Introduction

The atmospheric water vapor is an important component of the hydrological cycle and the global climate system. Water molecules transfers and redistributes solar energy through latent heat of fusion, vaporization and condensation in the course of its continuous transport through hydrological cycle (Hoffmann, Cuntz, Jouzel, & Werner, 2005; Lieberman, Ortland, & Yarosh, 2003). Water vapor, thus, governs both water availability on earth and the global temperature. With increasing surface air temperature of earth, the potential for evaporation is also increasing. With increased evaporation and consequent water vapor loading in atmospheric, the heat and water distribution over earth is expected to be affected, about which there is considerable uncertainty (Lawrence et al., 2004; Wild, Ohmura, Gilgen, & Rosenfeld, 2004).
In this backdrop, it is important to trace movement of water vapor, particularly at ground level in both oceanic and land areas. It is also important to understand spatio-temporal variability of out flux of vapor from ground level to lower troposphere and the relative proportion of transpired and evaporated flux. Vapor dynamics are highly variable in space and time because it is subject to availability of source water, vegetation and soil cover, and microclimate (rainfall, temperature, Rh and wind speed). It is therefore, important to characterise the vapor flux as densely as possible.
In addition to its use in inferring various processes affecting availability and redistribution of water and heat on earth, the ground level vapor is also very important in validating remotely sensed data about vapor, which are used for different studies involving computations and modelling.
Oxygen and hydrogen isotopic composition is an effective tracer of water molecules’ origin through evaporation, transpiration, sublimation and its movement and mixing in troposphere (R. Deshpande, Bhattacharya, Jani, & Gupta, 2003; Maurya et al., 2011). The oxygen and hydrogen isotopic composition is expressed in terms of per mil (‰) deviation of abundance ratio of heavier to lighter isotopes with reference to international standard reference material. Isotopic composition is defined in terms of δ (‰) notations as: [δ18O or δD = (Rsample /Rstd -1) x 1000]. Rsample refers to the abundance ratio (18O/16O or D/H) for the sample, and Rstd refers to similar ratio for international standard reference material VSMOW (Vienna Standard Mean Ocean Water) (Clark & Fritz, 2013; Gat, 1981).
Isotopic composition of ambient water vapor is one of the most important input parameters for isotope mass balance and isotope enabled atmospheric circulation and mass flux process models. It is also important for estimating relative contribution of water vapor from evaporation of surface water bodies (lakes, reservoirs, wet lands) and transpiration (Breitenbach et al., 2010; Purushothaman et al., 2014). Isotopic composition of ambient water vapor, unlike rainfall, can be obtained throughout the year and can be advantageously used to understand various processes affecting concentration and composition of atmospheric water vapor (Saranya, Krishan, Rao, Kumar, & Kumar, 2018).
From these considerations, monitoring ground level water vapor is of utmost importance but it is often a cumbersome and expensive process and there are numerous practical challenges involved in the large-scale vapor sample collection for mass spectrometric analysis (Lawrence et al., 2004). There are three options to estimate isotopic composition of water vapor, namely, from (1) Satellite Remote sensing and Radiosonde; (2) computations from isotopic composition of rainfall and (3) sampling of ambient water vapor for isotopic analyses.