Overview

The overall goal of this tutorial is to demonstrate how ensemble output from HEC-ResSim can be manually overwritten with a user specified release time-series. The example was developed for an HEC-ResSim model integrated within HEC-RTS that uses ensemble forecasts generated from the National Weather Service. This short tutorial aims to demonstrate how ensemble members respond when we apply rule overrides.

In the HEC-ResSim software, overrides are user-specified time-series that replace the model’s rule-based release decisions. Overrides allow operators to represent manual, conditional, or scenario-specific reservoir actions while preserving the system’s underlying physics and routing. Users may choose any of the available override operations, including reservoir elevation target overrides, reservoir release overrides, and diversion overrides. However, this tutorial focuses on the reservoir release override; similar steps and procedures apply to the other override options.

Prerequisite

Download and unzip this file within the C:\CWMS_EFP directory on your computer. HEC-RTS 3.5 was used to create this example

CWMS_EFP.zip

Tasks 

The three main tasks for this tutorial are:

  1. Confirm models are properly set up and required data are downloaded
  2. Configure the override set up
  3. Run the model and review ensemble results

1. Confirm models are properly set up and required data are downloaded

  1. Download the zipped folder containing the model and data to C:\CWMS\CWMS_EFP. Verify that the extracted directory contains the expected project structure (i.e., database, forecast, watershed folders), including a database folder. Ensure that the tutorial watershed opens in HEC-RTS with no missing file or path errors.
  2. Open HEC-RTS and navigate to the Modeling tab. Open the forecast named “RTS_Ensemble_ResSim” and run both forecast runs sequentially. Verify that both the deterministic NBM forecast simulation and the Ensemble forecast simulation complete successfully.
  3. The figure below shows snapshots of the model run, similar to those presented the Ensemble Forecasts In HEC-RTS Tutorial.

2. Configure the override set up

2.1 Select the model alternatives and override type

  1. Once the model and data setup are complete, select the ResSim model alternative in the Modeling tab. In the RTS-HEFS Actions panel, choose “Release Overrides” to open the “Overrides” editor, as shown in the figure below.

Note that the "Ensemble Member: 000001" has been selected on the top and “Release” tab shows corresponding release rates in the third column. There is text located at the right top corner of the page that states "Overrides entered apply to all members". This means the operator can choose any Ensemble member (shown on the top left) but the same override values entered into the table will be applied for all ensemble members. For detailed explanations of the menus and tabs available in this Editor, please consult the HEC-ResSim User's Manual or the relevant HEC-ResSim tutorial documentation.

2.2 Edit the Overrides Values 

  1. Specify override values by selecting a single cell or by revising multiple adjacent cells using the fill function.
  2. To modify a single value, double-click the selected cell and enter the new value.
  3. To modify multiple adjacent values, click the first cell and drag the mouse, or use Shift + click (hold the Shift key while selecting additional cells) to highlight the cells to be revised.
  4. Similarly, users can import time-series data from DSSVue using the "Import…" dialog command, or paste values copied to the clipboard.
  5. Select the appropriate "Fill" option in the "Table Fill Options" dialog (see the figure below).

The revised values change color from black to blue. For timesteps without user entered overrides, HEC-ResSim applies reservoir rules to determine release values.  After running a simulation, you can discontinue some or all overrides by selecting "Unspecify" or "Unspecify All " to revert to rule-based releases.  Click apply and OK after you have entered the override values. The selected overrides are now ready for the reservoir simulation.

3. Run the model and review ensemble results

  1. Once the release overrides are specified, compute the simulation to apply the override releases.
  2. After a successful compute, use the Map Window to navigate to the reservoir with the zoom and pan tools on the Map Toolbar. Click the reservoir element on the map to select the location of interest, then choose Plot Time Series to open the Time-Series Plot Window.
  3. Alternatively, visualization can be performed by opening the ResSim model from the CAVI menu or by using the Ensemble Viewer. In addition, users can export results to Excel (.xls) or CSV format through DSSVue and further analyze statistical properties. The figures below show the ensemble results for two different scenarios (at Lake Mendocino and downstream at the Hopland Gage). Scenario 1 included a user specified release of 2,000 cfs from December 25, 2025 at 1300 to December 26, 2025 at 0600 for the Lake Mendocino reservoir, and Scenario 2 had no override defined.

The figures below show ensemble results for the specified release overrides, along with corresponding results without overrides, at Lake Mendocino—focusing on reservoir outflow and elevation. 

Similarly, the figures below show ensemble results for the specified overrides release and corresponding results without release overrides at the downstream Hopland gage.

It is noted that overrides represent the highest level of the decision-making hierarchy. However, HEC-ResSim maintains "Physical Constraints" (e.g., maximum outlet capacity) to ensure that the user does not inadvertently force a release that is physically impossible given the headwater elevation and outlet capacity.