Complex networks of both natural and engineered flow paths control the hydrology of streams in major cities through spatio-temporal variations in connection and disconnection of diverse water sources. We used spatially extensive and temporally intensive sampling of water stable isotopes to disentangle the hydrological sources of the heavily urbanized Panke catchment (~220 km2) in the north of Berlin, Germany. The isotopic data enabled us to partition stream water sources across the catchment using a Bayesian mixing analysis. The upper part of the catchment streamflow is dominated by groundwater (~75%) from gravel aquifers. In dry summer periods, streamflow becomes intermittent in the upper catchment, possibly as a result of local groundwater abstractions. Storm drainage dominates the responses to precipitation events. Although such events can dramatically change the isotopic composition of the upper stream network, storm drainage only accounts for 10%–15% of annual streamflow. Moving downstream, subtle changes in sources and isotope signatures occur as catchment characteristics vary and the stream is affected by different tributaries. However, effluents from a wastewater treatment plant (WWTP), serving 700,000 people, dominate stream flow in the lower catchment (~90% of annual runoff) where urbanization effects are more dramatic. The associated increase in sealed surfaces downstream also reduces the relative contribution of groundwater to streamflow. The volume and isotopic composition of storm runoff is again dominated by urban drainage, though in the lower catchment, still only about 10% of annual runoff comes from storm drains. The study shows the potential of stable water isotopes as inexpensive tracers in urban catchments that can provide a more integrated understanding of the complex hydrology of major cities. This offers an important evidence base for guiding the plans to develop and re-develop urban catchments to protect, restore, and enhance their ecological and amenity value.
Bibliographical noteFunding Information:
We thank D. Dubbert and A. Dahlmann for running the isotopic analysis in the isotope laboratory of IGB. A. Smith is thanked for advice on aspects of the analysis and comments on an earlier draft. Additionally, we thank the Berliner Wasserbetriebe (BWB), and Berlin Senate for constructive information, expert knowledge, feedback and provision of data, as well as access to their groundwater wells. L. Kuhlemann and L. Kleine are thanked for constructive feedback and discussion. Funding from this study was through the project “Modelling surface and groundwater with isotopes in urban catchments” (MOSAIC) provided by the Einstein Foundation and CM is associated with the Research Training Group “Urban Water Interfaces” (UWI), GRK 2032/2 as a collegiate, financed by the German Research Foundation (DFG). Contributions from CS were also funded by the Leverhulme Trust's ISOLAND project. We also thank three anonymous reviewers for their constructive reviews.
Data Availability StatementStream data and groundwater level based in rating curves, presented in Figure 3b–e, are only available upon request from the Berlin Senate, the same applies for the wastewater effluent data which is available from the Berliner Wasserbetriebe (BWB). Precipitation data are available publicly from the Deutsche Wetterdienst (DWD), station Buch. Precipitation and stream isotopes are available upon reasonable request. Discharge and groundwater level data are available upon request from the Berlin Senate/BWB.
- end member mixing analysis
- urban hydrology