Abstract
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 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
here is dominated by groundwater from gravel aquifers underlying
surrounding agricultural land. In dry summer periods, streamflow becomes
intermittent; possibly as a result of local groundwater abstractions.
Urban storm drainage is also an important part of runoff generation,
dominating the responses to precipitation events. Although this
dramatically changes the isotopic composition of the stream, it only
accounts for 10-15% of annual streamflow. Moving downstream, subtle
changes in sources and isotope signatures occur as catchment
characteristic vary and the stream is affected by different tributary
inflows. However, effluent from a wastewater treatment plant (WWTP)
serving 700,000 people dominates the stream in the lower catchment where
urbanisation 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. As a result, only about
10% of annual runoff in the lower catchment comes from urban 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 the
ecological and amenity value of these important resources.