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Urban-runoff data were collected from 1975 to 1977 on three drainage basins in the Denver metropolitan area. Constituent concentrations in the urban runoff varied considerably. However, constituent concentrations in rainfall runoff generally were greatest during the initial parts of the runoff periods and then decreased with time. In snowmelt runoff, constituent concentrations generally peaked during the middle of the day corresponding with periods of maximum melting and runoff. Instantaneous loads of constituents were largely a function of discharge for both rainfall and snowmelt runoff. A first flush, defined as a disproportionately large part of the total storm-runoff load in the initial part of the storm runoff, was rare. Antecedent precipitation or the number of days since the last street sweeping had no apparent effect on rainfall-runoff quality. However, snowmelt-runoff loads apparently increased with the number of days snow had been on the ground. In addition, several instances were documented in which landuse activities, such as building construction or application of fertilizers and pesticides, had a discernible effect on the quality of storm runoff. Urban storm runoff may be a significant contributor of total ammonia nitrogen, total nonfiltrable residue, total copper, total iron, total lead, and total zinc to local receiving waters. During the winter months, snowmelt runoff may be a significant contributor of sodium and chloride to local receiving waters. Data from two of the basins were used for calibration and verification of the U.S. Environmental Protection Agency's Storm Water Management Model II for rainfall-runoff modeling of flow and total nitrogen. Using values for the accumulation and washoff parameters as suggested in the user's manual for the model and basin-characteristic parameters as determined from the model calibration for flow, the errors in simulated rainfall-runoff loads of total nitrogen ranged from +67 to +1,000 percent. The model assumption that landsurface loads of total nitrogen are directly proportional to the number of days prior to the storm during which the accumulated rainfall was less than 1.0 inch was not substantiated. Based on the results of this study, caution should be used when applying the Storm Water Management Model II or other similar models to assist in making water-quality-management decisions, particularly in the absence of local data for model calibration and verification.

Quantity and quality of urban runoff from three localities in the Denver Metropolitan area, Colorado