If you have repeated measures of D and O-18 at a site, then you can easily calculate D excess, since this is defined by the relation: D excess = d2H - 8 * d18O as first described by Dansgaard, 1964 (Tellus 16: 436-468). The D excess values of precipitation can provide insight to sources of moisture, atmospheric processes, and relative humidity (Merlivat and Jouzel, 1979, J. Geophys. Res. 84: 5029_5033). Time series of D excess values can inform understanding of how climate change may be impacting geographic source areas of precipitation (Puntsag et al., 2016, Scientific Reports, DOI: 10.1038/srep22647). D excess is also interesting to understanding the local water balance as reflected in surface waters and groundwater (Kendall and Coplen, 2001, Hydrol. Proc. 15: 1363-1393). Comparison of the local meteoric water line from precipitation with that obtained from local surface and groundwater can provide insight to ET as D excess tends to decrease with evaporative intensity. It is also important to consider other sources of moisture/recharge such as fog and cloud water, which can be locally important in coastal areas and in mountainous regions. My advice is that if you are measuring D and O-18 of precipitation and local waters through time, then develop the regression relationship for the local meteoric water lines of these respective waters and examine the D excess among other facets of these relationships. This may provide insight to ET, sources of recharge, and other elements of the water balance.

Well, basically, the stable isotopes as well as the deuterium excess are used to trace the water cycle ,and that's why they are called tracer. So if you say, you know d-excess could be used to trace moisture source conditions, you already know or exactly say, you include most of the work this parameter can finish.  But if you go insight, you will find d-excess is quite different to stable isotopes.

Thanks for your kind reply.

I agree that d-excess is different from oxygen-18 or deuterium in some specific applications. However, some reviewers always doubted that while oxygen-18 and deuterium can do most of the applications in hydrological cycle, why do you use d-excess. I don't have enough evidences.

For example, d-excess is commonly used to calculate sub-cloud evaporation, but oxygen-18 or deuterium can do, too. What's the advantages of d-excess here?

Heavy water is water in its molecular deuterium instead of normal hydrogen with oxygen bonded. The water compared to tap water in late spring and early freezes, Gilbert Lewis first sample of pure heavy water in 1933 won. Means heavy water, deuterium atoms from an oxygen atom and two (D) is formed. For the production of heavy water to water molecules containing heavy hydrogen (deuterium) from ordinary water molecules apart, or through hydrogen, or deuterium, heavy hydrogen atoms are isolated and purified.Molecular mass and molecular mass of 18 heavy water 20 is ordinary water. Heavy water heat exchanger to transfer the heat from the reactor core to the disposal of this water due to high heat capacity capable of storing more heat Radard. The heavy water is also a good option to cool nuclear reactors.

http://kimiagar85.blogfa.com/post-42.aspx

The isotope concentration ratios (D/H, and Oxygen-18/Oxygen-16) from a "representative"  sample of water turns out to be sensitive to a number of natural processes that cause these ratios to vary and can be measured with some accuracy. Hence these measurements give some information on  environmental processes when combined with conceptual models of the interactions of these processes.  The d-excess is calculated by means of these same two ratios, and the observation of an approximate near-global uniformity (the slope of the "global meteoric waters line", which turns out to be a useful parameter for inferring certain information about the "history" of the water sample, given the necessary assumptions. However, given the assumptions, the d-excess is obviously completely determined from the measured ratios, and cannot give new information not inherent in the original ratios, although it can replace either one in principal. 

Interpretation of results based on isotope ratios are only as good as the supporting models used to interpret the data; hence the preferred use of other tracers or measurements of other physical concentrations, and the use of alternative models based on these measurements, to increase confidence in the interpretations. Generally, the use of such information is used in "inverse problems" that do not have unique solutions except under special conditions.

Dear Prof. Barnes, Thanks for your kind consideration and reply. I got a lot from your explanation.

If you have repeated measures of D and O-18 at a site, then you can easily calculate D excess, since this is defined by the relation: D excess = d2H - 8 * d18O as first described by Dansgaard, 1964 (Tellus 16: 436-468). The D excess values of precipitation can provide insight to sources of moisture, atmospheric processes, and relative humidity (Merlivat and Jouzel, 1979, J. Geophys. Res. 84: 5029_5033). Time series of D excess values can inform understanding of how climate change may be impacting geographic source areas of precipitation (Puntsag et al., 2016, Scientific Reports, DOI: 10.1038/srep22647). D excess is also interesting to understanding the local water balance as reflected in surface waters and groundwater (Kendall and Coplen, 2001, Hydrol. Proc. 15: 1363-1393). Comparison of the local meteoric water line from precipitation with that obtained from local surface and groundwater can provide insight to ET as D excess tends to decrease with evaporative intensity. It is also important to consider other sources of moisture/recharge such as fog and cloud water, which can be locally important in coastal areas and in mountainous regions. My advice is that if you are measuring D and O-18 of precipitation and local waters through time, then develop the regression relationship for the local meteoric water lines of these respective waters and examine the D excess among other facets of these relationships. This may provide insight to ET, sources of recharge, and other elements of the water balance.

Dear Burnes,

Thank you very much for your reply and many applications of d-excess in water cycle.

They shed much light on the question I asked.