The on-going temperature rise is projected to alter the distribution of freshwater (rainfall, runoff, and groundwater) with some regions anticipating seasonal flooding while others severe droughts. Such changes to the hydrological system have significant ramifications for sustainability and socio-economic stability particularly in densely populated regions like Southeast (SE) Asia. Forecasting the impact of global climate change on local freshwater is of great importance to this region, as it will allow local governments to prepare appropriate mitigation and adaptation policies. However, such prediction requires a better understanding of the factors that control regional precipitation.

The global hydrological cycle as well as the past and present climate change have long been studied using water stable isotopes (i.e., d18O and d2H). As natural tracers of hydrological processes, water isotopes record climate parameters, such as temperature, evaporation/condensation processes, precipitation history, and atmospheric processes. A Global Network of Isotopes in Precipitation (GNIP) has been built by the International Atomic Energy Agency (IAEA) in cooperation with the World Meteorological Organization (WMO) since the 1950s to study regional to global hydrological processes and water resource management. Recently, isotope-enabled atmospheric general circulation models (i-GCMs) have provided reasonably accurate reconstructions of the global distribution of the mean annual isotope ratios of precipitation. However, model simulations are inconsistent with observations in the tropics, where the i-GCMs predict a clear negative relationship between stable isotope ratios and rainfall intensity, the so-called amount effect. In contrast, the amount effect is not observed in recent studies of daily precipitation isotopes, which suggests the existence of other factors such as moisture source, transport patterns and regional organized convection. This inconsistency shows our limited understanding of the spatial and temporal variations in stable isotopes of present-day precipitation and the complexities of atmospheric and climate processes in the tropics. Recently, with advancement in analytical techniques and instruments, 17O-excess (17O-excess = ln(d17O+1) - 0.528×ln(d18O+1)) has been used as a new tracer in hydrological studies. However, observations show that controls of 17O-excess are much more complicated than experiments and theory suggested. Further observation and modelling are needed to understand what controls the variability of 17O-excess, particularly in the tropics, where studies in 17O-exess are sparse.

While the GNIP is very useful for routine studies in hydrology and climate, most of its stations are located in the mid-latitudes of the Northern Hemisphere, and they mostly collect monthly precipitation. 

Such isotopic data suffer from insufficient temporal and spatial resolution in the tropics, thus limiting the extent of our knowledge of the climate system and atmospheric circulation. High temporal and spatial resolution of sampling in the tropics is therefore necessary but requires a collective effort internationally. In 2013, the IAEA launched a new Coordinated Research Project (CRP)–“Stable Isotopes in Precipitation and Paleoclimatic Archives in Tropical Areas to Improve Regional Hydrological and Climatic Impact Models (2013-2017)”–with the focus on present-day isotope ratios in 

(daily) precipitation and their use in interpretation and validation of climate and hydrological models and paleoclimatic proxy records from tropical regions. We were active members of this project, and in this proposed project, we will continue this international effort to investigate the stable isotopes of modern precipitation in the tropics, particularly in SE Asia. Our study will thus contribute to advancing our knowledge of key processes that govern the tropical precipitation under global climate change conditions. 

In our project, we aim to collect daily precipitation samples from a large area in SE Asia for three more years and beyond the duration of this proposed research, and to analyze the samples for commonly studied stable isotopes d18O, d2H, and d-excess (d = d2H - 8×d18O), as well as for until recently rarely analyzed d17O and 17O-excess. Specifically, we will focus on the following scientific questions:

  1. How do stable isotopes of precipitation in the tropics respond to monsoons, the El Niño–Southern Oscillation (ENSO), Madden-Julian Oscillation (MJO), Indian Ocean Dipole (IOD), and other climate systems on different time scales?
  2. What is the spatial variation of the influence from monsoons, the ENSO, MJO, and IOD on precipitation isotopes in SE Asia?
  3. Is there a strong correlation between 17O-excess of tropical precipitation and the relative humidity (RH) of moisture source regions? If so, to what extent can 17O-excess be used as a new tracer of hydrological processes in the tropics?
  4. How does microphysics during tropical convection affect stable isotope signals, particularly 17O-excess?

To obtain rain samples, we will draw on an already established network of rain stations across SE Asia, and expand it by setting up new stations, particularly in the critical Asian monsoon regions such as Myanmar. i-GCMs will also be utilized to assist us to identify the physical controls of stable isotopes of precipitation, including d-excess and 17O-excess. Our investigation will provide a much-needed unique data set from tropical regions to validate i-GCMs and help to improve the physical and chemical components of these models, and thus to improve future precipitation projections.

Funding Source:

Earth Observatory of Singapore

Project Years:

2020 to 2023

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The Team

HE Shaoneng River

HE Shaoneng River

Principal Research Fellow

WANG Xianfeng

WANG Xianfeng

Principal Investigator


Nathalie Goodkin, American Museum of Natural History 

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