Using δ18O signatures to trace evapotranspiration and partition ecosystem water fluxes
2 Helmholtz Center for Environmental Research
3 Agrarökosystemforschung
O 3.3 in Ökosysteme: Funktion und Leistungen
11.10.2012, 12:00-12:15, H8
Stable oxygen isotopes (δ18O) of water provide a valuable tracer for water movements within ecosystems and are used to estimate the contribution of transpiration to total ecosystem evapotranspiration (ft). We measured firstly the isotopic composition of evaporation (δ18OE) on bare soil plots with a cavity ring-down spectrometer connected to a soil chamber on a field site in central Portugal. We measured secondly the flux and corresponding isotopic composition of evapotranspiration (δ18OET) of an herbaceous understory layer. The Craig and Gordon equation was tested against bare soil measurements of δ18OE. We quantified specifically the variation in δ18OE arising from uncertainties in the determination of environmental input variables to the Craig and Gordon equation: the isotope signature at the evaporating site (δ18Oe), the temperature at the evaporating site (Te), and the kinetic fractionation factor (αk). We then calculated the isotopic composition of evaporation δ18OE on the vegetated plots from measurements of soil water isotopic composition at the evaporating site δ18Oe. We could hence quantify the contribution of transpiration to evapotranspiration (ft) based on measured δ18OET, modeled δ18OE from observed soil water isotopic composition at the evaporating site δ18Oe, and modeled δ18O of transpiration δ18OT from observed total soil water isotopic composition.
Our results demonstrate that predicting δ18OE using Craig and Gordon leads to good agreement with measured δ18OE given that the temperature and isotope profiles of the soil are thoroughly characterized. However, modeled δ18OE is highly sensitive to changes in temperature Te and the isotopic composition at the evaporating front (δ18Oe) as well as the formulation of the kinetic fractionation factor (αk) of soil evaporation. This has a strong impact on the partition of transpiration and evaporation from the total ET flux. The fraction of transpiration (ft) varied strongly using different formulations for αk and assuming steady or non-steady state transpiration. These findings provide a first comparison of laser-based and modeled isotopic compositions of evaporation, testing the Craig and Gordon equation under field conditions. Our findings are of special interest for studies using stable isotopes to separate soil evaporation and plant transpiration fluxes and highlight the need for a thorough characterization of the micrometeorological and isotopic constitution of the upper soil layer to locate the evaporating front. We also call on a better characterization of the kinetic fractionation factor of soil evaporation.
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