The rotary impact sprinkler is the most common form of sprinkler. At least 90% of all sprinkler systems use rotary impact sprinklers. These sprinklers are equipped with one, two or even three nozzles. Each nozzle contributes to the final distribution pattern of water delivered by the sprinkler.

Sprinklers are spaced in the field in such a way that the merged distribution pattern provides an even distribution of water across the field. This spacing is a function of the distribution pattern of single sprinkler. The delivery and distribution pattern of a single sprinkler will vary with changes in pressure, or i the nozzle size/s.

The rotation of the sprinkler is caused by the impact of an oscillating swing arm against the body of the sprinkler. By rotating in regular forward moves, the sprinkler can wet consecutive sectors of a circle. Each movement of the sprinkler irrigates the next consecutive sector of the circle until the entire area has received water. The nozzles of a normal impact rotary sprinkler are made of brass and wear relatively quickly (more so when fertilisers are applied through the sprinkler.)

Regular checks on nozzle wear and periodic replacement of nozzles are needed. It is important that nozzles are replaced with nozzles of the same type and outlet size, or sprinkler delivery and distribution patterns will also change.

Pressure plays an important role in the distribution pattern of the sprinkler. If the pressure is too low, the water jet does not break up easily and most water falls in a donut, some distance from the sprinkler. The correct pressure will produce a distribution pattern which will allow for the best possible final overlapped distribution of water between a series of sprinklers spaced at the chosen distance apart.

High pressures will cause the jet of water to break up too much. A fine mist will form, which can easily be driven away by wind. Most water will also fall close to the sprinkler.

This series is published with acknowledgement to the ARC Agricultural Engineering for the use of their manuals. Visit www.arc.agric.za for more information.