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.