Development of the Hydrologic Modeling System (HEC-HMS) was initiated as part of the Next Generation Software Project to succeed the aging HEC-1 program for simulating the rainfall-runoff process. However, it was not designed to simply add a graphical user interface to the old program. Instead it was designed to use advances in engineering and computer science wherever possible to improve the quality of simulation results. The modernization process has therefore resulted in some changes in how computations are performed. While these modernizations result in computation differences between the two programs, the HEC-HMS results are preferred because of the modern techniques that have been implemented. Changes in parameter specifications or computations are detailed in the following sections.