5.2 Insights into high-resolution urban
ecohydrology
Our results indicate that the urban grassland surface is contributing to
the atmospheric moisture affecting water partitioning with the main
drivers being high surface evaporation and/or high transpiration of the
grass, high surface temperatures as well as low atmospheric mixing. The
measurements from beneath canopy give useful insights for turbulent
mixing parameterisation of urban canopy layer vertical transport, but
direct transpiration imprint could not be measured.
Additional insights into the processes controlling isotopic composition
of urban green space δv were leveraged by having
simultaneous ecohydrological monitoring, i.e. measurements of soil
moisture, throughfall, sap flux and tree diameter. At both sites, the
overall low top soil moisture increased in response to precipitation and
then decreased with time reflecting drainage and evaporative losses that
contribute – at least in the grass plot – to increased evaporation
that affects the isotopic signal at 0.15 m. Potential normalized
ETnorm did not exceed total sap fluxnormof the maple tree during the phase of active leaves indicating no
drought stress of the tree. Additionally, dendrometer data revealed
normal stem growth for late summer and autumn. Considering the low top
soil moisture and constant sap flux, our results match with the findings
of Kuhlemann et al., from another urban green space site in Berlin in
that urban tree transpiration rates show a certain resilience to drought
(which is of course highly dependent on tree ages and species). Further,
the investigated tree stands were a group of trees which probably makes
a difference compared to individual urban trees in another study that
showed considerably higher sap flux densities .
Interestingly, despite interception evaporation and transpiration from
the urban tree canopy after events, there was no imprint on
δv captured at 10 m compared to lower heights (cf. .
δv variabilities did not correlate with certain
ecohydrological parameters throughout the whole sampling period; though
δv responded to changes in potential ET during the
warmer period until the end of September. A storm on Oct.
21st led to a quick decrease of δvsignatures that day.
These insights on high-resolution dynamics of ecohydrological fluxes and
partitioning can contribute to improved strategies of urban green space
management in the future. However, there is great potential for more
detailed monitoring of urban canopy ET by more distributed networks in
canopies, e.g. it would be interesting to measure at least 5 m over an
urban canopy or higher similar to Braden-Behrens et al. and compare this
with corresponding measurements above an urban grassland.