The stable isotopic composition of water vapour provides information about moisture sources and processes difficult to obtain with traditional measurement techniques. Recently, it has been proposed that the D-excess of water vapour can provide a diagnostic tracer of continental moisture recycling. However, D-excess exhibits a diurnal cycle that has been observed across a variety of ecosystems and may be influenced by a range of processes beyond regional-scale moisture recycling, including local evaporation (ET) fluxes. There is a lack of measurements of D-excess in evaporation (ET) fluxes, which has made it difficult to assess how ET fluxes modify the Dexcess in water vapour (dv). With this in mind, we employed a chamber-based approach to directly measure D-excess in ET (dET) fluxes. We show that ET fluxes imposed a negative forcing on the ambient vapour and could not explain the higher daytime dv values. The low dET observed here was sourced from a soil water pool that had undergone an extended drying period, leading to low D-excess in the soil moisture pool. A strong correlation between daytime dv and locally measured relative humidity was consistent with an oceanic moisture source, suggesting that remote hydrological processes were the major contributor to daytime dv variability. During the early evening, ET fluxes into a shallow nocturnal inversion layer caused a lowering of dv values near the surface. In addition, transient mixing of vapour with a higher D-excess from above the nocturnal inversion modified these values, causing large variability during the night. These results indicate dET can generally be expected to show
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: Stephen Parkes was supported by the Atmospheric Mixing and Pollution Transport (AMPT) project at the Australian Nuclear Science and Technology Organization (ANSTO) and the King Abdullah University of Science and Technology. The Baldry Hydrological Observatory field campaign was supported by Australian Research Council Discovery grants DP0987478 and DP120104718. Matthew McCabe acknowledges the support of the King Abdullah University of Science and Technology. We thank Peter Graham, Cecilia Azcurra, Jin Wang and Yingzhe Cai for their assistance during the campaign. We also appreciate the support of Diana and Jason Tremain for access to the Baldry Hydrological Observatory and surrounding farmland, Chris Dimovski for performing plant and soil water extractions and Barbara Neklapilova analysis of plant and soil water samples.