Comparing urban sub-catchment delineation approaches for dynamic hydrological modelling

In the face of rapid urban and population growth and with climate change altering precipitation patterns, urban water management is becoming increasingly demanding. Numerous software, tools and approaches to study urban water flow behaviour and model hydrological processes exist. However, the understanding of water movement in urban areas, especially during extreme events, and the physical principles behind them, as well as the interaction between the natural and the urban hydrological cycle is still incomplete. For decades, models suited for urban hydrological analysis greatly impacted the improvement of flood protection, public health and environmental protection, changing the way we look at urban water and stormwater management. In order to calculate accurate quantities of runoff in any rainfall/runoff model, information about urban sub-catchment characteristics plays an important role. Size, shape, topography, as well as land use influencing infiltration rates and evapotranspiration, are of great importance to calculate accurate runoff quantities on the urban scale. New implementations to reduce runoff towards the sewer system, such as decentralised stormwater techniques, increase the urgent need for accurate and high-resolution local/neighbourhood-scale information. Spatial and temporal developments require water management models to be connected with GIS (Geographical Information Systems). Initially not being developed to interact with each other, multiple approaches exist to combine GIS with water management models. Nevertheless, defining urban sub-catchments for rainfall-runoff modelling is often still performed manually using specific maps or using simple surface partitioning algorithms such as the Thiessen polygons. A significant disadvantage in generating urban sub-catchments manually is the fact that natural surface inclination is usually not considered, influencing the size and shape of the delineated sub-catchments. So far, only a few studies have devoted attention to improving the way urban sub-catchments are delineated and the information about their surface characteristics is generated. This study evaluates a GIS-based approach to automatically delineate urban sub-catchments accounting for the location of nodes (actual manholes or drain inlets) as sub-catchment outlets. In order to compare the influence of the sub-catchment delineation methods (1 to 3), we use (1) a digital surface model (DSM) and (2) a digital elevation model (DEM) to automatically delineate the urban sub-catchments and compare these two methods with each other as well as with (3) already manually derived sub-catchments of a specific case study. Furthermore, we compare hydraulic simulation results from the software SWMM with measured flow data to infer the most accurate sub-catchment delineation method.