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<br />A heavily surface-water-dependent nation like Thailand is also quite <br />reliant on the rainfall in each month, season, and year. The southwest <br />monsoon, or wet season (May-October), brings up to 80 percent of the annual <br />rain and it can begin late or end early or be weak; any of these factors <br />will reduce the annual rainfall by 10 to 35 percent. Within the wet season <br />and particularly in the rain shadow areas of northern and northeastern <br />Thailand, dry periods of 1 to 4 weeks length develop in the June-August <br />period. These often become disasters to heavy wate!r demand crops like <br />rice, the principal subsistence crop. <br /> <br />Within Thailand the major needs and uses for water can be classed into two <br />types, each geograpnically distinct. The first is in the central plains <br />where extensive irrigated areas have developed that. are supplied by water <br />from a series of major reservoirs; yet water is adequate to irrigate only <br />35 percent of the area in the dry season. These are multipurpose reser- <br />voirs operated to handle the floods from occasional tropical storms and to <br />provide water for urban supplies, for dilution in lower river pollution <br />episodes and to generate hydroelectric power (15 percent of the nation's <br />supply). The irrigated areas here represent only 15 percent of the <br />nationls agricultural lands, but they produce nearly 50 percent of the crop <br />value. The economy of the nation is strongly tied to this central area and <br />the needs served by these reservoirs. Hence, any added water in these <br />reservoir systems is of immense value. These reservoirs are in basins <br />which generally have relatively high average rainfall. <br /> <br />The second area of major water need is the northeast, a large rOlling pla- <br />teau with mountains to its west and south. These mountains and the area's <br />location farther from the Indian Ocean than other areas of Thailand lead <br />.. to the nationls lowest average rainfall (1000 to 1200 mm annually) and to <br />weather conditions in June-August with dry periods very detrimental to high <br />water demand crops. This area is largely devoted to rainfed agriculture <br />and contains one-third of all Thai farms. The area also has many low <br />moisture capacity soils, and when irrigated the soils often develop sali- <br />nity problems. Thus, the geology, soils and climate interact to provide <br />strong needs for additional rainfall. <br /> <br />The rapid transition of Thailand (shifts of agriculture to industry, low to <br />middle income, rural to urban population, and subsistence to cash crop <br />agriculture) has produced a series of water problems. These fall broadly <br />under two headings: too much water and too little 'water. The factors <br />helping to cause too little water include: <br /> <br />o Climatic conditions with inadequate rain amounts to meet crop <br />demands, and considerable temporal variabilit.y of rainfall between <br />years and within a season (dry periods and droughts). <br /> <br />o Salt (sea) water intrusion in major rivers. <br /> <br />o Water pollution due to soil erosion, and displJsal of various wastes. <br /> <br />o Loss of storage due to silting and lack of ma'lntenance. <br /> <br />o Over-pumpage of groundwater giving rise to subsidence of Bangkok and <br />resultant poor drainage and flooding. <br /> <br />10 <br /> <br />~ <br /> <br />~ <br />~ <br /> ~ <br />9 <br /> ~ <br />