5.3 Testing the assumption of equilibrium between vapour and
precipitation
The equilibrium assumption between the isotopic signatures of
δv and precipitation was not always held during our
field campaign. Previous studies showed that the equilibrium assumption
is more robust at subseasonal, longer time scales than for individual
rain events. Lee et al. found that during rain events, vapour in the
surface layer developed in general a state of equilibrium with the
falling raindrops. In our study, this assumption was not robust at the
subseasonal scale and did not confirm an establishment of an
equilibrium.
ΔRatm (i.e. difference in isotope ratios of water
vapour and precipitation in atmosphere) was greater and showed more
positive values in summer reflecting that vapour was more enriched than
precipitation during summer. Potential reasons for these results are (as
discussed by ): raindrops formed at high elevation , precipitation came
from convective events with big raindrops or high tree transpiration
rates from deeper sources prevented vapour from equilibrium with
precipitation. Additionally, high transpiration rates lead to isotopic
enrichment of δv and could generate higher deviation
from δv with precipitation.
Testing the equilibrium assumption is especially important for areas
with a distinct microclimate like cities as previous studies showed that
equilibrium estimates can be biased . Further, different regions of the
World show diverging results for ΔRatm depending
on climate, altitude and latitude. E.g. Mercer et al. showed that the
equilibrium assumption does not hold in continental mountain
environments. By going beyond the standard assumption of equilibrium in
urban ecohydrology, we can improve simple mixing models, complex
process-based, isotope-aided ecohydrological models like
EcH2O-iso , estimations in keeling plot and the Craig
and Gordon approach (cf. ).