How to pump constant flow in ICM/SWMM5/XPSWMM?


Modeling a pump can feel challenging if you are not familiar with the concept of how a pump works.

  • Blue: fixed pump
  • Red: Rotary pump
  • Green: screw pump

Wet Well

Most of the pumps in the real world has a wet well. The reason we need a wet well is that we want to operate a pump at optimal efficiency to save energy and ensure a long service life. Therefore, when the flow is too low, we would like to collect enough water in the wet well to keep the pump on continuously for 10 mins, instead of kicking on every other minute.

Fixed Pump

The most obvious way to model it will be using a fixed pump.

Screw pump

For a screw pump we define a curve, by setting the discharge at 10cfs, we are modeling a pump with a fixed pumping rate.

Rotary pump

The most common type of pump is rotary pump, and we’ll see how to setup this up in ICM. I devised an imaginary pump curve, so that during the simulation the pump will pump 10cfs.

  • The head is ordered in descending order
  • And the discharge is in ascending order


SWMM5 doesn’t have the fixed pump option. For screw pump, it is the type II pump.


XPSWMM results are similar to SWMM5 results with a few exceptions,

  • The screw pump option is less stable than SWMM5
  • The peak flow of rotary pump seems to be different from SWMM5, most likely small differences between the models.


Correctly setting up a pump can be tricky without accurate field data. As a modeler, you should be familiar with how the different type of pumps behave. As shown in the article, different software implements the pumps quite differently.

  • ICM seems try to stay true to the pump curve except when using a screw pump.
  • SWMM5 will automatically adjust the pumping rate to match the wet well condition, in a way working as an idea pump if the pump is too powerful while not enough water is stored in the wet well.
  • XPSWMM seems to work very similar to SWMM5



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