HEC-WAT 1.1 represents many new improvements over the HEC-WAT 1.0 software released in 2016.   The software has been under a beta release for the last several years and received extensive testing at HEC and by users in the field on a variety of studies.  Over the course of the last nine years, a significant number of enhancements to the software have been made, including many bug fixes in HEC-WAT and the accompanying modeling software.  If you find issues, please check the known issues page for a possible resolution, and report any bugs or other issues encountered to the HEC-WAT team.


A new Quick Start Guide for HEC-WAT 1.1 has been revised and significantly updated from the Quick Start Guide provided with HEC-WAT 1.0; in addition, the documentation for HEC-WAT has been moved to the HEC Confluence system, allowing for rapid updates and improvements in the documentation as new software features are released.

Documentation that has been updated includes:


Significant changes and enhancements from HEC-WAT 1.0

Updated modeling applications

HEC-WAT 1.1 includes plugins for HEC-HMS version 4.13, HEC-ResSim version 3.5.1, HEC-RAS version 6.5, and HEC-FIA version 3.3.  Testing has been performed with these versions of the modeling software using both test-case models developed to cover the range of a feature's functionality, and HEC-WAT studies provided by users to identify potential impacts to the end user.   Newer versions of each piece of software may be supported by modifying your HEC-WAT installation, but these have not been tested as thoroughly and should be used with care.  Refer to the application-specific appendix to the HEC-WAT users manual for current instructions to change each version of software.

Model Linking by Simulation

In coordination with the HEC-RTS/CAVI software used by the Corps Water Management System, HEC-WAT moved towards a "simulation-based model linking" scheme, where the connections between models are tracked by HEC-WAT as the integrating application and overwritten to the models at compute time.  The previous approach depended on each model keeping track of the linked inputs from other models in the compute, with the HEC-WAT Model Linking Editor providing a unified interface to update these.  This new simulation-based linking approach allows for additional flexibility without needing to create additional model alternatives just to change one of the linked inputs.  This approach, combined with other enhancements to the model linking editor and modeling application plugins (HEC-HMS, HEC-ResSim, and HEC-RAS) should reduce the number of duplicate models needed to successfully run a HEC-WAT model with different combinations of inputs.  This change has not been without challenges, and there are many combinations of model alternatives we have not been able to test.  If you encounter a bug or other issues while using the Model Linking Editor, or your model alternatives are trying to read incorrect or unexpected data, please do not hesitate to reach out for assistance.

Hydrologic Sampler Plugin Improvements

The Hydrologic Sampler Plugin that provides input to the Flood Risk Analysis (FRA) simulations for Monte Carlo-based evaluation of risk and uncertainty has been significantly improved since HEC-WAT 1.0, including the following features:

  • Stratified Sampling - this new feature allows the Hydrologic Sampler to weight the sampled exceedance probabilities (return intervals) for flood events such that the upper end of the frequency curve can be sampled more frequently, and thus reducing the number of events that must be sampled in order to adequately capture rare events.  This method is available for Correlated Flow Frequency Curve and Basin Average Precipitation event generation methods.  Stratified sampling breaks the range of exceedance probabilities into bins (one per year of the analysis period used) and samples a return interval within each bin.  Users are able to choose a probability distribution for these bins to further weight samples towards the more rare end of the frequency curve.  Outside of the Hydrologic Sampler, HEC-WAT's output reporting tools have been enhanced to properly process this data when generating frequency curves and other statistics on the model results.
  • The duration over which events are scaled by the hydrologic sampler and the accompanying frequency curves can now be varied by the sampled exceedance probability – this allows for a different frequency curve to represent each range of event and better capture the correct scaling duration for rare events, rather than choosing a single scaling duration to fit all events.
  • Basin Average Precipitation sampling method now allows associating a temperature sequence with each sub-basin in the shape sets that are used to represent temporal and spatial variation in each event.  This temperature sequence can be used by HEC-HMS to model snowmelt and evapotranspiration processes, or passed to other models where needed.  The shape set temperature sequences are used without modification, as the scaling process is only applied to precipitation.
  • The bootstrap method for sampling now supports an auto-regressive lag-1 approach to preserve correlation of flow volumes from year to the next, allowing for modeling behavior of reservoir systems and other processes that require multiple years of data to properly account for variability.  Enabling this process causes the sequence of years to replicate a serial-correlation value from one year into the next, either as volume for the whole year or limited to a season.   
  • The bootstrap method for sampling has also been modified to support sampling parameters other than flow, which can be defined by the user for each location used.  This allows sampling additional data for each event that may be used to drive reservoir operations or provide additional boundary conditions tied to the hydrologic data sampled.  This also enables the bootstrap method to sample precipitation and temperature timeseries to pass to a hydrologic model, allowing continuous simulation with many sequential years of data.

