Sprinkler irrigation is the most common form of mechanised irrigation. Water is normally supplied to the sprinkler under pressure.

The term “sprinkler” includes a wide variety of spray systems, from small orifice sprayers to large high-pressure rain-guns. When discussing agricultural irrigation in this article, “sprinkler” refers only to small rotary sprayers that operate at medium pressure (two to four bar) and discharge water through a rotating nozzle into the atmosphere in such a way that a fixed area receives uniform artificial rain.

Sprinkler irrigation equipment costs more than flood systems. Sprinkler systems are, however, extremely flexible and designs can be adapted to operate on any topography. There is no need for costly and ongoing land preparation and levelling associated with flood irrigation.

Sprinkler systems are also less sensitive to poor maintenance or control, though a well maintained and managed flood irrigation system can distribute water more efficiently than a good sprinkler system. If management and maintenance are not effective, a sprinkler system will be considerably more efficient than irrigation through flooding.

This is important. Small-scale and emerging farmers are not likely to use the computerised monitoring systems and laser levelling associated with good flood systems. For these farmers, sprinkler systems will save water and ensure improved crop production.

Labour usage with sprinkler systems will vary depending on the type of system layout selected. For a totally portable system where laterals are moved to new positions three or four times daily, a team of at least three labourers per 10 hectares covered will be needed. A solid-set sprinkler system can be totally automated. A hop-along system on fixed, buried laterals can be operated with one labourer, controlling between 20 and 50 hectares.

The ideal pressure at the nozzle will vary between two and four bar, (200 to 400 kPa) depending on the type of sprinkler and nozzle size. A range between 5% above and 10% below design pressure is normally allowed. If pressures exceed this range, the result will be poor application efficiency.

Certain crops, such as tomatoes, are sensitive and water should not be applied directly to the leaves or fruit while in full sunlight (burns on the skins damages tomatoes). Sprinkler systems cannot be used on these crops.

Next month we shall look at the types of sprinklers. 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.