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The HSPF (Hydrologic Simulation Program Fortran) model simulates the water quality impact of storm triggered point and non-point source loading. Although HSPF simulates both impervious and pervious hydrologic components of watershed runoff, the model cannot represent transport mechanisms common to suburban watersheds, including storm sewer runoff, spatially variable watertable triggered runoff, and runoff trapping in vegetative filters and detention basins. These limitations were overcome by loosely coupling HSPF with three other runoff models, Storm Water Management Model (SWMM), Hydrologic -Simulation Program - Fortran (HSPF), Program for Predicting Polluted Particle Passage through Pits, Puddles and Ponds (P8), and Topographic -based Land Atmosphere Transfer Scheme (TOPLATS). Combined, these four models simulate the following transport pathways of concern: 1) impervious processes of catch basin and storm sewer hydraulics, 2) pervious processes of infiltration, surface runoff, sub-surface storm flow, watertable dynamics, and 3) reach and reservoir processes of runoff routing and storm runoff detention. Each of these pathways may influence pollutant fate, and the total load leaving the watershed.