To calculate urban stormwater and combined sewer overflow pollution and means for its control, a comprehensive mathematical model is presented. The model (Quantity-Quality Simulation) operates in a continuous mode and accounts for the unsteady runoff and overflow behavior of total drainage systems. Lumping techniques that calculate the runoff from drainage areas are combined with detailed flow routing through main and interceptor sewers as well as other structures such as branches, overflows, basins, pump stations, control gates and treatment facilities. The computer program calculates the runoff in the storm or combined sewer system and in the receiving waters.