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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/hess-2019-679
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-679
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  16 Jan 2020

16 Jan 2020

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This preprint was under review for the journal HESS but the revision was not accepted.

Deep soil water 18O and 2H measurements preserve long term evaporation rates on China's Loess Plateau

Wei Xiang1, Bingcheng Si1,2, Min Li1, and Han Li1 Wei Xiang et al.
  • 1Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, North-west A&F University, Yangling, Shaanxi Province, 712100, China
  • 2Department of Soil Science, University of Saskatchewan, Saskatoon, SK, Canada

Abstract. Knowledge about the long-term average soil evaporation, especially the ratio of evaporation to precipitation (f), is important for assessing the total available water resources. However, determining the long-term f remains technically challenging because soil evaporation is highly dynamic. Here we hypothesize that the stable isotopes (2H and 18O) of deep soil water preserve the long-term evaporation effects on precipitation and can be used to estimate long-term f. Our results showed that the deep soil water (2–10 m) had a mean line-conditioned excess (lc-excess) less than zero (−13.1 ‰ to −3.8 ‰) at the 15 sites across China's Loess Plateau, suggesting that evaporation effects are preserved in the isotopic compositions of the deep soil water. We then estimated f by the new lc-excess method that combines lc-excess and the Rayleigh fractionation theory, because it does not require the initial source isotopic values of soil water, which has a distinct advantage over traditional isotope-based methods (e.g. Craig-Gordon model) that require such information a priori. The estimated f of the 15 sites varied from 11 % to 30 %, and over 60 % of the variability of f was explained by the well-known Budyko dryness index. These data are also comparable with available annual estimates under similar climate regions of the world. Furthermore, these data represent a long-term average value because soil water tritium profile shows that deep soil water has a long residence time on the order of years to decades. Our work suggests that isotopic compositions of deep soil water can be used to calculate long-term average f where water flow within the unsaturated zone is piston-like flow predominantly, and the new lc-excess method provides an effective tool to estimate f.

Wei Xiang et al.

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Wei Xiang et al.

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Short summary
Soil evaporation is an essential component of the terrestrial water cycle, but it is often difficult to assess over a long period. We, for the first time, combined deep soil water isotopes and line-conditioned excess to estimate evaporation to precipitation ratio. Our work underlines that deep soil water stable isotopes have the potential to estimate long-term average evaporation rates against the back of the increasing interest of evaporation estimation in eco-hydrological studies.
Soil evaporation is an essential component of the terrestrial water cycle, but it is often...
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