HEC-HMS — The Complete Guide: Free Rainfall-Runoff and Design Flow Modelling
HEC-HMS (Hydrologic Modeling System) is the hydrology model from the Hydrologic Engineering Center of the US Army Corps of Engineers — the sibling of HEC-RAS. If HEC-RAS answers "where will the water go", HEC-HMS answers the question before it: "how much water will come". It is free and runs on Windows, macOS and Linux.
HEC-HMS
US Army Corps of Engineers
Rainfall-runoff hydrological model — infiltration, unit hydrograph and routing. Pairs with HEC-RAS.
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HEC-RAS
US Army Corps of Engineers
1D/2D hydraulic model for rivers, bridges and culverts, with floodplain mapping. The world standard.
Get it from the vendorWhat a model is made of
- Basin model — subbasins, routing reaches, junctions and reservoirs, connected along the flow direction.
- Losses — how much rain becomes runoff: SCS Curve Number, Green-Ampt, Initial and Constant and more.
- Transform — unit hydrographs (SCS, Clark, Snyder) or kinematic wave.
- Routing — how the flood wave travels along channels (Muskingum, Muskingum-Cunge and others) and through reservoirs.
- Meteorological model — design rainfall, a hypothetical storm or gauge records.
How to build a model
1. Delineate the basin and subbasins — with the built-in GIS tools or in QGIS. 2. Set loss parameters and time of concentration for each subbasin. 3. Define design rainfall for a return period. 4. Run and get the hydrograph and peak flow at every point. 5. Pass the hydrograph to HEC-RAS to compute water levels and flood maps.
QGIS
QGIS.org
Open-source GIS for spatial analysis, layer work and map production.
Get it from the vendorWhat's missing
HEC-HMS computes runoff, not hydraulics — water levels, velocities and flooding are computed in HEC-RAS. The parameters (Curve Number, time of concentration) strongly affect the result, so adapt them to local data and guidelines rather than copying default values. The interface takes learning, and support is mainly documentation and community.
Who it suits
Drainage engineers, hydrologists, road and culvert designers, and drainage authorities — anyone who needs a design flow for a natural catchment.
Companion tools
HEC-RAS for hydraulics and flood maps, SWMM for urban drainage, and WhiteboxTools for elevation model processing and catchment delineation.
SWMM
US Environmental Protection Agency
Urban drainage model — runoff, sewers, detention and infiltration, including runoff quality.
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WhiteboxTools
Whitebox Geospatial
Hundreds of geospatial analysis tools, especially strong on terrain and LiDAR point clouds.
Get it from the vendorBottom line
HEC-HMS and HEC-RAS are the classic pair of drainage engineering: one computes how much water, the other where it goes — and both are free.