What controls the stable isotope composition of precipitation in the Mekong Delta? A model-based statistical approach
Authors: Le Duy Nguyen , Heidbüchel Ingo , Meyer Hanno , Merz Bruno , Apel Heiko 
Published: February 2018

Abstract. This study analyzes the influence of local and regional climatic factors on
the stable isotopic composition of rainfall in the Vietnamese Mekong
Delta (VMD) as part of the Asian monsoon region. It is based on 1.5 years of
weekly rainfall samples. In the first step, the isotopic composition of the
samples is analyzed by local meteoric water lines (LMWLs) and single-factor
linear correlations. Additionally, the contribution of several regional and
local factors is quantified by multiple linear regression (MLR) of all
possible factor combinations and by relative importance analysis. This
approach is novel for the interpretation of isotopic records and enables an
objective quantification of the explained variance in isotopic records for
individual factors. In this study, the local factors are extracted from local
climate records, while the regional factors are derived from atmospheric
backward trajectories of water particles. The regional factors,
i.e., precipitation, temperature, relative humidity and the length of backward
trajectories, are combined with equivalent local climatic parameters to
explain the response variables δ18O, δ2H, and d-excess
of precipitation at the station of measurement. The results indicate that (i) MLR can better explain the isotopic
variation in precipitation (R2 = 0.8) compared to single-factor
linear regression (R2 = 0.3); (ii) the isotopic variation in
precipitation is controlled dominantly by regional moisture regimes
(∼ 70 %) compared to local climatic conditions
(∼ 30 %); (iii) the most important climatic parameter during
the rainy season is the precipitation amount along the trajectories of air
mass movement; (iv) the influence of local precipitation amount and
temperature is not significant during the rainy season, unlike the
regional precipitation amount effect; (v) secondary fractionation processes
(e.g., sub-cloud evaporation) can be identified through the d-excess and take
place mainly in the dry season, either locally for δ18O and
δ2H, or along the air mass trajectories for d-excess. The
analysis shows that regional and local factors vary in importance over the
seasons and that the source regions and transport pathways, and particularly
the climatic conditions along the pathways, have a large influence on the
isotopic composition of rainfall. Although the general results have been
reported qualitatively in previous studies (proving the validity of the
approach), the proposed method provides quantitative estimates of the
controlling factors, both for the whole data set and for distinct seasons.
Therefore, it is argued that the approach constitutes an advancement in the
statistical analysis of isotopic records in rainfall that can supplement or
precede more complex studies utilizing atmospheric models. Due to its
relative simplicity, the method can be easily transferred to other regions,
or extended with other factors. The results illustrate that the interpretation of the isotopic composition
of precipitation as a recorder of local climatic conditions, as for example
performed for paleorecords of water isotopes, may not be adequate in the
southern part of the Indochinese Peninsula, and likely neither in other
regions affected by monsoon processes. However, the presented approach could
open a pathway towards better and seasonally differentiated reconstruction
of paleoclimates based on isotopic records.