Alternative approaches to setup CN and imperviousness in SWMM models

Hydrological models, such as SWMM 5 and the WinTR-55, and widely applied to represent urban watersheds. In this context, the computation of abstractions is a fundamental component. Among the different approaches to computing infiltration, the USDA-NRCS Curve Number (CN) method is one of the widely used approaches implemented in various hydrological tools. SWMM 5 hydrological guidelines indicate that the CN should be computed only for the pervious fraction of subcatchments, whereas the CN values for the impervious fraction should not be considered. Yet, the alternative of considering CN values for the entire subcatchment is possible in SWMM and adopted in other hydrological tools such as WinTR-55. To our knowledge, a comparison between these two approaches to assign CN values has not been performed. The present work evaluates the differences between these approaches in CN computation within SWMM through a comparison with field data collected in an urban watershed in Alabama. The work also compares SWMM 5 and WinTR-55 predictions for the same watershed. Four approaches to computing CN were considered in which the impervious fractions varied according to a threshold CN value. Results indicated that a Fully Composite approach, which computed CN from all subcatchment areas, yielded the best results for the sub-watershed with higher average CN. It was also observed that results from the approaches using CN Cut-off values of 90 and 93 were better for subcatchments with lower average CN. The comparison between SWMM 5 and WinTR-55 indicated that SWMM 5 hydrographs had larger peak flow rates, but these differences decreased with larger intensity rain events. Other related studies are needed to assess the performance of SWMM when CN values are averaged over entire subcatchment areas.

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