Problem 

The table of head and flow values have been entered in a pump definition with the Multiple Point (multipoint) option, but the graph below it does not seem to match the values entered (different shape, line doesn't follow values in table). Also, the curve is seen extending all the way to zero head and flow.

Solution

By default, WaterCAD and WaterGEMS uses a best-fit quadratic curve to generate the pump curve based on the table of head and flow that you enter - notice the coefficients shown below the graph. If you do not want this, open the calculation options for your current scenario and select "true" for "use linear interpolation for multi-point pumps." See more on that here. This will also cause the graph display in the pump definition to only plot the points in the multi-point pump curve table.

Note though that linear interpolation can sometimes make it more difficult to converge on a balanced solution, which is why quadratic interpolation is used by default.

There is one other option to consider, if you are certain the pump will only operate within a small duty flow range (such as Pumps with a steady discharge and suction HGLs eg. Reservoir/tank lift pumps). Simply input a large number of flow/head points into the pump curve in this operating range, and put zero or few points for the rest of the range.

While the derived curve will be inaccurate at flow ranges above and below the normal operating range, this doesn't adversely affect the modelling results since it never models flows through the pumps in these flow ranges.

See Also

User notification "Pump exceeds the maximum operating point specified for the pump curve"

Pump curve linear interpolation