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.