Using stable isotopes of water in evapotranspiration studies

The use of stable isotopes to estimate evapotranspiration (ET) fluxes from vegetated areas is increasing. By complementing conventional net flux measurements (gradient or eddy correlation techniques), isotope analyses can allow partitioning ET between its gross components, soil evaporation and leaf transpiration. Isotopic analyses of atmospheric water vapour above canopies can also constrain, or provide alternatives for estimating ET. A brief discussion of the isotope approach is aimed at highlighting some of the uncertainties that require further research. We also demonstrate first, the application of combined concentration and isotopic gradient analysis of atmospheric water vapour above crop fields in order to estimate ET fluxes and its gross components (soil evaporation was estimated at 1\5–3\5% of mid-day ET flux in a mature wheat field). Second, we demonstrate the potential in monitoring δss−δL, the difference between predicted and measured leaf water δ18O values, as an indicator of seasonal variations in canopy-scale transpiration in a desert ecosystem (linear correlation between this indicator and conventional ET measurements was observed). Improving our analytical capabilities for high-precision isotopic analysis of very small water vapour samples was a limiting factor in the above applications and a method for pyrolysis and on-line 18O analysis of 0\2–2 μL water samples is described. Copyright © 2000 John Wiley & Sons, Ltd.