Characterizing changing stream flow components and hydroclimate interactions in cities – implications for future management and restoration of urban ecosystems

In urbanized areas the natural flow paradigm from river management is being increasingly challenged by hydroclimatic changes and marked anthropogenic influences on flow regulation. Although the impacts of urbanization, increased runoff and reduced baseflow are increasingly well quantified; the characteristics of flow regimes that are sustainable and wanted for urban streams to preserve or restore their hydrologic and ecological integrity in the face of future climate change and rapid urbanization, remain less well understood. To that end, we conducted a paired catchment study of two streams, both sub-catchments of the Spree catchment – a more natural intermittent rural agricultural stream in Brandenburg (Demnitzer Mill Creek) and an anthropogenically impacted urban stream in Berlin (Panke).  We characterized contrasts in inter- and intra-annual streamflow variability, storm period responses, water ages and mixing processes. Through tracer-based analyses, using stable water isotopes, we identified the physical processes (sources, flow paths and age) sustaining stream flow over multiple years (2018-2023), and broadly linking them to biological dynamics obtained through environmental DNA, in order to estimate resilience to future hydroclimate interactions and land use changes. The higher specific discharge of the urban stream emerges as a clear artefact of artificially increased baseflow due to discharge of wastewater, while reacting primarily to convective summer storms with strong runoff reactions and short discharge peaks. In contrast, the rural stream shows a characteristically intermittent behavior with longer periods without baseflow and only limited runoff reactions with only temporary superficial accumulation of water after heavy rainfall. Water ages reflected the respective runoff contributions and mixing processes, with a low contribution of young water observed in the urban stream and a higher, more variable contribution of young water in the rural stream. The strong dichotomy of runoff responses and unmistakable influence of baseflow manipulation on streamflow dynamics and biological processes point to major uncertainties in the suitability of different approaches for the restoration of urban and management of naturally intermittent rivers. Effective stakeholder engagement will be necessary in seeking to manage flow regimes to maintain ecohydrological connectivity and future resilience in the face of urban growth and climate change.