Partitioning evapotranspiration – Testing the Craig and Gordon model with field measurements of oxygen isotope ratios of evaporative fluxes

Stable oxygen isotopes 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 tested the Craig and Gordon equation against continuous field measurements of isotopic composition of evaporation and assessed the impact for partitioning evapotranspiration. Therefore, evaporation (E) and its isotopic signature (δ18OE) on bare soil plots, as well as evapotranspiration (ET) and its corresponding isotopic composition of (δ18OET) of an herbaceous layer was measured with a cavity ring-down spectrometer connected to a soil chamber on a field site in central Portugal. We quantified the variation in δ18OE arising from uncertainties in the determination of environmental input variables to the Craig and Gordon equation: the isotope signature (δ18Oe) and the temperature at the evaporating site (Te), and the kinetic fractionation factor (αk). We could hence quantify 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.