Water sources for irrigation can be at a higher, the same, or lower elevation than the area to be irrigated.
Water from a higher elevation, when distributed through a closed conduit (a pipe), possesses potential energy. This may enable water to be supplied for irrigation at the required pressure without adding external power (pumping). However, for similar or lower elevations the water will have a zero or negative potential energy. This requires the addition of energy so the water can be supplied at pressure, a pump is needed to do this, which is a machine that transfers mechanical energy into pressure and velocity energy in flowing water.
The IrrigationNZ Hydraulics and Pumping resource book outlines some crucial considerations to take into account related to this topic.