04 Feb 2022
04 Feb 2022
Status: this preprint is currently under review for the journal HESS.

Technical note: Conservative storage of water vapour: a key to practical measurements of water stable isotopes in tree stems and soils

Ruth Magh1,2, Benjamin Gralher3, Barbara Herbstritt3, Angelika Kübert4,5, Hyungwoo Lim1, Tomas Lundmark1, and John Marshall1 Ruth Magh et al.
  • 1Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, 90183, Sweden
  • 2Terrestrial Ecohydrology, Friedrich Schiller University, Jena, 07749, Germany
  • 3Hydrology, Albert Ludwigs University, Freiburg, 79098, Germany
  • 4Ecosystem Physiology, Albert Ludwigs University, Freiburg, 79103, Germany
  • 5Micrometeorology, University of Helsinki, Helsinki, Finland

Abstract. Using water stable isotopes to track plant water uptake or soil water processes has become an invaluable tool in ecohydrology and physiological ecology. Recent studies have shown that laser absorption spectroscopy can measure equilibrated water vapour well enough to support inference of liquid stable isotope composition of plant or soil water, on-site and in real-time. However, current in-situ systems require the presence of an instrument in the field. Here we tested, first in the lab and then in the field, a method for equilibrating, collecting, storing, and finally analysing water vapour for its isotopic composition that does not require an instrument in the field. We developed a vapour storage vial system (VSVS) that relies on in-situ sampling into crimp neck vials with a double-coated cap using a pump and a flow meter powered through a small battery and measuring the samples in a laboratory. All components are inexpensive and commercially available. We tested the system’s ability to store the isotopic composition of its contents by sampling a range of water vapour of known isotopic compositions (from −95 to +1700 ‰ for δ2H) and measuring the isotopic composition after different storage periods. Samples for the field trial were taken in a boreal forest in northern Sweden. The isotopic composition was maintained to within 0.6 to 4.4 ‰ for δ2H and 0.6 to 0.8 ‰ for δ18O for natural-abundance samples. Although 2H-enriched samples showed higher uncertainty, they were sufficient to quantify label amounts. We detected a small change in the isotopic composition of the sample after long storage period, but it was correctable by linear regression models. We observed the same trend for the samples obtained in the field trial for δ18O but observed higher variation in δ2H compared to the lab trial. Our method combines the best of two worlds, sampling many trees in-situ while measuring at high precision in the laboratory. This provides the ecohydrology community a tool that is not only cost-efficient but also easy to use.

Ruth Magh et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Short comment on hess-2022-37', Matthias Beyer, 10 Mar 2022
    • AC1: 'Reply on CC1', Ruth Magh, 18 Apr 2022
  • RC1: 'Comment on hess-2022-37', Rachel Havranek, 16 Mar 2022
    • AC2: 'Reply on RC1', Ruth Magh, 18 Apr 2022
  • RC2: 'Comment on hess-2022-37', Anonymous Referee #2, 24 Mar 2022
    • AC3: 'Reply on RC2', Ruth Magh, 18 Apr 2022

Ruth Magh et al.


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Short summary
We developed a new approach to sample and store water vapour for isotope analysis. It allows to reliably measure at high temporal and/or spatial resolution even in remote areas. Further, we ensured all necessary components to be easily available, making this method cost-efficient and simple to implement. We found our method to perform well in the lab and in the field, enabling our method to become a tool for everyone aiming to resolve questions regarding the water cycle.