New Plugins: Scripting

HEC-WAT 1.1 includes a new plugin to add user-defined "scripts" between models in the program order.  These scripts can be used to pre-process or post-process data required for each model.   Example scripts can be found on HEC's GitHub page.  The scripting plugin uses the Jython programming language, a dialect of the Python programming language that works within the Java Virtual Machine environment used by HEC-WAT.  This scripting plugin is very similar to the scripting plugin provided by HEC-RTS, and can be used to call the standard Java libraries used by other HEC software including HEC-ResSim and HEC-DSSVue.

New Plugins: "Stochastic" Data Importer

The Hydrologic Sampler plugin for HEC-WAT provides one way to generate events to run through a Monte Carlo simulation to estimate risk and uncertainty on flood risk management metrics, but a number of hydrologic studies have needed additional or different approaches to generating inputs for their Monte Carlo or other ensemble-based simulations within HEC-WAT.  Thus, a new plugin called the "Stochastic Data Importer" has been provided with HEC-WAT that maps structured time-series data stored in HEC-DSS files into the realization/lifecycle/event sequence used by the HEC-WAT Flood Risk Analysis simulation framework.  This plugin has been used across a number of specialized studies to perform large-scale computes of hydrologic and hydraulic models using the HEC-WAT Distributed Compute architecture, while providing flexibility in the input data used.  We've also been planning a name change for this plugin, as few of the uses have involved stochastically generated data, and thus the acronym SDI may be changed to mean "static" or "structured" data importer.

Removed: Performance Metrics Plugin

This plugin was provided with HEC-WAT 1.0 to aggregate and summarize metrics within the Flood Risk Analysis simulation type when event-based economic consequences models are used.   Given that few HEC-WAT modeling efforts have taken the steps of performing event-based economic consequences, instead choosing to inform HEC-FDA modeling inputs from hydrologic or hydraulics results, we've decided to deprecate the use of the Performance Metrics Plugin.  If analysis of economic data generated though the use of HEC-WAT is required, please reach out to the HEC-WAT team to find out if other tools or post-processing techniques may be applicable for your study.

HEC-DSS version 7 as the default

HEC-WAT 1.1 uses version 7 of the HEC-DSS file format by default, and is configured to launch each plugin to use HEC-DSS version 7 files by default as well.  HEC-DSS version 7 brings a number of significant improvements for HEC-WAT users, including supporting files larger than 8 gigabytes, the use of mixed-case names in the labeling of records, and speed enhancements in opening files and saving data.  However, switching to HEC-DSS version 7 has caused a number of issues, particularly for existing studies upgrading from software versions that used HEC-DSS version 6.  In particular, the change to mixed-case record names, and the E-part (or Interval) of the record name no longer using abbreviations (e.g. 15Minute  instead of 15MIN ) has caused some issues for users.  If a model is failing to compute due to missing data or other issues with DSS pathnames are found, please check both the model linking editor and the standalone models to ensure they are using DSS version 7 compatible pathnames.  Model scripts and tools used for post-processing data that use string operations to process DSS pathnames may also need to be made case-aware or revised to use the new E-part strings.

Automated Testing for HEC-WAT Development

A major effort in the past year has been the development of automated compute testing for the HEC-WAT development team, intended to help the software developers on the HEC-WAT team more easily test proposed changes to the software, ensuring that these do not negatively impact existing features or change model results.  This suite of automated tests is continuing to be built out with future HEC-WAT releases and is expected to evolved into a validation and verification test suite that can be provided to our users to help document the intended and expected behavior of the HEC-WAT software.

