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.