21 Feb 2023
 | 21 Feb 2023
Status: this preprint is currently under review for the journal HESS.

Technical note: Isotopic fractionation of evaporating waters: effect of sub-daily atmospheric variations and eventual depletion of heavy isotopes

Francesc Gallart, Sebastián González-Fuentes, and Pilar Llorens

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)

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  • 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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Short summary
Normally the lighter Oxygen and Hydrogen isotopes are preferably evaporated from a water body, which becomes enriched in heavy isotopes. But we observed that, in a water body subject to prolonged evaporation, some periods of heavy isotope depletion instead of enrichment happened. Furthermore, the usual models that describe the isotopy of evaporating waters may be in error if the atmospheric conditions of temperature and relative humidity are time-averaged instead of evaporation flux-weighted.