Known Issues

A list of known issues that have been identified and not yet fixed, along with potential work-arounds, is being maintained by the HEC-WAT team.   Evaluation of each issue reported to us has been performed to determine what issues were fixed for the release of HEC-WAT 1.1, informed by a prioritization accounting for severity of each issue, likelihood of a user encountering the issue, and the cost to address each issue.  Unfortunately some of the core libraries used in HEC-WAT 1.1 are an older version that is no longer supported, and thus a large number of issues are targeted to be resolved in HEC-WAT 1.2 after upgrading to the latest version of those libraries.

Upgrading an existing model from HEC-WAT 1.0

If an HEC-WAT model ("study" or "watershed") is being upgraded from HEC-WAT 1.0, we recommend backing up the existing model for HEC-WAT 1.0, and then performing the upgrade over the following steps:

  • Open the model in HEC-WAT 1.1 but do not attempt to run any existing simulations. 
  • For each plug-in used by simulations within HEC-WAT, open each model alternative and upgrade that model within the standalone software user interface (by clicking the icon in the HEC-WAT toolbar).  Check each model in the standalone software and perform a validation compute, fixing issues encountered and comparing results against the previous model version used to ensure that no significant changes will impact your study results.  Differences between model versions are to be expected, and are likely due to bug-fixes or enhancements in the software, but it is still critical to assess that the updated standalone  computes are successful before using the models in HEC-WAT.
  • Then within HEC-WAT, check each simulation, confirming that model linking and output variables are properly configured.

For each plugin, additional notes about the upgrade process within HEC-WAT are documented below:

Hydrologic Sampler

While the Hydrologic Sampler plugin may upgrade without issue from the HEC-WAT 1.0 version, if any issues are encountered, we recommend rebuilding the model alternative from scratch.  There are a number of uncaught compatibility issues that have been found in testing, but we do not expect many users will encounter those while upgrading a HEC-WAT 1.0 Hydrologic Sampler alternative to HEC-WAT 1.1 Hydrologic Sampler.  New features in the Hydrologic Sampler or changes to existing features may require reviewing each input and ensuring that the appropriate choice is selected for each.

HEC-HMS, HEC-ResSim, and HEC-RAS 6.x

Upgrading the standalone models for HEC-HMS, HEC-ResSim, and HEC-RAS 6.x in a HEC-WAT study is usually performed when first opening the HEC-WAT study with the newer HEC-WAT 1.1 and accompanying plugins.  Standalone testing and review of each model is still recommended, but we do not expect any changes in the HEC-WAT study are required to use them.

HEC-RAS 5.0.x 

HEC-RAS 5.0 models should be imported into a standalone HEC-RAS 6.5 project before importing into HEC-WAT.  We are uncertain if HEC-RAS 5.0 models developed for use with HEC-WAT 1.0 will function correctly if this step is not performed, and have had few opportunities to test it.

HEC-FIA 3.0 or 3.1

If you encounter a dialog box saying this upgrade is necessary but the HEC-WAT model does not have any HEC-FIA components, you can ignore the dialog box, or delete the fia  folder from the watershed, as an old empty FIA project file may be present.

If the existing model does have an HEC-FIA model that needs to be brought forward, we recommend opening the study's fia  folder within the standalone software to perform the upgrade, as HEC-FIA 3.0 and 3.1 models cannot be used in HEC-FIA 3.3 without upgrading them to 3.2 in the standalone software first.  As there are significant changes in the HEC-FIA software between 3.1 and 3.2, we strongly recommend reviewing and validating the upgraded model before using it in a HEC-WAT study.

HEC-WAT 1.2:  What's coming next.

A major effort in the past year has been the development of automated computational tests and better documentation of manual testing processes for each HEC-WAT release.  As a result, we expect to have a more rapid HEC-WAT release cycle, with HEC-WAT 1.2 being released in the next year.   HEC-WAT 1.2 will not contain any significant new functionality, and be focused on replacing the older core Java libraries with the newer ones used by other Java-based applications developed by HEC. It will also include plugin upgrades to the latest release of HEC-HMS, HEC-ResSim 4.0, and HEC-RAS 6.7.  HEC-WAT 1.2 using the latest libraries may require changes to scripts used within HEC-WAT, but otherwise should be backwards compatible with existing HEC-WAT 1.1 watersheds.