Spatial and temporal distributions of stable isotopes in precipitation over Thailand

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.

Isotopic temporal and spatial variations of tropical rivers in Thailand reflect monsoon precipitation signals

“…Remarkably, the station NE1 had low slope values and similar LRWL (5.45) and LMWL (5.82, Figure 6) values, suggesting that river runoff is directly recharged from rainfall experiencing sub-cloud evaporation during precipitation (Wu, Zhang, Xiaoyan, Li, & Huang, 2015). This interpretation is also supported by previous studies that reported that sub-cloud evaporation had a significant influence on the precipitation isotopes over upper Thailand (Laonamsai et al, 2020;Laonamsai et al, 2021) and on the isotopic signatures of river waters.…”

“…For the northeast monsoon, moisture dominantly from the South China Sea and the Gulf of Thailand was precipitated in continental and coastal Thailand, respectively (Laonamsai et al, 2021). The S region showed a continuous similarity in δ 18 O and d ‐excess between river water and precipitation.…”

“…Tropical rivers differ in their isotopic characteristics compared to temperate rivers in mid‐latitude regions due to isotopic enrichment of monsoon precipitation water by river runoff generation mechanisms. This is associated with the greater evaporation that dominates in the tropics (Lindzen & Nigam, 1987) and the seasonal dynamics of precipitation sources between the Indian and Pacific Oceans (Laonamsai et al, 2021; Wei et al, 2018). Therefore, the relationship between the isotopic composition of precipitation (input) and river runoff (output) also depends on the extent of processes such as subsequent evaporation that fractionate the isotopes of the precipitation source (Gat & Tzur, 1967).…”

“…By contrast, the C and S regions received precipitation from surrounding seas carried by both the southwest monsoon (76% for C and 66% for S) and northeast monsoon (24% for C and 34% for S). The C and S regions are located in the windward direction and received moisture from both the Indian Ocean and the Gulf of Thailand during all seasons (Laonamsai et al, 2021).…”

“…Most rivers in upper Thailand have low to no runoff, while the rivers along the coast still have discharge. The isotopic spatial heterogeneity associated with altitude is thought to result from different water sources and orographic precipitation (Laonamsai, Ichiyanagi, Kamdee, Putthividhya, & Tanoue, 2021), both of which reflect the seasonally‐differing transport of water vapour due to the monsoons. However, it is not yet known how effectively these monsoons influence the isotopic compositions of tropical rivers.…”