This tutorial shows how to manually override HEC-ResSim ensemble release decisions with a user-defined release time series. The example uses an HEC-ResSim model integrated with HEC-RTS and National Weather Service ensemble forecasts, and demonstrates how ensemble members respond when a rule override is applied.

In HEC-ResSim, an override is a user-specified time series that replaces rule-based release decisions. Overrides let you represent manual, conditional, or scenario-specific operations while preserving model routing and physical behavior. HEC-ResSim supports multiple override types (elevation targets, releases, diversions). This tutorial focuses on Reservoir Release Overrides, but the workflow is similar for other override types.

Necessary Data

Download and unzip the CWMS_EFP.zip file to the C:\CWMS\CWMS_EFP directory on your computer. 

Software Version

HEC-RTS 3.5 was used to develop this tutorial.

Step 1: Verify Setup and Run the Forecasts

  1. Extract the tutorial data to C:\CWMS\CWMS_EFP on your computer.
  2. Confirm the extracted folder contains the expected structure:
    1. database
    2. forecast
    3. watershed 
  3. Launch HEC-RTS and confirm the watershed opens without missing file/path errors.
  4. Navigate to the Modeling tab.
  5. From the Forecast menu, click Open.
  6. Select the RTS_Ensemble_ResSim forecast and click OK.
  7. Compute both the NBM Forecast and Ensembles ResSim forecast runs (one at a time).
  8. Verify both runs compute successfully.

Step 2: Open the Release Override Editor

  1. Select the RTS-HEFS ResSim Alternative.
  2. From the Actions tab, click Release Overrides.

You may see a specific member selected (for example, Ensemble Member: 000001). Even if you select a different member, the editor indicates that Overrides entered apply to all members. So, any override values you enter are applied across the full ensemble.

Step 3: Enter Override Values

You can edit releases in several ways:

  • Edit a single value
    • Double-click a cell and type a new release value.
  • Edit multiple values
    • Drag across adjacent cells, or
    • Use Shift + click to select a range of cells, then apply a fill option.
  • Import or Paste a time series
    • Use Import to bring values in from HEC-DSSVue, or
    • Paste values from the clipboard.

  1. Set the release for Lake Mendocino (Coyote Reservoir) to equal 2,000 cfs from 25Dec2025 at 1300 to 26Dec2025 at 0600.
    1. Edited values change color (typically black to blue) to indicate user-specified overrides.
  2. Click Apply, then OK to save the overrides.

Use Unspecify (selected cells) or Unspecify All (entire table) to return to rule-based operations for those timesteps.

Step 4: Compute and Review Ensemble Results

  1. Compute the simulation again to apply the override releases.
  2. Review results using any of the following:
    1. Select the reservoir within the Map Window and choose Plot Time Series.
    2. Open the HEC-ResSim model directly.
    3. Use the Ensemble Viewer.
    4. Export results from the forecast.dss file to Excel or CSV for further analysis.

The figures below show the ensemble results for two different scenarios:

  • Scenario 1 (override applied): Lake Mendocino release set to 2,000 cfs from December 25, 2025 at 1300 to December 26, 2025 at 0600.
  • Scenario 2 (no override): Model uses standard rules for releases.

Similarly, the figures below show the results at the Hopland gage for both Scenarios.

Final Review

Overrides are the highest level operational input in the decision hierarchy, but HEC-ResSim still enforces physical constraints (such as outlet capacity and available head). If an override requests an impossible release, the model will limit releases to what the reservoir/outlet configuration can physically deliver.