The purpose of the Pulse Flow Options is to allow you to specify a new value for the Default Pulse Flow and/or to specify a reservoir-specific Pulse Flow value for one or more of the reservoirs in your network. ResSim uses the Pulse Flow value in its algorithm for determining the routing window that will be used in the downstream control operations logic.

In order to identify when the value of the pulse flow needs to be changed and what value should be used, you need to understand how ResSim estimates the routing time window.

The Routing Time Window

When downstream control (DSC) rules are active in an alternative, ResSim adds a special compute section, labeled Evaluating Routing for Downstream Operations, to the beginning of the compute of the alternative. During this compute section, ResSim routes a "pulse" of water from each reservoir (with downstream control rules) to that reservoir's downstream control points. It then compares the hydrograph of the pulse as it left the reservoir to the routed version of the pulse at each control point in order to determine the peak to peak "lag time" and the overall spread of the pulse at the control point. The routing time window for each control point starts with the timestep when the pulse left the reservoir and ends at the end of the "spread". The routing time window computed by ResSim for each active downstream control rule will be reported in the compute log at the start of the Computing Regulated Flow section—look for the messages that start with "Pulse Width Steps =". An example of these messages is shown in "Figure: Example Compute Log Messages - Computed Window Size for each DSC Rule".

Figure: Example Compute Log Messages - Computed Window Size for each DSC Rule

To view the routed hydrographs of the pulse flow produced by your model:

  • Open DSSVue from the Tools menu of your simulation
  • Filter the DSS pathnames for a C-part of Flow-Routing-YourReservoir-1 where YourReservoir is the name of the reservoir you are interested in.
  • Select the dataset generated at the outflow junction of the reservoir and the dataset generated at the downstream-most control point. Then,
  • Plot the selected datasets.

An example plot of the two suggested pulse flow hydrographs is shown in "Figure: Plot of Pulse Flow and Routed Pulse Flow". Did you notice that the pulse illustrated above is actually an inverted pulse? Routing an inverted pulse of water is numerically more stable for the hydrologic routing methods used by ResSim. It is also more consistent with how reservoir operations actually work; reservoirs rarely send out pulses of water, instead they often maintain a relatively constant release then cut back when necessary.

Figure: Plot of Pulse Flow and Routed Pulse Flow

If you look carefully at the hydrograph leaving the reservoir, you'll see that ResSim is routing a constant flow and then cutting that flow in half for one timestep before returning the flow to the original constant value. The inverted pulse is the one-timestep cutback. Although it is a bit of a misnomer, the Pulse Flow Options editor labels the constant flow value Pulse Flow. The default value for the Pulse Flow is 5000 cfs (142 cms).

The Pulse Flow Options is the only other option currently available in ResSim for influencing the estimate of the Routing Time Window used by the downstream control operation logic. However, this option is not located in the Downstream Control Function rule editor. Instead, it is located in the Dam menu or the Dam's context menu ("Figure: Dam Context Menu - Pulse Flow Options") on the Physical tab of the Reservoir Editor.

Figure: Dam Context Menu - Pulse Flow Options

The Pulse Flow Options is the only option, other than those included in the downstream control rule's Advance Options, that is currently available in ResSim for influencing the estimate of the Routing Time Window used by the downstream control logic. However, this option is not located in the Downstream Control Function rule editor. Instead, it is located in the Dam menu or the Dam's context menu on the Physical tab of the Reservoir Editor:

To edit the Pulse Flow value(s):

  • Open the Reservoir Editor and click on the Physical Tab.

  • Right-click on the Dam in the Reservoir Tree, then select Pulse Flow Options from the context menu. The Pulse Routing Options editor will open: ("Figure: Pulse Routing Options Editor").

    Figure: Pulse Routing Options Editor


  • Enter a new Pulse Flow value in the Default Pulse Flow textbox.

Remember, the value you are entering as the Pulse Flow is actually the constant flow of the pulse flow hydrograph and the magnitude of the inverted pulse is half of that. As a rule of thumb, use a pulse flow value that is 1.5—2 times larger than the average value of the limit that the reservoir should usually be operating for.

  • And/or uncheck the Use Default box for each reservoir for which you want to specify a new Pulse Flow value, then enter the new Pulse Flow value in the last column of the table.

If your model is using a linear routing method in your reaches, then there should be no need to change the Pulse Flow value(s) from the default since the routing window will not change as a function of flow. However, if you are using a non-linear routing method, then determining an appropriate value for the pulse flow can be a challenge, especially if you are operating for both a minimum and a maximum limit at the control point. Decide which limit is the most important, then, as a rule of thumb, set the pulse flow to 1.5—2 times the average value of the downstream limit defined at the control point. See how that value affects the routing window and the downstream operation. If necessary, adjust from there. It won't take long to zero-in on an appropriate value.

  • When you have finished setting the Pulse Flow value(s), click OK to save your setting and close the Pulse Flow Options editor.
  • Be sure to click Apply in the Reservoir Editor to save your changes to the reservoir data in the network before proceeding to another tab or reservoir.