Enhancing urban runoff modelling using water stable isotopes and ages in complex catchments

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 is 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 Hydrologic Engineering Center Hydrological Modelling System (HEC-HMS) 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 217 km(2) catchment in Berlin, Germany. Deeper groundwater was the primary flow component in the upper reaches of the catchment within fewer urbanized regions, while the addition of wastewater effluent in the mid-reaches of the catchment was the dominant water supply to sustain baseflow in the lower main stem stream, with additional direct storm runoff and shallow subsurface contributions in the more urbanized lower reaches. Water ages from each modelling approach mirrored flow contributions and water age mixing potential in subsurface storage; with older average water and lower young water contributions in less urbanized sub-catchments and younger average water and higher young water contributions in more urbanized regions. The results from the first step towards more integrated tracer-aided hydrologic modelling tools for similar peri-urban catchments, given the potential limitations of simpler model frameworks. The results have broader implications for assessing the uncertainty in evaluating urban impacts on hydrological function under environmental change.

Automated summary

Increased urbanization and climate change require further evaluation of the impact of urban development on water flow paths for sustainable land-use planning. The complex interaction of water fluxes within urban water stores and their effects on streamflow regimes over time need to be quantified. This study utilized hydrological and water stable isotope datasets to evaluate water flow paths and ages in a heavily urbanized catchment in Berlin. The results provide valuable information for assessing the uncertainty of urban impacts on hydrological function under environmental change.