The local meteroric evaporative line (LMEL) was built by the water samples collected from the ponds or reserviors in the catchment. The slope and intercept of δD vs. δ18O equation was fit from the isotopic data of water samples. How to consider the effect of water evaporation on the calculations? Thank you so much for your time and help!
Dear Pei Zhao,
As you write, the evaporation line is generated by fitting a linear regression to the isotope data in the dual isotope plot (d2H over d18O). There is no need to account for the effect of evaporation that experienced the water samples. Instead, the regression line will tell you about evaporation, since the evaporation induces kinetic fractionation. This way, the slope of the evaporation line will be smaller than 8 (slope of the global meteoric water line) and also smaller than the slope of the local meteoric water line (LMWL). The deviation of the water samples from the LMWL will indicate evaporation.
The following literature provides more background information:
Craig, H.; Gordon, L. I.; Horibe, Y. Isotopic exchange effects in the evaporation of water: 1. Low-temperature experimental results, J. Geophys. Res. (Journal of Geophysical Research), 1963, http://dx.doi.org/10.1029/JZ068i017p05079
Gibson, J. J.; Edwards, T. W.D.; Bursey, G. G.; Prowse, T. D. Estimating evaporation using stable isotopes: quantitative results and sensitivity analysis for two catchments in northern Canada, Hydrology Research, 1993
Gat, Joel R.The isotopic composition of evaporating waters - review of the historical evolution leading up to the Craig-Gordon model, Isotopes in Environmental and Health Studies, 2008 doi: 10.1080/10256010801887067
Also the duterium excess may give some hints on evaporation conditions.
Fabio
I agree with Fabio that deuterium-excess (see Dansgaard, W. (1964), Stable isotopes in precipitation, Tellus, 16(4), 436–468, doi:10.1111/j.2153-3490.1964.tb00181.x.) can tell you about evaporation fractionation.
However, I prefer to account for the local precipitation, and thus the local meteoric water line, from which the water samples will deviate according to its evaporation fractionation. This is described by 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.
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