PWM water pump flow control - in percent
Expanding on dannyf's answer, if the pump datasheet does not feature a graph, then you will have to experiment. If a flowmeter is not readily available, an excellent approximation can be calculated by pumping water into a large, known receptacle (e.g. 10 litre bucket) and timing how long it takes. Do this for 20%, 40%, 60%, 80% and 100% Pulse Width Modulation modes, and you will have a plot of flow rate vs PWM that will help you interpolate the PWM required for a desired flow rate (e.g. 50% flow rate).

Below is an example of Rotations Per Minute of a 12V computer CPU cooler water pump for a given PWM input. It is assumed that this would be under load (of pumping water), but not stated explicitly.

Note that it may not be physically possible for the motor to operate with 10% or less PWM duty cycle. At this point I imagine that the internal friction of the motor and forces required to push the water along are greater than the force provided by a "10%" duty cycle.



Source: ekwb