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Short answer: HEC-HMS (Hydrologic Modeling System) is rainfall-runoff modelling software from the US Army Corps of Engineers Hydrologic Engineering Center. It simulates the complete hydrologic processes of dendritic watershed systems, and non-Corps users may use it at no charge, without user support from HEC.
Definition and scope
HEC-HMS is developed by the Hydrologic Engineering Center (HEC) of the US Army Corps of Engineers. HEC describes it as software that simulates the complete hydrologic processes of dendritic watershed systems: a watershed is represented as connected elements, and the program computes how precipitation becomes runoff and how that runoff moves downstream.
The software bundles an integrated database, data entry tools, a computation engine and a graphical interface for reporting results, which are stored in HEC-DSS. For the underlying science, see our explainer on what hydrology is; for the professional context, what civil engineering is.
How a model is built
A HEC-HMS model is assembled from components, each with a choice of computational method. Following the HEC feature list, a typical runoff model chains these steps:
- Precipitation: the rainfall input, either historical or synthetic.
- Loss: the part of rainfall that infiltrates or is otherwise abstracted.
- Transform: conversion of excess rainfall to direct runoff at the sub-basin outlet.
- Baseflow: the slower groundwater contribution added to direct runoff.
- Reach routing: movement of the flow through channels to the next element.
Snowmelt, evapotranspiration, impoundments, pumps, diversions and canopy or surface components can be added where the watershed needs them.
Method options by component
This table reproduces the method names listed on HEC’s HEC-HMS features page (read 2026-11-08). It shows the choices that exist; it does not say which is best for a given watershed.
| Component | Methods listed by HEC |
|---|---|
| Historical precipitation | User-specified hyetograph, gage weights, Thiessen, inverse distance, gridded precipitation |
| Synthetic precipitation | Frequency storm, standard project storm, hypothetical storm, HMR52 |
| Loss (event) | Initial and Constant, SCS Curve Number, Exponential, Green and Ampt, Smith Parlange |
| Loss (long or multi-event) | Deficit and Constant, Layered Green and Ampt, Soil Moisture Accounting; gridded versions of Deficit and Constant, Green and Ampt, SCS CN and SMA |
| Transform | Clark, Snyder, SCS, user-specified unit hydrograph, user-specified S-graph, ModClark, kinematic wave |
| Baseflow | Recession, Constant Monthly, Linear Reservoir, Nonlinear Boussinesq, Bounded Recession |
| Reach routing | Lag, Muskingum, Straddle Stagger, Lag and K, Modified Puls, Normal Depth, Kinematic Wave, Muskingum-Cunge |
| Evapotranspiration | Monthly average, Hamon, Hargreaves, Penman-Monteith, Priestley-Taylor, user-specified |
| Snowmelt | Temperature index method with cold content, applied by elevation bands or grid cells |
Two details in the same source are easy to miss. Channel losses can be a constant loss with any routing method, but percolation is available only with Modified Puls, Normal Depth and Muskingum-Cunge routing. And gridded runoff uses the linear quasi-distributed ModClark transform.
Event versus continuous simulation
HEC separates two modes. Event analysis covers short, discrete storms using infiltration, unit hydrograph and hydrologic routing methods. Continuous simulation adds evapotranspiration, snowmelt and soil moisture accounting so the model can run over long periods and multiple events. This is why the loss list above splits into event methods and long or multi-event methods.
Design-storm work relies on synthetic precipitation; see our guide to NOAA Atlas 14 for a United States source of rainfall frequency estimates.
Supporting tools
Beyond runoff, the HEC feature list includes parameter optimization (seven objective functions and two search methods), forecast zones and alternatives for streamflow forecasting, depth-area reduction, Monte Carlo uncertainty analysis, erosion and sediment (MUSLE and build-up/wash-off), nutrients, and GIS watershed delineation from a digital elevation model (HEC-HMS 4.4 and later).
Access, versions and system requirements
- Current version: HEC’s download page lists version 4.14 for Windows, macOS and Linux (read 2026-11-08).
- Tested systems: Windows 11 (64-bit), macOS Sonoma 14.4.1 (the app is not digitally signed) and Ubuntu 20.04 LTS.
- Cost and support: the software was developed for the Corps and its use is not restricted. Non-Corps users may use it at no charge, but HEC provides no user assistance or support to them.
Where it matters for research and design
Because the program exposes every method choice, a study can document its model as a set of named selections (for example, SCS Curve Number loss, Clark transform, Muskingum-Cunge routing). Recording those choices alongside the HEC-DSS output, and archiving both in a repository such as CUAHSI HydroShare, makes a hydrologic study easier to reproduce. That is an editorial recommendation, not an HEC requirement.
Common misunderstandings
- Free does not mean supported. The no-charge terms for non-Corps users come without HEC user assistance.
- A long method list is a menu, not a recommendation. The features page lists what is available; it does not rank the methods, so selecting and justifying them remains the modeller’s responsibility.
FAQ
What does HEC-HMS stand for?
Hydrologic Engineering Center’s Hydrologic Modeling System, produced by the US Army Corps of Engineers.
Is HEC-HMS free?
Per HEC’s download page, non-Corps users may use it at no charge, though HEC does not provide user assistance to them.
Which operating systems does it run on?
Version 4.14 is offered for Windows, macOS and Linux; HEC’s tested systems are Windows 11 64-bit, macOS Sonoma 14.4.1 and Ubuntu 20.04 LTS.
Can it simulate snowmelt and long periods?
Yes. HEC lists continuous simulation with evapotranspiration, snowmelt and soil moisture accounting, plus a temperature index snowmelt method.
Where are results stored?
In HEC-DSS, according to HEC’s software page.
Sources
- US Army Corps of Engineers Hydrologic Engineering Center, HEC-HMS software page, read 2026-11-08.
- HEC, HEC-HMS features, read 2026-11-08.
- HEC, HEC-HMS downloads, read 2026-11-08.
