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| Titel |
Long-term and high-frequency non-destructive monitoring of water stable isotope profiles in an evaporating soil column |
| VerfasserIn |
Y. Rothfuss, S. Merz, J. Vanderborght, N. Hermes, A. Weuthen, A. Pohlmeier, H. Vereecken, N. Brüggemann |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1027-5606
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| Digitales Dokument |
URL |
| Erschienen |
In: Hydrology and Earth System Sciences ; 19, no. 10 ; Nr. 19, no. 10 (2015-10-06), S.4067-4080 |
| Datensatznummer |
250120820
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| Publikation (Nr.) |
 copernicus.org/hess-19-4067-2015.pdf |
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| Zusammenfassung |
The stable isotope compositions of soil water (δ2H and
δ18O) carry important information about the prevailing soil
hydrological conditions and for constraining ecosystem water budgets.
However, they are highly dynamic, especially during and after precipitation
events. In this study, we present an application of a method based on
gas-permeable tubing and isotope-specific infrared laser absorption
spectroscopy for in situ determination of soil water δ2H and δ18O. We
conducted a laboratory experiment where a sand column was initially
saturated, exposed to evaporation for a period of 290 days, and finally
rewatered. Soil water vapor δ2H and δ18O were
measured daily at each of eight available depths. Soil liquid water
δ2H and δ18O were inferred from those of the vapor
considering thermodynamic equilibrium between liquid and vapor phases in the
soil. The experimental setup allowed for following the evolution of soil water
δ2H and δ18O profiles with a daily temporal
resolution. As the soil dried, we could also show for the first time the
increasing influence of the isotopically depleted ambient water vapor on the
isotopically enriched liquid water close to the soil surface (i.e., atmospheric
invasion). Rewatering at the end of the experiment led to
instantaneous resetting of the stable isotope profiles, which could be
closely followed with the new method.
From simple soil δ2H and δ18O gradients
calculations, we showed that the gathered data allowed one to determinate the
depth of the evaporation front (EF) and how it receded into the soil over
time. It was inferred that after 290 days under the prevailing experimental
conditions, the EF had moved down to an approximate depth of −0.06 m.
Finally, data were used to calculate the slopes of the evaporation lines and
test the formulation for kinetic isotope effects. A very good agreement was
found between measured and simulated values (Nash and Sutcliffe efficiency,
NSE = 0.92) during the first half of the experiment, i.e., until the EF
reached a depth of −0.04 m. From this point, calculated kinetic effects
associated with the transport of isotopologues in the soil surface air layer
above the EF provided slopes lower than observed. Finally, values of kinetic
isotope effects that provided the best model-to-data fit (NSE > 0.9)
were obtained from inverse modeling, highlighting uncertainties
associated with the determinations of isotope kinetic fractionation and soil
relative humidity at the EF. |
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