This Is AuburnElectronic Theses and Dissertations

Evaluating alternatives for the implementation of Curve Number in SWMM for an urbanized watershed




Xiao, Han

Type of Degree

Master's Thesis


Civil and Environmental Engineering


Modeling hydrological abstraction, particularly infiltration, in urbanized areas is a complex task. Without field validation data, researchers and engineers often rely on readily available methodologies on semi-empirical formulations such as the Curve Number (CN) method. Curve Number is a factor that depends on land cover, antecedent moisture conditions, and hydrologic soil groups to determine the maximum soil moisture storage capacities derived by Soil Conservation Service (SCS). Many hydrological models have implemented the CN method in their formulation, including Storm Water Management Model (SWMM). The CN method in SWMM is used to determine only the cumulative infiltration changes with cumulative rainfall during the rainfall events instead of direct runoff in the original SCS CN method. The CN method is highly suggested to apply only to the pervious area of the subcatchments with assigning the corresponding percent of impervious area of the subcatchments in SWMM. However, it is relatively complex and sophisticated to specify the percentage of impervious area by subjective assumptions and average the CN only for the rest of pervious area. This research tested two alternative ways of setting up the CN method in SWMM based on the QGIS plugin “CurveNumberGenerator” to find the shortcut. The model results were compared with field data collection performed at the headwater reaches of Moore’s Mill Creek in Opelika and Auburn, AL. Research results indicated that the Fully Composite CN approach yielded better for more impervious watersheds. Regarding the CN Cut-off approach, a cut-off value of 90 presented the optimum results for most cases of the mixed urban areas and undeveloped areas with hydrologic soil group A/B.