A quantification of transit times and subsurface flow paths of peri-urban baseflow – AQUASUBURB

Increasing urbanisation has led to extensive soil sealing, reducing infiltration and evapotranspiration while increasing stormwater runoff, which leads to more intense high flows in peri-urban streams. However, the impact of urbanisation on baseflow – the portion of streamflow coming from delayed sources, e.g. groundwater – remains complex due to uncertain subsurface flow paths. This project addresses two research questions: 1) How does urbanisation impact the age of baseflow (defined as the time needed for rainfall to exit a catchment as baseflow)? 2) Can incorporating timeseries of the stable isotopes of water (18O, 2H) in a process-oriented hydrological model improve its accuracy? We hypothesize that urbanisation accelerates underground water transport, and that isotopic data integration will enhance the model representation of baseflow and groundwater components. A first task will develop a methodology to sample baseflow specifically in urban streams. Precipitation and baseflow will then be sampled weekly across peri-urban catchments with varying levels of urbanisation, to measure water isotopes, in order to fit transit time distributions to collected data. A second task will focus on modeling baseflow with a process-oriented model (J2000). Data from Task 1 will be used to test whether the integration of isotope data in the model enhances model calibration and whether adding a conceptual “urban karst” (the network of underground pipes and trenches) module improves subsurface flow paths representation. Expected outcomes are 1/ experimental estimates of the impact of urbanization on baseflow transit times, 2/ a better understanding of subsurface flow in urban environments and 3/ enhanced hydrological modeling. These findings will strengthen our understanding of the resilience of urban streams to urbanisation and climate change, offering guidance for water management strategies in peri-urban settings.