Geographic effects on stable isotopic composition of precipitation across Thailand^*

This study investigated the spatial variation of isotopes in precipitation over Thailand in relation to geographic condition. Monthly data (2013–2015) of stable isotopes (δ²H and δ¹⁸O) in precipitation within the region 5°37′–20°27′N, 98°04′–105°31′E were analysed using the least squares regression method to determine the possible effects of latitude and longitude on δ¹⁸O and d-excess in precipitation. Results showed that the spatial variability of isotopes in precipitation is affected by both continental and amount effects and that the variation is slightly greater in terms of latitude than longitude. Latitude and longitude effects both exhibited negative and positive correlations. Negative correlation, whereby isotopic values decreased with increasing latitude, was found during the June–August and September–November periods. Conversely, a positive relationship appeared in the December–February and March–May seasons. These effects had opposite correlation owing to the various moisture sources and different precipitation patterns between seasons. Furthermore, weak correlation for both latitude and longitude effects was revealed during September–November because of the intraseasonal variation of the monsoon. Consequently, the isotopic composition of precipitation in Thailand is controlled not only by geography but also by moisture source and precipitation pattern.