Stable isotope composition of water vapor as an indicator of transpiration fluxes from rice crops

Abstract

Measurements of the stable isotope composition (δ2H and δ18O) of water vapor and associated micrometeorological parameters were made before and after full establishment of a rice crop in southeastern Australia. The aim of the experiment was to gain a better understanding of stable isotope variations of water vapor near the ground surface in response to local evaporation, local transpiration, regional scale vapor transport and the vertical stability of the atmospheric boundary layer. Vapor samples were collected at several heights within 9 m of the water surface during two separate sampling periods. The δ2H values of the water vapor ranged over more than 60‰, reflecting major rapid changes in regional air mass sources, as well as variations in the stability of the lowest 10 m of the atmosphere. The influence of tropical and higher‐latitude air masses resulted in local vapor compositions which were relatively enriched and depleted, respectively, in heavy isotopes. Vertical gradients in heavy isotope abundances were very large during stable conditions (as much as Δδ2H = –27‰ from 0.8 to 8.4 m), as the result of mixing between transpired water and regional air vapor. Transpiration fluxes calculated from the water vapor δ2H gradient ranged from 5 to 7 mm d−1, which was in good agreement with one‐dimensional aerodynamic energy budget calculations of daytime vapor fluxes.