Urban water systems under climate stress: An isotopic perspective from Berlin, Germany

Large urban areas are typically characterized by a mosaic of different land uses, with contrasting mixes of impermeable and permeable surfaces that alter “green” and “blue” water flux partitioning. Understanding water partitioning in such heterogeneous environments is challenging but crucial for maintaining a sustainable water management during future challenges of increasing urbanization and climate warming. Stable isotopes in water have outstanding potential to trace the partitioning of rainfall along different flow paths and identify surface water sources. While isotope studies are an established method in many experimental catchments, surprisingly few studies have been conducted in urban environments. Here, we performed synoptic sampling of isotopes in precipitation, surface water and groundwater across the complex city landscape of Berlin, Germany, for a large ‐scale overview of the spatio‐temporal dynamics of urban water cycling. By integrating stable isotopes of water with other hydrogeochemical tracers we were able to identify contributions of groundwater, surface runoff during storm events and effluent discharge on streams with variable degrees of urbanization. We could also assess the influence of summer evaporation on the larger Spree and Havel rivers and local wetlands during the exceptionally warm and dry summers of 2018 and 2019. Our results demonstrate that using stable isotopes and hydrogeochemical data in urban areas has great potential to improve our understanding of water partitioning in complex, anthropogenically‐affected landscapes. This can help to address research priorities needed to tackle future challenges in cities, including the deterioration of water quality and increasing water scarcity driven by climate warming, by improving the understanding of time‐variant rainfall‐runoff behaviour of urban streams, incorporating field data into ecohydrological models, and better quantifying urban evapotranspiration and groundwater recharge.Seasonal isotope and hydrogeochemical dynamics of surface‐ and groundwater in a large urban area following the dry summer of 2018, which was characterized by a temperature anomaly and precipitation deficit.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165