Evapotranspiration contribution to the atmospheric boundary layer determined from stable isotope observations in a semi-arid environment
Abstract
To assess the impact climate change will have on water resources it is important to quantify how fluxes of water at the land-atmosphere interface will be affected. Monitoring the stable isotopes of water in different reservoirs is a way to trace hydrological processes occurring at the surface and the within the Atmospheric Boundary Layer (ABL). Recently introduced instrumentation allow in-situ measurements of water vapour isotopes on time scales conducive to studies of the transition between the stable nocturnal and convective atmospheric boundary layers as well as changes in the synoptic state of the atmosphere. We present measurements from a short term campaign to a semi-arid site during an extended dry spell. Combining in-situ water vapour isotope, meteorological and surface radon concentration measurements we investigate the relationships between water vapour mixing ratios and atmospheric processes. The H2O mixing ratio, δ2H and d-excess of water vapour all show variations that are affected by both synoptic and local scale processes. The δ2H of water vapour is equally affected by local processes (evapotranspiration fluxes and diurnal mixing processes) and changes in air mass history (predominantly precipitation history). In contrast, local processes are the dominant feature for the collected d-excess time series, thus making d-excess a better tracer of the local ABL water vapour budget. Plant sample d-excess measurements, in-situ d-excess water vapour observations and ABL height measurements are combined in an isotopic mass balance used to study the diurnal variation of the local ET contribution to the ABL. Shortly after sunrise, before the breakup of the nocturnal inversion, ET fluxes can account for up to 35% of the moisture contained within the ABL. In the middle of the day when the convective ABL has fully developed, ET fluxes account for less than 1% of ABL moisture. In the late afternoon, when the surface layer begins to decouple from the rest of the convective ABL, the ET flux contribution to ABL moisture increases to 8.3%. The isotope budget method is shown to compare well with other boundary layer budget methods, although due to the effects of entrainment the isotope based method overestimates ET contributions to the convective ABL. Future work should investigate whether the d-excess based isotopic budget is successful under a range of soil moisture and climatic regimes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.H53A1510P
- Keywords:
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- 0315 ATMOSPHERIC COMPOSITION AND STRUCTURE / Biosphere/atmosphere interactions;
- 0394 ATMOSPHERIC COMPOSITION AND STRUCTURE / Instruments and techniques;
- 1818 HYDROLOGY / Evapotranspiration;
- 1840 HYDROLOGY / Hydrometeorology