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| Titel |
Determination of kinetic isotopic fractionation of water during bare soil
evaporation |
| VerfasserIn |
Maria Quade, Nicolas Brüggemann, Alexander Graf, Youri Rothfuss |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250143227
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| Publikation (Nr.) |
 EGU/EGU2017-6931.pdf |
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| Zusammenfassung |
| A process-based understanding of the water cycle in the atmosphere is important for
improving meteorological and hydrological forecasting models. Usually only net fluxes of
evapotranspiration – ET are measured, while land-surface models compute their raw
components evaporation –E and transpiration –T.
Isotopologues can be used as tracers to partition ET, but this requires knowledge of the
isotopic kinetic fractionation factor (αK) which impacts the stable isotopic composition of
water pools (e.g., soil and plant waters) during phase change and vapor transport by soil
evaporation and plant transpiration. It is defined as a function of the ratio of the transport
resistances in air of the less to the most abundant isotopologue. Previous studies determined
αK for free evaporating water (Merlivat, 1978) or bare soil evaporation (Braud et al. 2009) at
only low temporal resolution. The goal of this study is to provide estimates at higher temporal
resolution.
We performed a soil evaporation laboratory experiment to determine the αK by applying
the Craig and Gordon (1965) model. A 0.7 m high column (0.48 m i.d.) was filled with silt
loam (20.1 % sand, 14.9 % loam, 65 % silt) and saturated with water of known
isotopic composition. Soil volumetric water content, temperature and the isotopic
composition (δ) of the soil water vapor were measured at six different depths. At each
depth microporous polypropylene tubing allowed the sampling of soil water vapor
and the measurement of its δ in a non-destructive manner with high precision and
accuracy as detailed in Rothfuss et al. (2013). In addition, atmospheric water vapor was
sampled at seven different heights up to one meter above the surface for isotopic
analysis.
Results showed that soil and atmospheric δ profiles could be monitored at high temporal
and vertical resolutions during the course of the experiment. αK could be calculated by using
an inverse modeling approach and the Keeling (1958) plot method at high temporal resolution
over a long period. We observed an increasing δ in the evaporating water vapor due to more
enriched surface water. This leads to a higher transport resistances and an increasing
αK.
References
Braud, I., Bariac, T., Biron, P., and Vauclin, M.: Isotopic composition of bare soil
evaporated water vapor. Part II: Modeling of RUBIC IV experimental results, J. Hydrol., 369,
17-29.
Craig, H. et al., 1965. Deuterium and oxygen 18 variations in the ocean and marine
atmosphere. In: E. Tongiogi (Editor), Stable Isotopes in Oceanographic Studies and
Paleotemperatures. V. Lishi, Spoleto, Italy, pp. 9-130.
Keeling, C. D.: The Concentration and Isotopic Abundances of Atmospheric Carbon
Dioxide in Rural Areas, Geochim. Cosmochim. Acta, 13, 322-334.
Merlivat, L., 1978. Molecular Diffusivities of H216O, HD16O, and H218O in Gases. J
Chem Phys, 69, 2864-2871.
Rothfuss, Y. et al., 2013. Monitoring water stable isotopic composition in soils using
gas-permeable tubing and infrared laser absorption spectroscopy. Water Resour. Res., 49, 1-9. |
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