Variable source area modelling in urbanizing watersheds

A variable source area model called TOPMODEL was modified and tested for its applicability on urbanizing watersheds. The model was modified to consider urban land uses by altering the topographic index and the mechanism of surface runoff generation. A Geographic Information System was utilized to delineate the catchment, determine the urban areas and produce Digital Elevation Models. The model was applied to a small catchment of approximately 8 km2 in southern Ontario that is characterized by mild slopes, well-draining soils and a semi-humid climate. Testing was conducted using a partially revised model with four calibration parameters (TOPURBAN v. 1) applied to three separate time periods; and a fully revised model with five calibration parameters (TOPURBAN v. 2) applied to six time periods. The model performed well with calibrated model efficiencies of greater than 70%. Verification efficiencies ranged from 20 to 70%. TOPURBAN v. 2 increased Nash and Sutcliffe efficiency in comparison to v. 1 by anywhere from 2 to 8% as it accounted for storage in the urban areas. This model is recommended for urbanizing catchments of small to moderate size. Some calibrated parameter interaction was observed between parameters dealing with the urban areas and parameters dealing with the natural areas.