The time window for an HEC-ResSim simulation has two parts, a "lookback period" and a "simulation or forecast period". The lookback period is used by HEC-ResSim to "warm up" a HEC-ResSim model alternative. During this period, routing of flows through the river and reservoir network are performed but no release decisions are made nor are any mass balance computations evaluated. The user should think of the lookback period as the time to establish the state of the river network and the starting conditions of the reservoirs. The simulation period is the portion of the time window during which HEC-ResSim simulates reservoir operations, and makes release decisions based on the operation scheme defined by the user. Releases are then routed through the river network, combining with local inflows along the way.

Since HEC-WAT and most of the models treat the analysis period as a single, continuous time window, a setup step is required for each HEC-ResSim alternative in an HEC-WAT study. The setup step lets each HEC-ResSim model alternative know how much of the analysis period to use for the lookback period.  

To setup the lookback period: 

  1. From the HEC-WAT main window, from the Edit menu, point to ResSim, click Lookback Window.

    Edit Menu – ResSim – Lookback Window



  2. The Lookback Window dialog opens. The table in the Lookback Window displays the HEC-ResSim alternatives that are available in the HEC-WAT study. Each alternative that is being used in an HEC-WAT simulation will need to have a lookback window set. 

    Lookback Window Dialog



  3. The time step can be selected from the TimeStep column, in each cell there is a list of available time steps for the user to choose from. 

    The selected time step has to match the time step that was specified for the HEC-ResSim alternative as part of the alternative's original run control data (which is located in the HEC-ResSim Alternative Editor). The number of lookback steps must be entered in a cell in the Lookback Steps column. At least one lookback time step is required for a HEC-ResSim alternative to compute, however more lookback steps may be needed, depending on a variety of factors in the HEC-ResSim alternative's operating scheme. The HEC-ResSim modeler should be consulted to determine the minimum lookback window needed by each alternative.

  4. For each ResSim alternative the Russian River example study, (e.g., calibrate), set the TimeStep to 1 Hour and the Lookback Steps to 6.
  5. Click OK.
    • Note: A Missing Lookback Info window will open if any of the ResSim alternatives do not have lookback information entered. (e.g., if C:calibrate and B:calibrate had missing lookback information). 
    • Click OK and the Missing Lookback Info message window and Lookback Window dialog will both close. If lookback information is not entered, reopen the Lookback Window dialog to enter the missing information.

      Missing Lookback Info Window