The mathematical model described is adequately detailed for use in at least preliminary design. Developed by The Metropolitan Sanitary District of Greater Chicago, the computer program was used in the analysis of streamflows and sewer systems and for sizing several reservoirs. Using the model, adequate approximations of observed hydrographs for several catchments in the Chicago region were obtained without modification of the programming or the constants involved. The model appears to have a good potential for satisfactory application to watersheds in other parts of the country with possible alteration of the constants being the only change necessary. The computer program in the form of a FORTRAN IV card deck is available.