Spatial and temporal variation in the stable isotope composition (δ¹⁸O and δ²H) of rain across the tropical island of Sri Lanka

Abstract

Seasonal and spatial variation in δO and δH in rainwater was determined in three selected transects across Sri Lanka, the tropical island in the Indian Ocean. Local meteoric water lines (LMWLs) for three distinguished climatic zones; wet, dry and intermediate were constructed. LMWLs show slight variations in their gradients and respective d-excess values, depending on the air moisture origin, circulation and environmental conditions of each climatic zone. The elevation effect and amount effect could be identified but the continental effect is not significantly seen in the isotope composition of rain in the concerned areas. The results reasonably revealed that the distinct rainfall regimes; two monsoonal rains and two convectional (inter-monsoon) rains have characteristic isotopic signatures. Also the impact of (i) terrestrial and oceanic moisture sources, (ii) depression and cyclonic conditions of the Bay of Bengal, and (iii) topography of the country on the variation of the isotopic composition of rain in Sri Lanka could be satisfactorily identified.

Spatial Variations in the Stable Isotopic Compositions of Surface and Groundwaters across Central Sri Lanka

“…1(a)] (Dissanayake & Weerasooriya 1985). The central highlands, reaching 2,524 m above sea level at Pidurutalagala's peak, consist of a number of ridges, peaks, plateaus, basins, valleys, and escarpments (Edirisinghe et al 2017a).…”

“…Sri Lanka's climate is controlled by the tropical monsoon system and is classified into two monsoon periods and two intermonsoon periods: the Southwest Monsoon (SWM, from May to September), Northeast Monsoon (NEM, from December to February), First Intermonsoon (FIM, from March to April), and Second Intermonsoon (SIM, from October to November). During the SWM, moist southwesterly winds orographically lift as they cross the central highlands, resulting in a considerable rainfall on the windward side of the highlands, whereas little rain falls on the lee side (Edirisinghe et al 2017a). The NEM is dry and stable compared to the SWM, and windward rainfall during January and February is relatively small (Song et al 1999).…”

“…The relationship between δ 18 O and δD values in our samples is shown in Figure 4. The local meteoric water line (LMWL) illustrated in Figure 4 was identified by the long-term observations of rainwater across Sri Lanka (Edirisinghe et al 2017a). Groundwaters from wells and spring, tap, and bottled waters plot near the LMWL.…”

“…The ratio of stable isotopes 18 O/ 16 O and D/ 1 H (defined as the conventional delta notation, δ 18 O and δD) of water is widely used as an environmental tracer in hydrological and hydrogeological studies and has been applied for investigating the interactions between surface water and groundwater (Blumstock et al 2015, Dogramaci et al 2015, separating climatic zones of the Deduru Oya river basin. Edirisinghe et al (2017a) revealed seasonal and spatial δ 18 O and δD variations in rainwater due to monsoon and convectional rainfall and identified an isotopic fractionation on the basis of their long-term observations across central and northern Sri Lanka. Basin-scale stable isotopic analyses have also been used to understand the evolution of groundwater quality (Song et al 1999), assess groundwater recharge mechanisms (Edirisinghe et al 2014), and demonstrate the relationship between groundwater and chronic kidney disease of unknown etiology (CKDu) (Edirisinghe et al 2017b).…”

Characteristics of Water Isotopes and Water Source Identification During the Wet Season in Naqu River Basin, Qinghai–Tibet Plateau

“…For example, based on the isotopic values of river waters, significant spatial and temporal variations of the Xijiang River were investigated [26]; Kong et al found that the snowmelt water accounted for more than 57% of runoff of the Kumalak River [1], and more than 53% during the wet season [6]. Based on isotopes and geochemical tracers, streams in plateau regions are mainly replenished by snowmelt and groundwater [27][28][29][30][31][32][33][34][35][36][37][38]. There has been relatively little research on the composition and mechanism of water sources on the Qinghai-Tibet Plateau.…”