2.3.1 Local Meteoric Water Line
The Global Meteoric Water Line (GMWL) [Craig (1961)] provides the
relationship between the global isotopic contents of
δ18O and δ2H, referred to the Vienna
Standard Mean Ocean Water (VSMOV), as:
δ2H= 8δ18O+10‰ VSMOV. (1)
At the local scale, this relationship is represented by the Local
Meteoric Water Line (LMWL) which can be used to determine how (i) the
spatial variability of the linear statistical relationship between
isotopic ratios, and (ii) the variation in slope that provides
information about the seasonal climatology of a particular site
(Rozanski, Araguás-Araguás, & Gonfiantini, 1993; Putman, Fiorella,
Bowen, & Cai, 2019). Therefore, the extent (and direction) of the
deviation of the LMWL from the GMWL is an indicator of the thermodynamic
state of vapor formation processes, as well as the
strengthening/weakening of processes or mechanisms involved in the
atmospheric vapor transport. We constructed the LMWL for the study area,
based on the linear regression of the monthly average of
δ18O and δ2H isotope composition for
the period between 1971 and 2016. Data of isotope composition of local
precipitation were obtained from the Global Network of Isotopes in
Precipitation (GNIP) project (IAEA, 2020). We included up to as many as
33 sampling points (shown in Fig 1.)
The deviation of LMWL from GMWL explains the depletion/enrichment of
isotopic composition of precipitation concerning the GMWL and how it can
be interpreted in terms of processes and water vapor formation
conditions that air masses suffer in their evolution before becoming
precipitation. This information is useful to infer the terrestrial or
oceanic origin of water vapor. Pairs of δ18O and
δ2H in the LMWL result from the interaction of
fractionation generated in the advance into the continent of moisture
flow and meteorological conditions at the site (Dansgaard,1964). Since
the GMWL is used as the ‘expected’ equilibrium relationship (Putman et
al., 2019), the LWML is often evaluated in the context of its deviation
from the GMWL, in terms of the variability of δ18O and
δ2H pairs. Pairs of δ18O and
δ2H in the LMWL bring information of the history of
fractionation of water in the air mass due to phase changes such as
evaporation, condensation, and transpiration, as well as variations due
to meridional and altitudinal changes that are arising from the
progressive rainout of heavy isotopes during the evolution of a
precipitating air mass (Rayleigh distillation, Gat, 1996). Depletion or
enrichment of isotopes in precipitation is expressed by the deviation
from the VSMOW. In general, expected values from terrestrial regions
exhibit depletion in isotopic composition due to Rayleigh distillation,
and expected oceanic values from cold sources (for instance the Pacific
Ocean or the South Atlantic Ocean for the study area) are more depleted
than the isotopic composition originated from warmer sources (for
example the Tropical Atlantic).
We compiled the main findings from (Putman et al., 2019) and (Clark &
Fritz, 2013) in order to produce a conceptual scheme for the
interpretation of moisture origin based on the comparison between the
relative position of isotopic observations in precipitation and the GMLW
(Fig. 3). When δ18O and δ2H pairs
are located in the upper-right (lower-left) portion of the line (Fig.
3), indicates that moisture was originated in warmer (colder) regions,
low (high) altitude, low (high) latitude, and coastal (terrestrial)
zones (Clark & Fritz, 2013). Similarly, terrestrial (and water) sources
in this scheme are located above (below) the GMWL (Putman et al., 2019).
[Insert Figure 3]