Measure Water Flow in a Swimming Pool
One very simple method if you know the pump specs (gal/min) and assuming pump runs at a constant speed, would be to time the on period of the pump and calculate total from there.
For example (just for completeness) if the pump pumps at 0.5 gal/min and it runs for 5 mins you have a total of 2.5 gal.
Just hack into the pump and tap into the on/off circuit then send to micro.