Irrigation pumps installed above ground are the most used type in irrigation systems. They must be filled with water, or primed, prior to operation and can draw water from reservoirs, lakes, streams and wells. Centrifugal irrigation pumps offer a wide range of capacities while providing a fairly constant flow. The capacity is measured in gallons per minute (GPM) and is a function of the motor's horsepower (HP), discharge pressure (measured in pounds per square inch, or PSI) and the distance above water the pump is located. As the flow rate increases, pressure decreases, and vice-versa. If you live in a climate those experiences freezing temperatures in the winter, you'll need to winterize your irrigation system by disconnecting the suction and discharge lines connected to the pump, emptying water from all of the pipes and storing the pump indoors during the cold season.