HEC-ResSim (Reservoir System Simulation) version 4.0 (Sep 2025) is available for download for general use. This version supersedes version 3.5.1 (Dec 2024). 

Note to CWMS Modelers

Corps Water Management System users will use version 3.5.1 until the release of RTS 3.6, which will be compatible with ResSim 4.0.

About the Release of ResSim 4.0

HEC-ResSim 4.0 is a major step forward in how we build and deliver ResSim. It's the first version to be built using HEC's DevOps platforms and practices. This branch of ResSim has been in development since 2017, in parallel with the code that resulted in ResSim 3.5, allowing parts to diverge significantly from earlier versions and warranting the new major version.

What changed in 4.0

  • New capabilities: 4.0 introduces and validates many new features. Some are early iterations—foundational pieces that will continue to evolve in upcoming releases.
  • Modernized delivery: Under‑the‑hood changes to how we build, test, and release ResSim improve reliability and set the stage for faster improvements.
  • Major-version reset: Because parts of the codebase have substantially changed since 3.5, 4.0 represents a clean line for future development.

What to expect next

  • Faster, more frequent releases: We’re moving to a quicker cadence. You’ll see incremental improvements shipped regularly instead of waiting for complete, large feature sets. See the ResSim 3.5 Release Notes for more about our transition to faster releases: HEC-ResSim 3.5 Release Notes
  • Faster, more frequent updates to the user documentation. Our web-based user documentation is updated continuously. All web-based HEC documentation now has a feedback feature; please provide feedback if you find opportunities for user documentation enhancement. 
  • Iterative feature development: New features may appear in stages. We’ll expand and refine them over time based on real‑world use and feedback.
  • Focus on stability and maintainability: In the near term, expect fewer big new features as we prioritize addressing technical debt to make the program easier to maintain and build upon.

What this means for you

  • More responsive software and user documentation products: The DevOps foundation enables smoother updates and quicker fixes.
  • Evolving functionality: Some 4.0 features—and upcoming ones—are the first step toward a larger vision. Details and workflows may improve iteratively.
  • Clearer upgrade signal: The jump to 4.0 reflects meaningful differences from 3.5. Please review accompanying documentation and release notes when planning upgrades.

Thank you for your continued feedback and support as we modernize HEC-ResSim and accelerate delivery. Your experience and input directly shape our priorities for the next iterations.

Major New Feature - Water Quality

HEC-ResSim 4.0 comes with a new water quality feature which can be turned on when building a new watershed. 

See the HEC-ResSim Water Quality User's Manual for more information.

Other New Features and Updates

Several new features, as well as enhancements to existing features, have been added to the program since the release of version 3.5. The new and updated features are listed below, along with links to pages in the online User’s Manual where you can find more detailed information.  

Underlying file structure changes

Some structural changes have been made to how model information is filed.  The most significant change is that ResSim networks and associated files are now saved in their own "networks" folder within the main rss/ folder and within the rss/ subfolder under each simulation. Each network within the network folder has its own folder and contains the network dss file, the rsys file, etc.

The network's state variable and scripted rule scripts are now stored within a /scripts/ folder under the network folder.  This makes them easier to review and change external to the program or network rsys file. See ResSim Model File Structure and Scripts Saved External to Network File starting with v4.0.


New Documentation Option – Watershed Info

It is essential for modelers to maintain quality documentation on their modeling decisions, data, assumptions, and changes. This becomes even more important when a model is used by multiple people or has a life beyond the original modeler. To assist ResSim modelers with a consistent location to store summary watershed information, a new Watershed Info... feature has been added to the File Menu. This feature builds a text file in the main watershed directory called WatershedInfo.md. ResSim will automatically log some actions to the file, and there are plans to expand that functionality, but the primary use at this time is for the modeler to log comments each time they update their model. Logging comments via the WatershedInfo dialog will attach the watershed name, date and time, user name, ResSim version, and type of action (e.g., "Created", "Comment"). Modelers are strongly encouraged to use this feature to document their models and changes, in addition to putting info in the description fields throughout the interface.  

Default DSS Version 7 Filetype

HEC-ResSim 4.0 now reads and writes DSS version 7 (DSS7) files by default, regardless of the file version of the DSS input.  While ResSim 4.0 can still read DSS version 6 (DSS6) and can be made to write DSS6 (with a modification to the configuration file), we do not recommend this. DSS6 is being phased out and is no longer supported.  A utility is available for assistance with converting files from DSS6 to DSS7.

Water Supply Enhancements

Water supply features have been expanded in ResSim 4.0. The yield analysis calculations have been improved and made more resilient. Two new features have been added:

Reverse Yield Analysis 

The Reverse Reservoir Yield Analysis and Reverse Water Account Yield Analysis allow the modeler to solve for a volume of storage needed to fulfill a give demand. This is the opposite of the yield analysis feature, which solves for the demand that can be reliably met, given a volume of storage.

Reservoir Storage vs. Yield Analysis

The Reservoir Storage vs. Yield Analysis feature helps the modeled develop a trade-off curve between the reservoir storage and firm yield. 

Initial Release of Smart Diversions Feature

ResSim 4.0 introduces the new Smart Diversion feature. Smart Diversions allow modelers to create different operation sets for different alternatives, and create a rule stack and conditional terms, similar to reservoir operation sets. Smart Diversions are in an early phase of development and are envisioned to have further functionality (especially related to water supply) in the future. See Diversion Editor: Operations Tab for more information.

Variable Routing Settings Based on Timestep

Routing calculations for reaches and diversions may require different settings, depending on the compute time step being used. In the past, modelers needed to use different networks if they wanted to set different routing calculations for simulations with different timesteps. New in ResSim 4.0, is the ability for reach and diversion routing methods and parameters to vary with the compute timestep.  See Reach Editor: Routing Tab.

Variants Improvements for CWMS and HEC-WAT

The Alternative Editor shows default and variant information on the Time Series tab and the Global Variables tab.  Alternative "Variants" are spawned when an HEC-ResSim alternative is used with the CWMS or HEC-WAT platforms.  Variants allow the program to save different time series mapping for the HEC-ResSim standalone program versus when HEC-ResSim is utilized within CWMS or HEC-WAT.  See ResSim Variants for more information.

  • Time Series Variants
  • Global Variable Variants

Features that were new or updated in 3.5 and have been polished in 4.0

Release Allocation Sets - Polished

Release Allocation Sets and If Blocks feature, new in v3.5 has been polished for v4.0 and several bugs have been addressed.  See Specifying Release Allocation.

Operation Support Interface (OSI) - Polished

Improvements were made to the look and feel of the OSI in v3.5. Further polishing and bug fixes .  See OSI Menu Bar and Customizing your View. 

Lookback Changes 

Expanded Lookback Options (for Elevation and Release) - feature has been polished.

Known Issues

Review the most significant known issues here: ResSim 4.0 Known Issues. 

Removed Features

The Vertical Datum feature has been removed.

ResSim's vertical datum feature was conceived to help users view data in their preferred datum and was present in early beta versions of HEC-ResSim 3.5, and some beta users reported successful use of the feature. However since that time, changes to the HEC shared code required this feature to be vastly reworked and the ResSim feature would need to be redesigned before it could be reliably used again. ResSim 3.5 was released with strongly worded warnings urging users to beware of the risk involved with vertical datum. By the time we released version 3.5.1, a few months later, it was clear that any vertical datum info stored with the DSS records was liable to cause much grief, especially if users updated to DSS 7. After considerable investment into finding a solution, the vertical datum feature was turned off, because user scripts work reliably.

A running list of ResSim's removed features can be found here: Removed Features