Validation of Changing Water Origins over Indochina during the Withdrawal of the Asian Monsoon using Stable Isotopes

Daily rainfall and delta O-18 were observed from August to November 2001 at three stations in Thailand. Water origins were estimated using a tagged-water transport model. The temporal and spatial distributions of water origins showed a clear transition from the Indian Ocean to the Pacific Ocean as the Asian monsoon retreated from Indochina. A new definition of the withdrawal date of the Asian monsoon over Indochina is proposed that considers the transition of the origin of water vapor. The daily variability of precipitation isotopes at three stations in Thailand was reproduced at statistically significant levels using a Rayleigh-type isotope circulation model. Therefore, the atmospheric water balance in the model can be validated reliably with these precipitation isotopes. Nevertheless, the simulated delta O-18 values were obviously underestimated, perhaps because of uncertainties in the evaporating isotope ratios and the lack of isotope physics at small scales.