What controls the stable isotope composition of precipitation in the Mekong Delta? A model-based statistical approach

Abstract

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 δ 18 O, δ 2 H, and d-excess of precipitation at the station of measurement.The results indicate that (i) MLR can better explain the isotopic variation in precipitation (R 2 = 0.8) compared to single-factor linear regression (R 2 = 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 δ 18 O and δ 2 H, 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...

Tracing the Relationship between Precipitation and River Water in the Northern Carpathians Base on the Evaluation of Water Isotope Data

“…Both the slope and the intercept were lower than the corresponding values of the GMWL (Global Meteoric Water Line), which could result partly from sub-cloud evaporation of the falling raindrops through dry air during summer [4]. When samples that show clear signs of evaporative enrichment (either in the atmosphere or during collection) were removed, the LMWL is defined by the equation:…”

“…The stable oxygen and hydrogen isotope composition of precipitation and/or river water exhibit large spatial and temporal variation across the Earth's surface [1], thus being useful tracers of the processes in the water cycle, being linked to air moisture sources. Also, they contribute to advancing the understanding of the atmospheric hydrological cycle, which is related to climate and linkages between precipitation surface and groundwater flow [2][3][4][5]. The stable isotopes of oxygen and hydrogen in precipitation also reflect the combination of source-region rainout effects and recycling effects which are affected by air masses bringing water vapor from different geographic regions [1].…”

“…The results presented in this study emphasize that the δ 18 O (and δ 2 H) and d-excess variability are strongly influenced by temperature, precipitation and the prevailing large-scale atmospheric circulation. precipitation and river water could give further information on regional and global scale hydrological processes, thus enabling better planning for water usage in the context of future potential changes in the climate system [12].In order to better understand the dynamics of water in a given watershed using the stable isotopes of oxygen and hydrogen, a two-step approach is required: First, identification of the contribution of regional and local moisture sources to precipitation generation, and second, identification of links between stable isotopic composition and atmospheric and hydrologic processes affecting surface water [13].A remarkable number of studies which investigate the relationship between hydrogen and oxygen isotopes in precipitation and river water have been carried out in Europe [3,[13][14][15][16][17], and elsewhere in the world [2,4,12,[18][19][20], but currently, similar studies in the Eastern Carpathian Mountains, the main recharge area for rivers in Southeast Europe, are missing. To fill this gap in knowledge, we have designed a study aiming: i) To investigate the temporal and spatial variations of the stable isotope compositions of precipitation and river water from the northeastern part of Romania, in order to identify the impact of different local, regional and large-scale factors on the variability of oxygen and hydrogen isotopic composition using the available isotopic data set; ii) to define the local meteoric water line (LMWL) over the analyzed region; and iii) to investigate the relation between air moisture sources and the prevailing large-scale atmospheric circulation.…”

“…A remarkable number of studies which investigate the relationship between hydrogen and oxygen isotopes in precipitation and river water have been carried out in Europe [3,[13][14][15][16][17], and elsewhere in the world [2,4,12,[18][19][20], but currently, similar studies in the Eastern Carpathian Mountains, the main recharge area for rivers in Southeast Europe, are missing. To fill this gap in knowledge, we have designed a study aiming: i) To investigate the temporal and spatial variations of the stable isotope compositions of precipitation and river water from the northeastern part of Romania, in order to identify the impact of different local, regional and large-scale factors on the variability of oxygen and hydrogen isotopic composition using the available isotopic data set; ii) to define the local meteoric water line (LMWL) over the analyzed region; and iii) to investigate the relation between air moisture sources and the prevailing large-scale atmospheric circulation.…”

Relating Moisture Transport to Stable Water Vapor Isotopic Variations of Ambient Wintertime along the Western Coast of Korea

“…A number of studies have been conducted on the tracking of water vapor sources with precipitation isotope ratio and local weather and spatial variables (e.g., temperature, relative humidity, latitude, altitude, precipitation amount and so on) [15][16][17][18][19]. However, little has been reported on isotope distributions in the water vapor phase around the Korean peninsula because of limitations in instruments and techniques for atmospheric water vapor isotopic measurement.…”