Pump performance (capacity or pressure) is highest when the pump is operated close to the water surface. Increasing the suction head will DECREASE the discharge head and consequently the discharge capacity of the pump. Most importantly, suction head should be kept to the smallest vertical distance possible to reduce the likelihood of cavitation. Cavitation (the formation of an air space in the pump head) can also occur if the suction hose is restricted. The intake hose or pipe should be the same size or larger than the intake port of the pump. Cavitation can quickly damage a pump.