Isotopic composition of bare soil evaporated water vapor. Part I: RUBIC IV experimental set up and results

Stable water isotopes such as oxygen 18, are natural tracers of water movement within the soilvegetationatmosphere continuum. They provide useful information for a better understanding of evaporation and water vapor transport within soils. This paper presents a novel controlled experimental set up. It is dedicated to detailed measurements of the evaporation fluxes from bare soil columns, as well as to the corresponding isotopic composition of the water vapor, under non steady state conditions. The experiment allowed an accurate determination of these quantities. The formulae encountered in the literature were used to estimate the isotopic composition of the evaporated water vapor. None of them was able to correctly reproduce the measured isotopic composition of water. The data were then used to estimate the value of the isotopic composition of the soil liquid water, which should be used to get the right results for the isotopic composition of the evaporated water vapor. Results suggest that, when liquid transfer is dominant within the soil, the isotopic composition of evaporation was controlled by the isotopic composition of the liquid water within very thin soil surface layers. When there is a peak in the isotopic profile, i.e. when water vapor is dominant close to the surface, the isotopic composition of the evaporated water seems to be governed by the isotopic composition of the soil liquid water at the peak. The data were also used to estimate the kinetic fractionation factor. The results suggest that the latter is not constant in time. The values seem to depend on the shape of the isotopic profile. In both cases, the uncertainty on the results is very large. The estimation of the kinetic fractionation factor is studied more in details using the modeling results presented in Part II of a companion paper where the data set is modeled using the SiSPAT_Isotope model.