Using in-situ stable isotope observations to study rapid changes in land atmosphere exchange within a chamber environment

The exchange of latent heat (e.g., evapotranspiration) between the land and the atmosphere is highly dependent on prevailing environmental conditions. Response of the hydrological processes responsible for latent heat fluxes to changing environmental conditions is not well characterised. This is in part due to the difficulty in separating latent heat fluxes into their components. Stable isotope observations of water vapour provide a unique tool to separate and quantify component evapotranspiration fluxes. However, before stable isotope observations can provide insight into the driving factors of latent heat fluxes, the variability of the stable isotope signature of evapotranspiration fluxes must be first understood. Here we look at the feasibility of using the combination of field chambers and in-situ stable isotope observations to determine the isotopic composition of evapotranspiration from bare and vegetated soil plots. An open chamber design was used to avoid condensation during chamber closure and the isotopic composition of the outflow from the chamber was monitored using an Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) based system. The chambers were fitted with Photosynthetic Active Radiation (PAR) and humidity/temperature sensors to investigate the influences of the chamber environment on the stable isotopic composition of the evapotranspiration flux. Three different methods, including mass balance and Keeling plot approaches were used to determine the isotopic composition of evapotranspiration within the chambers. Results reveal that the relationship between the different methods varies with changes in the chamber environment.