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" peak at the onset of the southwest monsoon; (2) a shift in rainfa~l during the June-August period; and (3) a major second peak in September. The two peaks of rain occur as the Intertropical Convergence Zone (ITCZ) moves across Thailand with an associated increase in large-scale con- vergence. In addition, the September rainfall peak is often enhanced by the occurrence Of tropical storms and an occasional typhoon that penetrates Thailand during late sumner' and early fall. The total seasonal rainfall in these periods can vary between years because of the late onset of the season in Mayor an early ending in October, a lack of tropical disturbances or the predominance of a generally weak monsoon. There are important spatial differences in the rainfall climate of Thailand due to latitudinal differenCI!S, topographic effects, and land- water interfaces. Major north-south finountains extend along Thailandls western boundary wi th secondary rangeli across the center of Thai 1 and and extending eastward to Cambodia. During the southwest monsoon, these ranges enhance clouds and intensify rain development since they are generally perpendicular to the prevailing moist airflow. They also produce "rain shadows" in their lee and account for the areas of least average rainfall (1000 mn) found in the north and northeast. High rain- fall occurs along the Malay Peninsula during both the southwest and northeast monsoons. This area has an equatorial climate with no distinct dry season. The coastal areas in the southeast of Thailand receive a great deal of rain during the southwest monsoon when they intercept the moisture-laden air tranversing the Gulf of Thailand. Rain in the wet season occurs on 50 percent (June) to 70 percent (September) of the days, but is often rather light. Thunderstorms, occurring on 40 to 50 days per season in most localest bring heavier localized rains. Extremely heavy rains (300 to 600 mm/clay) are usually a resul t of major tropical storms that migrate into Thailand from the South China Sea, Gulf of Thailand, or Andaman Sea. Prolonged (2 to 7 day) periods of moderate to heavy rains can occur in May and September as the ITCZ and its attendant trough and associated 1 CIW centers move across the country. High damaging winds sometimes accompany typhoons along the coastal area, and small hail occurs infrequently frclm thunderstorms in the north. 2. Water resources T. In most regions of the world the water' resources reflect the climate with the influence modified by the surface topography, soils, and geology. Climate is particularly critical to Thailand since surface water is the prevalent source of water' used. Hence, rainfall is criti- cal in terms of both timing and distribution~ The average annual surface runoff for Thailand is 171,206 million cubic meters (MCM) with a variation in average annual values of + 27,000 MeM. Almost 50 percent, or 81,000 MCM, of the total runoff is ciptured in 17 major reservoirs, and 90,000 MCM is in river flows and many smaller impoundments. Development of surface water storage has followed two directions: (1) the construction of 17 major multipurpose projects on major streams with large reservoirs and distribution systems that pri- marily serve irrigation and hydropower'; and (2) thousands of local small-scale impoundments (1,000 to 12,000 hectares) to serve the agri- 7