How do I use the GMWL equation to get the points to plot the line. Secondly, how do I plot the evaporation line
Dear Nangamso Tuswa,
You are not supposed to apply the equation for the GMWL with your own data. Instead, the GMWL represents the general trend in the d2H to d18O relationship of precipitation isotope data. If you have precipitation isotope data, you should rather use this data to generate a local meteoric water line (LMWL). This represents the regression line through the precipitation isotope data in a dual isotope plot (d18O versus d2H).
The evaporation line can be derived by a regression line through the data points from an evaporating pool (e.g., lake water, soil water) in the dual isotope plot (d18O versus d2H). Isotope samples from a water pool that experienced evaporation loss will plot below the LMWL in the dual isotope plot due to non-equilibrium fractionation.
I recommend looking into the text books:
for example, Isotope Tracers in Catchment Hydrology by Kendall and McDonnell
https://books.google.es/books/about/Isotope_Tracers_in_Catchment_Hydrology.html?id=kiLo1U0cmlIC&redir_esc=y
In order to describe the evaporation (non-equilibrium) fractionation, the deuterium-excess (see Dansgaard, W. (1964), Stable isotopes in precipitation, Tellus, 16(4), 436–468, doi:10.1111/j.2153-3490.1964.tb00181.x.) or the "line-conditioned excess" (short lc-excess) as described by Landwehr, J. M., and T. B. Coplen (2006, Line-conditioned excess: a new method for characterizing stable hydrogen and oxygen isotope ratios in hydrologic systems, in International Conference on Isotopes in Environmental Studies, pp. 132–135, IAEA, Vienna) can be helpful.
I hope this could help a bit.
Best,
Matthias
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