Technical note: Isotopic fractionation of evaporating waters: effect of sub-daily atmospheric variations and eventual depletion of heavy isotopes
Abstract. Isotopic fractionation of evaporating waters has been studied constantly in recent decades, particularly because it enables calculation of both the volume of water evaporated from a water body and the isotopic composition of its source water. We studied the stable water isotopy of an artificial pan filled with water in a sub-humid environment, in order to put into practice an operational method for estimating the time since disconnection of riverine pools when these are sampled for the quality of aquatic life.
Results indicate that: (i) when about 70 % of pan water had evaporated and its isotopy became enriched in heavy isotopes, some subsequent periods of depletion instead of enrichment happened, and (ii) the customary application of isotopic fractionation equations to determine the isotopic composition of the water in the pan using weekly averaged atmospheric conditions (temperature and relative humidity) strongly underestimated the changes observed, but predicted an early depletion of heavy isotopes. The first result, rarely reported in the literature, was found to be fully consistent with the early studies of the isotopy of evaporating waters. The second one could be attributed to that weekly averages of temperature and relative humidity strongly overestimated air relative humidity during daylight periods of active evaporation. However, when the fractionation equations were parameterized using temperature and relative humidity weighted by potential evapotranspiration at sub-hourly time steps, they adequately reproduced the observed isotopic composition of the water in the pan, including the late periods of heavy isotope depletion. Our results should be taken into account when fractionation equations are applied in areas with relatively humid climates.
Francesc Gallart et al.
Status: open (until 18 Apr 2023)
- RC1: 'Comment on hess-2022-427', Anonymous Referee #1, 09 Mar 2023 reply
Francesc Gallart et al.
Francesc Gallart et al.
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The authors measured the isotopic composition of water in an evaporation pan that was shielded against incoming precipitation. They compared their observations against theory and found that diurnal variations matter.
This is an odd paper. The subject is very old and well treated in the literature because evaporation pans were examined extensively in the 1970th to 1990th. The results mirror exactly theory and are deviating only if the theory is applied wrongly. The authors were not successful to convince me that there is enough literature that applies the theory wrongly to warrant publication of the data with such simple analysis.
There are some odd and formulations in the manuscript such as "rapid molecular exchange of isotopes between the water body and the atmospheric vapour, which predominates over the net isotopic effect of a simple evaporation process". This is a rather layman's notion that evaporation is a simple one-way process, which it is not. The old papers never assumed that. The authors could probably profit from reading some literature in plant sciences, for example Farquhar and Gan (Plant, Cell & Environment 2003), where this subject was treated extensively.
Their final remark "delta* [...] become detached from precipitation and atmospheric moisture isotopic content" is simply wrong. It is actually their Eq. (2) so it depends on isotopes of water vapour and it does not depend on precipitation. But it depends also on relative humidity and so might change even that one assume constant water vapour isotopes.
The authors do not really explain why they have decreasing isotopic composition at the end. But if the evaporating water gets more enriched than would be the actual new maximum enrichment given the new relative humidity, then it has to decrease. The way the paper is written, the environmental conditions are assumed very static, a priori.
So I was also astonished about the surprise that a average relative humidity is not appropriate. If one thinks about isotopic exchange, one sees that it is not relative humidity but absolute humidity (vapour pressure) that is relevant. So any other notion of relative humidity such as mid-day, ratio of mean vapour pressure over mean saturated vapour pressure, relative humidity weighted by saturated humidity, etc. would probably have been fine. If ever, one should probably use the Penman formulation rather than Penman-Monteith. The latte includes plants, the former does not.
I have not understood how the time series in the figures were constructed, especially when delta* was changing. Was delta_0 always constant? Did you go from one to the next observational time step? In latter case, I would have guessed that you could simply go one 5-min interval after the next and do not have to average anything.
So I would recommend publication if the authors would write down the dynamic theory, where their fraction of water volume that evaporated, i.e. evaporation, is calculated rather than assumed. This would probably be similar to Farquhar and Cernusak (Functional Plant Biology 2005) or any other non-steady state formulation of leaf water enrichment.
I would have a variety of minor to medium comments but I leave them out because I think the paper needs a thorough rewrite anyway.