Spatial and temporal distributions of stable isotopes in precipitation over Thailand |
| |
| Authors: | Jeerapong Laonamsai Kimpei Ichiyanagi Kiattipong Kamdee Aksara Putthividhya Masahiro Tanoue |
| |
| Institution: | 1. Department of Earth and Environmental Sciences, Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto, Japan;2. Thailand Institute of Nuclear Technology, Nakhon Nayok, Thailand;3. Department of Water Resources Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok, Thailand;4. Center for Global Environmental Research, National Institute for Environmental Studies, Tsukuba, Japan |
| |
| Abstract: | Spatial and temporal variations of the isotopic composition of precipitation over Thailand were investigated. The local meteoric water line for Thailand deviates slightly from the global meteoric water line, with lower slopes (7.62 ± 0.07, 7.59 ± 0.08) and intercepts (6.42 ± 0.39, 6.22 ± 0.42) using ordinary and precipitation weighted methods. Differences in spatial and temporal δ18O distributions between the tropical monsoon and tropical savanna climate zones were found due to differing moisture source contributions and seasonal precipitation patterns. The temporal data reveals that the northeast monsoon rains originate from isotopically-enriched local moisture with isotope values of ?9.36 to ?0.09‰ (mean ? 3.73 ± 0.42‰), whereas the southwest monsoon clouds had a more significant rainout effect from Rayleigh distillation, with isotope values of ?9.56 to ?1.78‰ (mean ? 5.40 ± 0.38‰). The precipitation amount at each site was negatively correlated with δ18O (?0.24 to ?3.20‰ per 100 mm, R2 = 0.1–0.9). Furthermore, δ18O was negatively correlated with geography (latitude, altitude) for the southwest monsoon periods, as expected based on other observed correlations. However, an inverse correlation was seen in the northeast monsoon due to differing moisture transportation as part of the continental effect. The correlation coefficient (R) was higher in the southwest monsoon (?0.84 for latitude effect, ?0.64 for altitude effect) than the northeast monsoon (0.67 for latitude effect, 0.35 for altitude effect). The spatial pattern of isotopic composition reflects the southwest monsoon more clearly than the northeast monsoon, but the two monsoons also have a cancelling impact on orographic patterns. An agreement of the δ18O and deuterium excess (d-excess) was a negative correlation and found to reflect precipitation sources and re-evaporation processes. The d-excess was slightly higher for the northeast monsoon, bringing moisture from the Pacific Ocean and travelling across the continent before reaching the observed stations. By contrast, the d-excess was relatively lower for the Indian Ocean's moisture in the southwest monsoon. |
| |
| Keywords: | d-excess monsoon rainfall stable isotopes in precipitation isotopic fractionation effect δ18O |
|
|
