Characterizing and Understanding the Effects of Spatial Resolution in Urban Hydrologic Simulations

Model subdivision is used to capture spatial heterogeneity in input parameters and it is well-established that spatial resolution affects model output. Although previous research has observed scale effects in urban hydrology, there is no general consensus about it or the underlying mechanism(s). The objective of this study was to investigate the effects of spatial resolution on model predictions in an urban catchment, and to understand the mechanism(s) responsible for the scale effect. The study area is the Faneuil Brook sub-basin of the lower Charles River watershed in Boston. The general approach used in this study is to develop models at various spatial resolutions (from 4 to 616 subcatchments), perform simulations and compare the predictions of total outflow volume and peak flow. Models were developed based on actual drainage networks, and artificial ones generated based on a fractal algorithm using the program Artificial Network Generator (ANGel), which was written as part of this research. Simulations were performed using the EPA Storm Water Management Model (SWMM) and model output was compared for 90 different storms.;There was very little difference in the total annual outflow volumes predicted by the different resolutions. However, peak flows showed a dual scale effect based on storm characteristics differentiated by total rainfall depth. For the larger storms, model aggregation significantly reduced peak flows, which can be explained by differences in infiltration and runoff. This effect was attributed mainly to the spatial distribution of the soil saturated hydraulic conductivity and the length of overland flow. For the smaller storms, aggregation significantly increased peak flows, which can be explained by the combined effects of overland flow and conduit routing. The results were consistent using actual and artificial networks, and also for storms in other water years. This study illustrates that a scale effect can be introduced by different processes, which can go in different directions (i.e. increase or decrease peak flows) and depend on the storm characteristics. This study also illustrates that the peak flows and the effects of spatial resolution are comparable using actual and artificial networks, which implied that artificial sewer networks can be a useful tool for spatial scaling analysis.