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]