There are three basic steps in creating an ensemble alternative in ResSim:

  • Set the Alternative Type to Ensemble
  • Map one or more required inflows to a collection time.
  • Select the members of the collection to use/compute in the alternative

These steps are described in more detail in the following sections.

Set the Alternative Type to Ensemble

Although you can create a new alternative and make it an ensemble alternative, the easiest way to create an ensemble alternative is to start with a standard alternative that is already working as desired. That's where these instructions are going to start.
To create an Ensemble Alternative in ResSim,

  • In the Reservoir Network module of ResSim, open the Alternative Editor.
  • Select an existing alternative that you have tested and validated in the Simulation module.
  • Select SaveAs… from the Alternative menu of the Alternative Editor.
  • Give your new alternative a unique name and click Ok.
  • Your new alternative is now the active alternative in the Alternative Editor.
  • Select the Run Control tab (Figure G.20) and change the Alternative Type radio button to Ensemble.
  • Select Save from the Alternative menu to save your change.

    Figure: Alternative Editor—Run Control Tab, Alternative Type 

Map Required Inflows to Collections

Typically, the boundary conditions that are associated with ensembles are the inflows to your watershed. However, you should remember that your model may have other boundary conditions that could or should be considered for associating with ensembles. These include diversions, zone definitions, and any rule or other element that is defined as a function of an external time-series.
To associate (map) a boundary condition to a collection:

  • Be sure that the current/active alternative in the Alternative Editor is an Ensemble alternative (check the Alternative Type on the Run Control tab).
  • Select the Time-Series tab in the Alternative Editor. When the Alternative Editor's active alternative is an Ensemble alternative, the Time-Series tab will allow selection of collections for any input Time-Series (Figure G.21).

    Figure G.21 Alternative Editor—Time-Series Tab,Replacing Single Time-Series with Collections 

  • Click the Select DSS Path button. A DSS Pathname Selector window will open. This window looks and acts a lot like DSSVue.

    Figure G.22 DSS Pathname Selector Window 

  • Select Condensed—Group Collections from the View menu. The collection member pathnames will condense as illustrated in Figure G.22. Filter the pathnames to the specific collection F part you need.
  • Move the Pathname Selector window so that you can see both it and the Time-Series tab of the Alternative Editor at the same time.
        • On the Time-Series tab, select the first boundary condition row that you want to associate with a collection.
        • In the pathname selector window, find and highlight the collection you want to associate with the selected boundary condition row on the Time-Series tab. Click the Set Pathname button or double-click the highlighted pathname. The pathname of the first member of the selected collection will appear in the selected row on the Time Series tab (Figure G.21).
        • Repeat these steps for each boundary condition that should be mapped to a collection.
  • When finished, close the pathname selector window and Save the alternative.

Select the Collection Members to Use in the Ensemble Alternative

Since collections can contain a large number of members, it may be desirable to use only a subset of the available members in your ensemble alternative. To specify which members of the collection to use:

  • Select the Ensemble tab of the Alternative Editor
  • In the Use Ensemble Members:* text box, enter the ID numbers of the collection members use/compute as part of the ensemble alternative (Figure G.23). If you want to use all the member of the collection(s), use the asterisk  character. If you want to compute only a subset of the members of your collection(s), enter a comma-separated list of member IDs with each entry consisting of one member ID or a range of member IDs. For example:

[] = all ensemble members
"3, 7, 10" = ensemble members with IDs 3, 7, and 10
"1, 3-12, 15" = ensemble members with IDs 1, 3 through 12, and 15

Figure G.23 Alternative Editor—Ensemble Tab 

NOTE: An additional option is available on the Ensemble tab of the Alternative Editor. This option is labeled Use Multithreaded Compute. If you turn on (check) this option and your computer and operating system is capable of multithreading, then ResSim will determine the number of threads that can be run on your computer and will divvy-up the ensemble alternative's runs across the available threads. This can substantially reduce compute time for the ensemble alternative. For example, HEC's test computer had a dual-core CPU capable of "hyper threading" making it appear that the computer had a quad-core CPU. The multithreading logic in HEC-ResSim believed the computer was capable of 5 threads. It took an ensemble alternative of 30 members and assigned 6 members to each of the 5 threads. The resulting compute time was approximately 1/4th of the compute time that the same 30-member ensemble alternative took without multithreading.

  • Look back at the Figure. If you look at the row of buttons at the bottom, you will notice a new box that shows the set of ensemble members identified on the Ensemble tab.