American Society of Civil Engineers

Effects of Spatial Resolution in Urban Hydrologic Simulations

by Indrani Ghosh, (Kleinfelder/SEA Consultants, 215 First St., Suite 320, Cambridge, MA 02142; formerly, Ph.D. Student, Civil & Environmental Engineering Dept., 400 Snell Engineering Center, Northeastern Univ., Boston, MA 02115. E-mail: and Ferdi L. Hellweger, (corresponding author), (Assistant Professor, Civil & Environmental Engineering Dept., 400 Snell Engineering Center, Northeastern Univ., Boston, MA 02115. E-mail:

Journal of Hydrologic Engineering, Vol. 17, No. 1, January 2012, pp. 129-137, (doi:

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Document type: Journal Paper
Abstract: Model subdivision is used to capture spatial heterogeneity in input parameters and it is well-established that spatial resolution (i.e., degree of aggregation) affects model output. However, a general consensus about the effect does not exist. 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 general approach is to develop models at various spatial resolutions, perform simulations, and compare the predictions of total outflow volume and peak flow. Models were developed on the basis of actual drainage networks, and artificial ones generated on the basis of a fractal algorithm by using the Artificial Network Generator (ANGel). Simulations were performed by using the EPA Storm Water Management Model (SWMM), and model output was compared for 50 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. For the larger storms, model aggregation reduced peak flows, which can be explained by differences in infiltration. This effect was attributed primarily to the spatial distribution of the soil-saturated hydraulic conductivity and the length of overland flow. For the smaller storms, aggregation 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. 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.

ASCE Subject Headings:
Hydrologic models
Urban areas
Scale effects

Author Keywords:
Hydrologic model
Urban area
Artificial sewer networks
Scale effect
Distributed parameter