5.1 Vapour stable water isotopes in different urban
vegetation
This study showed that extended (i.e. >2 months) periods,
continuous sequential in-situ monitoring of δvproduces reliable ad novel data in urban green space environments with a
temperate climate. Our distributed network of inlet ports sampling the
atmospheric boundary at different heights above contrasting urban green
space vegetation produced reliable high-resolution data with a 2-hourly
resolution for each inlet. However, there is no doubt that the method is
very labour intensive and requires almost daily maintenance including
checking the correct operation of the CRDS, ventilation systems and
pumps. Detailed, biweekly data checks of the different inlets are also
necessary to detect condensation in the tubes or other unwanted memory
effects in the CRDS. In particular, the in-situ setup requires a
secured environment for the CRDS and vapour tubing (e.g. a locked and
fenced box and securing pipes adapted to the study site). Overall,
however, in-situ monitoring of δv needs less
regular maintenance than in-situ soil water monitoring due to
greater condensation issues from deep soil compared to atmospheric
vapour (cf. ).
Monitored δv data fluctuated around the LMWL in equal
distribution over the entire study period indicating no dominance of
non-equilibrium fractionation , but disequilibrium occurred at shorter
time scales. We found limited difference between the two vegetation
covers reflecting a generally well mixed boundary. δv of
grassland showed a slightly higher temporal variability and also higher
variance along the height profile compared to the tree site. The only
significant difference was that the surface air (at 0.15 m height) above
the grassland was more enriched, though this was rapidly attenuated with
height. An in-situ study by Griffis et al. found similar effects
of surface evaporation enriching surface boundary layer water vapour and
atmospheric loss of light vapour fraction above grassland through the
underlying process of kinetic fractionation during evaporation , while
tree canopy protects from such loss.
At the event scale, δv showed clear isotopic responses
after rain. The response timing was dependent on the time of day being
more marked around noon when radiation input is elevated. This is due to
the fact that δv at hourly timescale is controlled by
airmass advection which increases with higher solar radiation . At
seasonal scale, lc-excess was low in summer and higher in autumn
reflecting higher ET in warmer months. Griffis et al. explained the
seasonal amplitude of δv to be driven by Rayleigh
processes that are strongly modulated by evaporation and entrainment,
i.e. inflow of an air parcel to another.