Urban Moisture (Re)cycling: quantifying canopy effects using in-situ water stable isotope monitoring

Urban green spaces are valuable infrastructure in the urban environment because they facilitate natural stormwater management through rainwater retention, decelerated runoff and enhanced evapotranspiration which can also mitigate heat stress. Investigating the complex interactions of water flux partitioning of incoming precipitation into “green” (i.e. evaporation and transpiration) and “blue” (surface runoff and groundwater recharge) water fluxes through urban vegetation is crucial to understand what types of landcover might best balance water re-distribution for a particular geographical setting and provide a cooling effect whilst not compromising groundwater recharge.

Stable water isotopes are very useful tools to investigate these complex processes. So far, studies investigating high-resolution ecohydrological process dynamics at the urban soil-plant-atmosphere interface, e.g. canopy evaporation, with stable isotopes are rare. Here, we conducted novel field experiments using direct in-situ monitoring of the isotope composition of evaporated atmospheric moisture at different heights above the soil surface, plant xylem and soil water in different types of urban greenspaces in Berlin, Germany. Results show a more homogenous spatio-temporal distribution of water vapour signals within the elevation profile of urban trees compared to grasslands, reflecting continuous interplay of interception evaporation, transpiration and soil evaporation. Additionally, grasslands showed a lower impact on the isotopic composition of atmospheric water vapor, mainly reflecting higher evaporative losses close to the ground surface. Complex patterns of precipitation fractionation under contrasting urban vegetation canopies were also revealed. Topsoil moisture rates strongly depended on the soil type and less on the above vegetation type.

The collected data on the redistribution of urban water in different types of green spaces is very helpful for the development of isotope aided ecohydrological models. This knowledge can further support valuable decision-making for sustainable urban development across scales.