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

Abstract. 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 (d_v  =  δ²H − 8  ×  δ¹⁸O) 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 in the soil moisture pool. 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 d_v 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 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. Spatial and temporal variability of D-excess in ET fluxes therefore needs to be considered when using d_v to study moisture recycling and during extended dry periods with weak moisture recycling may act as a tracer of the relative humidity at the oceanic moisture source.