Impact of drought hazards on flow regimes in anthropogenically impacted streams: an isotopic perspective on climate stress

Abstract. Flow regimes are increasingly impacted by more extreme natural hazards of droughts and floods as a result of climate change, compounded by anthropogenic influences in both urban and intensively managed rural catchments. However, the characteristics of sustainable flow regimes that are needed to maintain or restore hydrologic, biogeochemical and ecological function under rapid global change remain unclear and contested. We conducted an inter-comparison of two streams in the Berlin-Brandenburg region of NE Germany, which are both mesoscale sub-catchments of the River Spree; an intermittent rural agricultural stream (the Demnitzer Millcreek) and a heavily anthropogenically impacted urban stream (the Panke). Through tracer-based analyses using stable water isotopes, we identified the dominant physical processes (runoff sources, flowpaths and age characteristics) sustaining streamflow over multiple years (2018–2023), including three major drought years (2018–20, 2021–22). In the urban stream, low flows are regulated through artificially increased baseflow from treated waste water effluent (by up to 80 %), whilst storm drainage drives rapid, transient high flow and runoff responses (up to 80 %) to intense convective summer rainfall. The intermittent groundwater-dominated rural stream experienced extended no-flow periods during drought years (⁓ 60 % of the year), and only moderate storm runoff coefficients (<10 %) in winter along near-surface flows paths after heavy rainfall. In both streams, groundwater dominance with young water influence prevails, with low water ages in the urban stream (<10 %) despite significant urban runoff, and higher ones in the rural stream (⁓15 %). Urban cover resulted in mean transit time of ⁓4 years compared to arable land with ⁓3 years, highlighting the interlinkages of landuse and catchment properties on catchment transit times. Understanding seasonal and interannual variability in streamflow generation through a tracer-based hydrological template, has potential for assessing the impacts of natural hazards on the sustainability of future baseflow management, including wider water quality and ecological implications across anthropogenically impacted environments.