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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 22, issue 5 | Copyright
Hydrol. Earth Syst. Sci., 22, 2881-2890, 2018
https://doi.org/10.5194/hess-22-2881-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 May 2018

Research article | 15 May 2018

Effects of climatic seasonality on the isotopic composition of evaporating soil waters

Paolo Benettin1, Till H. M. Volkmann2, Jana von Freyberg3,4, Jay Frentress5, Daniele Penna6, Todd E. Dawson7, and James W. Kirchner3,4,8 Paolo Benettin et al.
  • 1Laboratory of Ecohydrology ENAC/IIE/ECHO, École Polytechinque Fédérale de Lausanne (EPFL), Lausanne, Switzerland
  • 2Biosphere 2, University of Arizona, Tucson, AZ, USA
  • 3Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland
  • 4Research Unit Mountain Hydrology and Mass Movements, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Birmensdorf, Switzerland
  • 5Faculty of Science and Technology, Free University of Bolzano, Bolzano, Italy
  • 6Department of Agricultural, Food and Forestry Systems, University of Florence, Florence, Italy
  • 7Department of Integrative Biology, University of California, Berkeley, CA, USA
  • 8Department of Earth and Planetary Science, University of California, Berkeley, CA, USA

Abstract. Stable water isotopes are widely used in ecohydrology to trace the transport, storage, and mixing of water on its journey through landscapes and ecosystems. Evaporation leaves a characteristic signature on the isotopic composition of the water that is left behind, such that in dual-isotope space, evaporated waters plot below the local meteoric water line (LMWL) that characterizes precipitation. Soil and xylem water samples can often plot below the LMWL as well, suggesting that they have also been influenced by evaporation. These soil and xylem water samples frequently plot along linear trends in dual-isotope space. These trend lines are often termed "evaporation lines" and their intersection with the LMWL is often interpreted as the isotopic composition of the precipitation source water. Here we use numerical experiments based on established isotope fractionation theory to show that these trend lines are often by-products of the seasonality in evaporative fractionation and in the isotopic composition of precipitation. Thus, they are often not true evaporation lines, and, if interpreted as such, can yield highly biased estimates of the isotopic composition of the source water.

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Evaporation causes the isotopic composition of soil water to become different from that of the original precipitation source. If multiple samples originating from the same source are available, they can be used to reconstruct the original source composition. However, soil water is influenced by seasonal variability in both precipitation sources and evaporation patterns. We show that this variability, if not accounted for, can lead to biased estimates of the precipitation source water.
Evaporation causes the isotopic composition of soil water to become different from that of the...
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