440,000 hectares had been irrigated. Supply problems held up projects during World War II, but work resumed with renewed vigor in the late 1940s. By 1950 the irrigated area totaled nearly 650,000 hectares. In 1950 Thailand secured the first of a series of loans from the World Bank (see Glossary) for the construction of the vital Chainant Diversion Dam on the Chao Phraya and a number of major canals. By 1960 over 1.5 million hectares had been irrigated, almost entirely in the Center and in the North.

Systematic development of the irrigation system began with the First Economic Development Plan (1961-66) and was continued in later plans. New assistance from the World Bank included financing of the important multipurpose Phumiphon (Bhumibol) Dam (completed in 1964) on the Mae Nam Ping and the Sirikit Dam (completed in 1973) on the Mae Nam Nan. These dams, both of which have associated hydroelectric power-generating facilities, impound water at two large reservoir locations in the Chao Phraya Basin. Other World Bank-financed projects were also carried out in this basin during the 1970s, and by the end of the decade nearly 1.3 million hectares had controlled water flow in the rainy season, and about 450,000 hectares had it in the dry season.

The Chao Phraya Basin's natural features, as well as its size, made it the most important area for irrigation development. The topography and water systems of the Northeast, by contrast, were not well suited to large-scale irrigation projects (except on the Mekong River, which would involve major resettlement problems). Controlled irrigation potentially could encompass about 10 percent of the Northeast's 3.5 million hectares of paddy. Beginning in the 1960s, the Royal Irrigation Department, founded in 1904 and largely responsible for development and maintenance of the country's main irrigation systems, constructed 6 large and about 200 small dams in the region. The associated irrigation system contained design defects, and in the mid-1970s improvement was undertaken with World Bank assistance 75c. Part o of the irrigable area was receiving water in the early 1980s, but completion of necessary additional work was not anticipated before the late 1980s, at which time about 160,000 hectares would have irrigation throughout the year.

Irrigation work also began in the 1960s in the Mae Nam Mae Klong Basin, which contained nearly 400,000 irrigable hectares of paddy. Regulated wet-season irrigation was furnished during the 1970s for roughly 175,000 hectares. A multiple dam completed in the late 1970s and a distribution system under way in the 1980s was expected to provide adequate water for double cropping on over 250,000 hectares. Small irrigation projects also were started in the 1960s in the South, on the east coast where more than 500,000 of the region's 600,000 hectares of paddy were located. About 75,000 hectares had supplementary wet-season water, and work under way in the 1980s in the Mae Nam Pattani Basin was expected eventually to serve about 52,000 hectares.

Data as of September 1987