Response of water vapour D-excess to land-atmosphere interactions in a semi-arid environment.

The stable isotopic composition of water vapour provides information about moisture sources and processes that is difficult to obtain with traditional measurement techniques. Recently, it has been proposed that the D-excess (d_v = δ₂H - 8 × δ¹⁸O) 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 D-excess in water vapour (d_v). With this in mind, we employed a chamber based approach to directly measure D-excess in ET (d_ET) fluxes. We show that ET fluxes imposed a negative forcing on the ambient vapour and could not explain the higher daytime d_v values. The low d_ET observed here was sourced from a soil water pool that had undergone an extended drying period, leading to low D-excess of the soil moisture. A strong correlation between daytime d_v and locally measured relative humidity was consistent with an oceanic moisture source, suggesting that remote hydrological processes were the major contributor to daytime d_v variability. During the early evening, ET fluxes into a shallow nocturnal inversion layer caused a lowering of the 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 within night variability. These results indicate d_ET can generally be expected to show large spatial and temporal variability and to depend on the soil moisture state. For long periods between rain events, common in semi-arid environments, ET would be expected to impose negative forcing on the surface d_v. The variability of D-excess in ET fluxes therefore needs to be considered when using dv to study moisture recycling and during extended dry periods may act as a tracer of the relative humidity of the oceanic moisture source. [ABSTRACT FROM AUTHOR]