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 measu rements (gradient or eddy correlation techniques), isotope analyses can all ow 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 dis cussion of the isotope approach is aimed at highlighting some of the uncert ainties that require further research. We also demonstrate first, the appli cation of combined concentration and isotopic gradient analysis of atmosphe ric water vapour above crop fields in order to estimate ET fluxes and its g ross components (soil evaporation was estimated at 1.5-3.5% of mid-day ET f lux in a mature wheat field). Second, we demonstrate the potential in monit oring delta(ss)- delta(L), the difference between predicted and measured le af water delta(18)O values, as an indicator of seasonal variations in canop y-scale transpiration in a desert ecosystem (linear correlation between thi s indicator and conventional ET measurements was observed). Improving our a nalytical capabilities for high-precision isotopic analysis of very small w ater vapour samples was a limiting factor in the above applications and a m ethod for pyrolysis and on-line O-18 analysis of 0.2-2 mu L water samples i s described. Copyright (C) 2000 John Wiley & Sons, Ltd.