Using both Rayleigh-type Isotope Circulation Model (ICM) and Colored Moisture Analysis (CMA), the origin of rainwater was estimated at three stations in Thailand. The daily variability of isotopic compositions in precipitation from August to November in 2001 were well reproduced by the ICM, but the simulated value was constantly underestimated compared with the observation. The origin of rainwater during 2001 was estimated by the CMA that is ICM with a tagged-water transport scheme. Especially during the rainy season (from May to September), most of the precipitable water was derived from the Indian Ocean (IDO), the mixing ratio of IDO to total amount were about 60%, 74% and 87% at ChiangMai, Bangkok and Phuket, respectively. The mixing ratio of Java Sea and Pacific Ocean at Bangkok and Phuket were much greater than that for ChiangMai during from January to April. At ChiangMai, however, the mixing ratio of Land (LND) was greater than those for Bangkok and Phuket during from October to December. The mixing ratios of precipitable water from IDO were increasing from north to south depend on the strength of Asian summer monsoon. Also, the mixing ratios of precipitable water from LND were increasing from coastal to inland depend on the evaporation of land surface.
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