Temporal Variations of d-excess in Water Vapor above a Subtropical Conifer Plantation in Southeastern China

The intercept parameter of the Global Meteoric Water Line (GMWL), d (=δD-8*δ¹⁸O), termed deuterium excess, is a combination of the oxygen (δ¹⁸O) and hydrogen (δD) isotope ratios and its variability is an indicator of environmental conditions during phase changes under non-equilibrium conditions. Direct measurement of d-excess in the vapor phase may offer constraints on how to separate local evaporative contribution to the atmospheric water vapor and to partition evapotranspiration flux into its component fluxes. The goal of this paper is to improve our understanding of the temporal variations of d-excess in water vapor and to identify the main drivers of its variability above a subtropical conifer plantation in southeastern China. Continuous measurements were made on the δD and δ¹⁸O of atmospheric vapor and evapotranspiration using an off-axis integrated cavity output spectroscopy analyzer (model DLT-100, Los Gatos Research, Mountain View, CA). Our results demonstrate that the d-excess in water vapor showed a robust diurnal cycle with maximum values during mid-day and minimum values at the moring. The d-excess of evapotranspiration showed also strong diurnal variations with a similar time phase. The most likely mechanism underlying the diurnal patterns is different non-steady state behaviors of δ¹⁸O and δD of plant transpiration. Diurnal variations in the d-excess of atmospheric vapor have been reported by other research groups, and our study shows that the main driver for these diurnal patterns (at least in the surface layer) is linked to water vapor originated from surface evapotranspiration and not from other non-local sources.