When a pump is pumping water from a level that is below the pump it is called suction head. If the water to be pumped is at a level above the pump, it is positive head. The pump maybe pumping from the base of a tank to a higher level such as an elevated tank or it may be pulling water from a tank that is below the pump. The distance from the center of the pump to the top of the water level would be the foot of head. It takes 2.31 feet to make 1 P.S.I. So if it is 10 feet from the center line of the pump to the water level, you would have about 4.33 P.S.I. of head.