An innovative technique has been applied to develop a physically distributed and calibrated hydrologic model using EPA HSPF software. HSPF is basically a lumped hydrologic model. The present HSPF model is discretized into subwatershed, catchment and subcatchment. It uses land use/cover, Hydrologic Soil Group (HSG) and small size climate zone to represent physical and spatial distribution of hydrologic responses. Each of the subcatchment is further discretized into 46 Unit Response Functions (URFs), which are developed from combinations of land use/cover and HSG. The URFs represent both rural and urban land use/cover in Ontario. The rural URFs represent agricultural crop/pasture, forest, hummocky, open place, water body and wetland land use/cover. The urban URFs include Low/Medium/High Density Residential, Industrial, Commercial, Education/other Institution, Park/Open Place, Highways and Roads land uses. The model can simulate several types of urban runoff such as sloped roof, flat roof, driveway, parking and storm sewer. To capture spatial distribution of climate data, the subwatersheds are divided into 64 climate zones. It uses over 50 climate stations’ data to estimate climate data at centroid of each climate zone.
The PEST auto-calibration (Doherty, 2008) technique is applied to calibrate HSPF parameters. A total of 375 parameters were calibrated using PEST and simulated streamflows of 17 WSC gauge stations. The calibration shows excellent results where simulated streamflow matching very well with observed streamflows. A number of FORTRAN codes are created to do pre and post model processing.
As each of the URFs is effectively a “sub-model”, its HSPF parameters are calibrated, and the URFs can be of any size/area, the model is applied to estimate pre and post development flows for 2 to 100 year storm including the Regional Storm flows. Similar to the HSPF BMP-toolkit (EPA) in US Unit, the model has HSPF-SWM module in Metric Unit, which calculates stage-area-storage-outflow relationships and simulates water storage in channel, pond, reservoir, wetland and infiltration basin for Low Impact Development (LID). The calibrated HSPF model and HSPF-SWM are applied for a subdivision SWM study. It compares pre and post development flows with those estimated by uncalibrated VO2 software. The tool is applied for both the conventional Stormwater Management and Low Impact Development (LID) practices