Quantitative knowledge about ecohydrological partitioning across the
critical zone in different types of urban green space is important to
balance sustainable water needs in cities during future challenges of
increasing urbanization and climate warming. We monitored stable water
isotopes in liquid precipitation and atmospheric water vapour (δ
v) using in-situ cavity ring-down spectroscopy
(CRDS) over a two-month period in an urban green space area in Berlin,
Germany. Our aim was to better understand the origins of atmospheric
moisture and its link to water partitioning under contrasting urban
vegetation. δ v was monitored at multiple heights (0.15,
2 and 10 m) in grassland and forest plots. The isotopic composition of δ
v above both land uses was highly dynamic and positively
correlated with that of rainfall indicating the changing sources of
atmospheric moisture. Further, the isotopic composition of δ
v was similar across most heights of the 10 m profiles
and between the two plots indicating limited aerodynamic mixing. Only
the surface at ~0.15 m height above the grassland, δ
v showed significant differences, with more enriched
values indicative of evaporative fractionation immediately after
rainfall events. Further, disequilibrium between δ v and
precipitation composition was evident during and right after rainfall
events with more positive values (i.e. vapour more enriched than
precipitation) in summer and negative values in winter, which probably
results from higher evapotranspiration and more convective precipitation
events in summer. Our work showed that it is technically feasible to
produce continuous, longer-term data on δ v isotope
composition in urban areas from in-situ monitoring using CRDS,
providing novel insights into water cycling and partitioning across the
critical zone of an urban green space. Such data has the potential to
better constrain the isotopic interface between the atmosphere and the
land surface and to improve ecohydrological models that can resolve
evapotranspiration fluxes.