"The Hydrological Simulation Program -- FORTRAN is a mathematical model that simulates hydrology and water quality in natural and man-made water systems. This report describes the work involved in maintaining and testing HSPF over a one-year period following its initial development. An account is given of the chronology of major events during the maintenance work. The testing included work with hypothetical data and checks against outputs produced by three predecessor models, the ARM, NPS, and HSP-QUALITY models. Through this process it was determined that the HSPF model functioned as designed."--Abstract.