6. Conclusion
We monitored stable water isotopes in liquid precipitation and
atmospheric water vapour (δv) using in-situcavity ring-down spectroscopy (CRDS) over a two-month period in an urban
green space area in Berlin, Germany. δv was monitored at
multiple heights (0.15, 2 and 10 m) in different vegetation: grassland
and forest plots. Our distributed sampling network of inlet ports
produced novel, reliable high-resolution data with a 2-hourly resolution
for each inlet.
We have shown that the isotopic composition of δv above
both land uses was highly dynamic and positively correlated with that of
rainfall indicating the changing sources of atmospheric moisture. The
isotopic composition of δv 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 showed significant differences in
δv, with more enriched values indicative of evaporative
fractionation immediately after rainfall events.
We combined this isotope monitoring with hydroclimatic monitoring and
measurements of sap flow, stem size, soil moisture, throughfall. At both
sites, the overall low top soil moisture increased in response to
precipitation and then decreased after the events reflecting drainage
and evaporative losses. Potential normalized ETnorm did
not exceed total sap fluxnorm of the maple tree during
the phase of active leaves indicating no drought stress on the tree.
Dendrometer data revealed normal stem growth for late summer and autumn.
Despite interception evaporation and transpiration from the tree canopy
after events, there was no imprint on δv captured at 10
m compared to lower heights. Our results indicate occasional
dis-equilibrium between water vapour and precipitation isotopes.
Our set up provided novel insights into high-resolution dynamics of
water cycling and partitioning in across the Critical Zone of an urban
green space can contribute to improved urban planning strategies
providing new evidence-base. Such data has the potential to better
constrain the isotopic interface between the atmosphere and the land
surface. Importantly, it can be incorporated into tracer-aided
ecohydrological models that can resolve evapotranspiration fluxes and
improve these estimations.
However, more research is needed to upscale these findings to canopy and
city scale. More detailed monitoring of urban canopy ET by more
distributed networks in and above canopies will benefit further
investigations.