Investigating land-atmosphere exchange using observations of the stable isotopes in water vapour during a short term field campaign

Thursday, 18 December 2014
Stephen David Parkes1, Alan Griffiths2, Lixin Wang3, Matthew F McCabe4, Scott D. Chambers2, Alastair G Williams2, Adrian Element2 and Josiah Strauss5, (1)King Abdullah University of Science and Technology, Thuwal, Saudi Arabia, (2)Australian Nuclear Science and Technology Organization, Kirrawee, Australia, (3)Indiana University Purdue University Indianapolis, Indianapolis, IN, United States, (4)King Abdullah University of Science and Technology, Biological and Environmental Sciences and Engineering, Thuwal, Saudi Arabia, (5)Duke University, Durham, NC, United States
Evaporation sources and meteorological conditions at the source of evaporation are important variables affecting the stable isotopes in water vapour. Isotopes therefore provide complimentary information to more conventional techniques used in land-atmosphere exchange studies. Augmenting in-situ water vapour isotope measurements, soil and plant water isotopic analysis, and meteorological measurements collected during a 2 week field campaign to a semi-arid grassland site in NSW, Australia, the relationship between land-atmosphere exchange processes and the water vapour isotopic composition was investigated. Back-trajectory analysis was used to provide insight into the role of large scale hydrological processes on the water vapour isotopic composition. During the campaign clear dry conditions were observed leading to strong diurnal cycles for the evapotranspiration fluxes. For diurnal times, the d-excess of the water vapour showed a strong relationship with the evapotranspiration fluxes. During the morning transition period when the convective boundary layer was being established and evapotranspiration fluxes were increasing, the d-excess increased sharply. Plant and soil water samples indicated non-steady state transpiration fluxes led to these higher values. In the afternoon when the evapotranspiration fluxes decreased and the humidity approached saturation, the d-excess of water vapour declined approaching values approximately in equilibrium with the soil water. On the other hand, the diurnal cycle of the δ2H did not show a consistent trend with the local meteorology, but showed a stronger relationship with the sea surface temperature of the oceanic moisture source and dehydration pathways during transport of atmospheric moisture to the site. The interpretation from these results indicates that the d-excess variable may be a better variable for investigating local land-atmosphere exchange as the δ2H is strongly influenced by changes in the synoptic scale meteorology.