One method for using ensemble forecasts in HEC-RTS is through the Ensemble Forecast Run, which enables HEC-RTS to automatically manage the ensemble members. More information can be found in the User Manual. An Ensemble Forecast Run is similar to a standard (deterministic) forecast run, with one key difference: you’ll need to select the ensemble members you want to compute. Instead of running a single set of input conditions, ensemble forecasting allows you to simulate multiple possible futures by running the model across a range of predefined input scenarios (called ensemble members). Each member represents a unique set of inputs, helping you evaluate forecast uncertainty and improve decision-making.

This guide walks you through the steps to create an ensemble forecast run in your HEC-RTS watershed.

Step 1: Open the Forecast Run Editor

To open the Forecast Run Editor:

  1. Launch HEC-RTS and open your watershed.
  2. Navigate to the Setup tab.
  3. From the Models menu, select Forecast Runs.
  4. The Forecast Run Editor dialog will appear.

Step 2: Create a New Ensemble Forecast Run

  1. In the Forecast Run Editor, click New Ensemble Run.
  2. Enter a Name for the run.
  3. Optionally, add a Description.

Step 3: Select Model Alternatives

  1. In the Models tab, select the Program Order for the run.
  2. Use the dropdown menus to select model alternatives.
    1. As with standard forecast runs, you can leave any model unselected if it's not needed for the run.

Step 4: Choose Ensemble Members to Compute

This step is what sets ensemble forecast runs apart from standard (deterministic) forecast runs in HEC-RTS.

To select ensemble members:

  1. In the Use Ensemble Members field, enter a list of member IDs or ranges.
    1. For example:
      1. 2, 5, 11 — uses members with IDs 2, 5, and 11
      2. 1, 5-8, 12 — uses members 1, 5 through 8, and 12
      3. 000001-000060 — processes all 60 members in the range

Selecting a subset of ensemble members can help reduce computation time or focus on specific scenarios of interest, while running all members offers the most comprehensive look at forecast variability.

Step 5: Save the Ensemble Forecast Run

  1. Review your selections and make any necessary adjustments.
  2. Click Apply to save your changes. 
  3. Your new ensemble forecast run will appear in the list at the top of the dialog.
  4. Click OK to close the Forecast Run Editor dialog. 

Forecast Run Editor, Ensemble Forecast Run Example

You’ve now successfully created an Ensemble Forecast Run for your HEC-RTS watershed.