Enhancing urban runoff modelling using water stable isotopes and ages in
complex catchments
Abstract
Increased urbanization, coupled with the projected impacts of climatic
change, mandates further evaluation of the impact of urban development
on water flow paths to guide sustainable land use planning. Though the
general urbanization impacts of increased storm runoff peaks and reduced
baseflows are well known; how the complex, non-stationary interaction of
the dominant water fluxes within dynamic urban water stores sustain
streamflow regimes over longer periods of time are less well quantified.
In particular, there is a challenge in how hydrological modelling should
integrate the juxtaposition of rapid and slower flow pathways of the
urban ‘karst’ landscape and different approaches need evaluation. In
this context, we utilized hydrological and water stable isotope datasets
within a modelling framework that combined the commonly used HEC urban
runoff model along with a simple hydrological tracer module and transit
time modelling to evaluate the spatial and temporal variation of water
flow paths and ages within a heavily urbanized 217km 2
catchment in Berlin, Germany. Deeper groundwater was the primary flow
component within less urbanized regions of the catchments, with
increased direct runoff and shallow subsurface contributions in more
urbanized areas near the catchment outlet. The addition of wastewater
effluent in the mid-reaches of the catchment was the dominant water
supply to the lower stream, and sustained baseflows during the summer
months. Water ages from each modelling approach imitated flow
contributions and opportunity for mixing with subsurface storage; with
older water and lower young water contributions in less urbanized
sub-catchments and younger water and higher young water contributions in
more urbanized regions. The results form a first step towards more
integrated modelling tools for similar peri-urban catchments, given the
potential limitations of more simple model frameworks. The results have
broader implications for assessing the uncertainty in evaluating urban
impacts on hydrological function under environmental change.