TEMPORAL VARIATION IN THE STABLE ISOTOPES IN PRECIPITATION RELATED TO THE RAINFALL PATTERN IN A TROPICAL RAINFOREST IN PENINSULAR MALAYSIA

Abstract

We present the temporal variability of the oxygen (δ¹⁸O) and hydrogen (δ²H) isotope signatures in precipitation at Pasoh Forest Reserve (FR), a tropical rainforest in Peninsular Malaysia. We investigated the daily and seasonal variability of stable isotope signatures in precipitation, particularly in relation to the effects of monsoon seasons, rainfall characteristics and larger scale trends compared with those at nearby Global Network of Isotopes in Precipitation (GNIP) monitoring stations. The isotope signatures did not differ between monsoon seasons but were correlated with amount of rainfall, its intensity and duration. The effect of amount of rainfall on isotope composition was clearly detected and comparable with long-term mean monthly statistics of nearby GNIP stations. Unfortunately the effect was obscured at the daily timescale and, for monthly rainfall, not averaged over the long-term. No large deuterium excess was detected at the daily timescale for small-scale rainfall events. The δ¹⁸O in precipitation water was more closely correlated with the 60-day antecedent rainfall index than with the amount of rainfall each day. These findings suggested that the isotopic composition in the study area was the result of a rainout on a larger scale in addition to the local scale and specific rain events.