Stable water isotopes to understand sources, pathways and ages of water in complex urban settings

The hydrology of major cities is controlled by complex networks of both natural and engineered flow paths and characterised by spatio-temporal variations in connection and disconnection of water sources. Here, we traced the transformation of stable water isotopes through the urban critical zone, in Berlin, Germany. The Panke catchment is heavily influenced by a wastewater treatment plant (~700,000 inhabitants), and legacy effects of water management during the past century. Two and a half years of daily stream isotopes revealed the complicated interactions between the groundwater fed-stream and urban impacts, such as wastewater effluents and “imported” transboundary water sources, and urban stormwater overflows. To mitigate the effects of the latter, urban greenspaces are important to store and release water more naturally to imitate the “sponge-city” concept and retain water in the urban landscape. We therefore also investigated stable water isotopes at the plot scale in three parks in Berlin. We sampled grassland and urban forest sites during the dry year of 2020. Soil and xylem isotopes of different tree species and under grassland revealed shallower root water uptake from grasslands and greater recharge by younger waters. As evapotranspiration accounts for about 90% of rainfall, ecohydrological dynamics in urban green spaces were shown to be largely disconnected from urban runoff generation. Isotopes were shown to be invaluable tools in multi-scale understanding of urban hydrology and have great potential in contributing to the evidence base needed to develop policies for more sustainable urban water management in the face of increased urban growth and climate change.