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.