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. Put simply, a pump is a machine that transfers mechanical energy into pressure and velocity energy in flowing water.
The INZ Pumping and Hydraulic Resource Book provides an overview of pumping and hydraulic considerations for successful irrigation.