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Response of water vapour D-excess to land-atmosphere interactions in a semi-arid environment.
- Source :
- Hydrology & Earth System Sciences Discussions; 2016, p1-44, 44p
- Publication Year :
- 2016
-
Abstract
- 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<subscript>v</subscript> = δ<subscript>2</subscript>H - 8 × δ<superscript>18</superscript>O) 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<subscript>v</subscript>). With this in mind, we employed a chamber based approach to directly measure D-excess in ET (d<subscript>ET</subscript>) fluxes. We show that ET fluxes imposed a negative forcing on the ambient vapour and could not explain the higher daytime d<subscript>v</subscript> values. The low d<subscript>ET</subscript> 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<subscript>v</subscript> and locally measured relative humidity was consistent with an oceanic moisture source, suggesting that remote hydrological processes were the major contributor to daytime d<subscript>v</subscript> 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<subscript>ET</subscript> 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<subscript>v</subscript>. 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]
Details
- Language :
- English
- ISSN :
- 18122108
- Database :
- Complementary Index
- Journal :
- Hydrology & Earth System Sciences Discussions
- Publication Type :
- Academic Journal
- Accession number :
- 116613615
- Full Text :
- https://doi.org/10.5194/hess-2016-271