1.   Purpose and objectives

     The program computes individual-event or continuous hydrographs for use in planning and design of flood control and water conservation projects.

2.   Description

     HEC-HMS is the U.S. Army Corps of Engineers’ new generation software for precipitation-runoff simulation and supersedes the HEC-1 Flood Hydrograph Package. HEC-HMS is a significant advancement over HEC-1 in terms of both computer science and hydrologic engineering. Streamflow from basins of any size is computed using the four processes of watershed runoff, river routing, combining at confluences, and diversions. Runoff hydrographs are computed from rainfall, loss rates (infiltration), unit hydrographs or kinematic wave, and baseflow data.

     Infiltration is computed using: initial and uniform, SCS curve number, Green & Ampt, or soil-moisture-accounting loss functions. Unit hydrographs may be input directly as ordinates or are computed from Clark, Snyder, or SCS synthetic methods. Baseflow decreases logarithmically from a given value on the recession limb of an event hydrograph or is computed from soil moisture accounting. Basin-averaged rainfall may be input directly or computed from point rainfall data using gage weightings, standard project or probable maximum storm criteria, or from gridded radar rainfall. Stream
routing methods include Muskingum, Muskingum-Cunge, modified Puls, or kinematic wave. Modified Puls storage routing is used for reservoirs. The model can operate on a subbasin-lumped basis or on a spatially distributed, -1>gridded basis. A digital elevation model and geographic information system preprocessor, HEC-GeoHMS, can be used to delineate subbasins, calculate watershed and river characteristics, and formulate the HEC-HMS basin schematic. It also produces the gridded subbasin definition. HEC-GeoHMS is an ArcView application package and will be available in 2000.

     Internal computations are in metric system; input and output may be in either metric or English system.

3.   Input

     A Graphic User Interface prompts the user for information needed. Required data are rainfall, infiltration, unit hydrograph or kinematic wave excess transform, baseflow, channel and reservoir routing parameters, and stream connectivity for hydrograph calculations.

4.   Output

     Hydrographs are available in graphical, tabular, and/or electronic file format for every computation node in a river basin. A tabular summary is available for the entire simulation. All hydrographs are written to the HEC Data Storage System, HEC-DSS, for further use in this or other models.

5.   Operational requirements and restrictions

     a.  Persons using the programs need to have knowledge of hydrological 
         processes and river basin hydraulics and a basic understanding of
         computers.
     b.  The program was developed in C, C++, and Fortran for use on PC 
         Windows 95 or better and Sun Solaris systems.
     c.  River basins of any size and hydrographs of any time duration may
         be simulated.
     d.  The following assumptions have been made in the methods used in
         the program: (1) a subbasin, or grid cell, is homogeneous and
         rainfall and infiltration occurs uniformly over that area;
         (2) downstream backwater effects are not significant in hydrograph
         routing; (3) precipitation is for an event and/or long continuous
         records; and (4) a constant time interval is used for all
         hydrograph calculations but the program may be stopped and
         restarted at a different time interval with automatic interpolation.

6.   Form of presentation

     The software is available on CD or may be downloaded from www.hec.usace.army.mil. Support documentation consists of Release Notes, User’s Manual, Technical Reference Manual, Short-Course Lecture and Workshop  Documents, and technical papers. Instructional videotapes are also available 
for the hydrologic methods.

7.   Operational experience

     Extensive; worldwide, with HEC-1 predecessor to this new software. ISO 9000 validation documentation is under preparation for HEC-HMS.

8.   Originator and technical support

     The Hydrologic Engineering Center, HEC, U.S. Army Corps of Engineers, developed the software. Technical support is provided by numerous engineering institutions worldwide.

9.   Availability

     Direct download via the Internet at www.hec.usace.army.mil or from the HOMS National Reference Centre for USA.

10.  Conditions on use

     The Internet download is free; a standard charge is made for supplying the CD.