The Hydrologic Modeling System (HEC-HMS) is designed to simulate the precipitation-runoff processes of dendritic watershed systems.  It is designed to be applicable in a wide range of geographic areas for solving the widest possible range of problems.  This includes large river basin water supply and flood hydrology, and small urban or natural watershed runoff. Hydrographs produced by the program are used directly or in conjunction with other software for studies of water availability, urban drainage, flow forecasting, future urbanization impact, reservoir spillway design, flood damage reduction, floodplain regulation, and systems operation.

The program features a completely integrated work environment including a database, data entry utilities, computation engine, and results reporting tools.  A graphical user interface allows the user seamless movement between the different parts of the program.  Program functionality and appearance are the same across all supported platforms.

This model is public domain and can be downloaded for FREE using this link.

See also the Watershed Modeling System (WMS) software
WMS provides a high qaulity graphical user interface for HEC-HMS.
WMS performs automated basin delineation from digital elevation data and determines basin parameters needed for HEC-HMS. 
WMS automatically generates HEC-HMS basin files, or your can run the full featured HEC-1 (the predecessor to HEC-HMS) fully within WMS.  

Retail Price: FREE