The time window for an HEC-ResSim run has two parts, a "lookback period" and a "simulation or forecast period". The lookback period is used by HEC-ResSim to "warm up" an 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 logic defined by the user. Releases are then routed through the river network, combining with local inflows along the way.

HEC-WAT and most of the models treat the analysis period as a single, continuous time window. For this reason, a setup step is required for each HEC-ResSim model 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 set up an HEC-ResSim lookback period:

  1. From the HEC-WAT main window, from the Edit menu, point to ResSim, and click Lookback Window. 
    HEC-WAT main window, Edit menu, ResSim sub-menu commands.
  2. The Lookback Window dialog box opens.
    Lookback Window dialog box, example TimeStep selection and Lookback Steps value.
  3. The table on the Lookback Window dialog box displays the HEC-ResSim model alternatives that are available in the HEC-WAT study. Each model alternative that is being used in an HEC-WAT simulation will need to have a lookback window set. For example, the HEC-ResSim model alternative calibrate has a TimeStep of one (1) hour and and entered Lookback Steps value of 6.
  4. For each Alternative listed, a TimeStep (compute interval) and number of Lookback Steps must be specified. 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 model alternative as part of the alternative's original Run Control data (which is located in the HEC-ResSim Alternative Editor).

  5. 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 an HEC-ResSim model alternative to compute; however, more lookback steps may be needed depending on a variety of factors in the HEC-ResSim model alternative's operating rules.

    The HEC-ResSim modeler should be consulted to determine the minimum lookback window needed by each HEC-ResSim model alternative. As mentioned, the lookback period takes place within the analysis period (time window) that HEC-WAT provides to the HEC-ResSim model alternative. No reservoir operations will be simulated during the lookback period, which could impact model results.

  6. Click OK, to close the Lookback Window.