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

摘要

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) = delta H-2 - 8 x delta O-18) 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 (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 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). 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.