Water displacement by sewer infrastructure in the Grote Nete catchment, Belgium, and its hydrological regime effects

Abstract. Urbanization and especially impervious areas, in combination with wastewater treatment infrastructure, can exert several pressures on the hydrological cycle. These pressures were studied for the Grote Nete catchment in Belgium (8.18% impervious area and 3.89% effective impervious area), based on a combination of empirical and model-based approaches. The effective impervious area, combined with the extent of the wastewater collection regions which do not coincide with the natural catchment boundaries, was used as an indicator for the urbanization pressure. Our study revealed changes in the total upstream areas of the subcatchments between −16% and +3%, and in upstream impervious areas between −99% and +64%. These changes lead to important inter-catchment water transfers. Based on simulations with a physically-based and spatially-distributed hydrological catchment model, profound impacts of effective impervious area on infiltration and runoff were found. The model results show that the changes in impervious areas and related water displacements in and between catchments due to the installation of the wastewater treatment infrastructure severely impacted low flows, peak flows and seasonal trends. They moreover show that it is difficult, but of utmost importance, to incorporate these pressures and artificial processes in an accurate way during the development of hydrological models for urbanized catchments.