Water Transducer

What about the water do you wish to measure? You could measure flow rate, pressure, level etc. and the type of sensor depends on what you wish to measure. For example you may have a float switch that turns the pump on when the water reaches a particular level. You could also do the same with a pressure sensor operating the pump when a specific pressure that is associated with a level is reached. It all depends on what you want the system to do and if you intend to buy an off the shelf sensor or build one yourself. If you can give me some more details I may be able to help.