Sprinkler characteristics greatly influence sprinkler performance in achieving a satisfactory application of water.
Types of sprinklers.Sprinklers vary according to their method of generating droplets, and die water pressure at which they normally operate.
Medium pressure rotary knocker.Most agricultural sprinklers are of this type, designed with an operating pressure in the range 200-400 kPa (30-60 psi). Water is forced out of single or dual nozzles, breaking up into droplets as it leaves the nozzle. The water hits a rotating striker arm, which performs two functions:
• As the arm strikes the water stream, driven by a spring, it deflects some of the water to a smaller radius, improving the distribution of water near the sprinkler.
• The force with which the arm hits the sprinkler causes the sprinkler body to rotate part of a turn, thereby accomplishing rotation. Spring tension can be adjusted to vary the speed of rotation.
Various sized nozzles are available. Nozzle size(s) and water pressure at the nozzle can be selected to provide the required discharge rate and wetted diameter. This in turn determines the sprinkler spacing for optimum uniformity and application rate over the field. A sample sprinkler performance table is shown in Figure 6.15
The angle of trajectory of the sprinkler is normally from 20-25° from the horizontal. Special low trajectory types are available for watering under trees or in high wind sites.
Nozzle blockage can be a problem; for example insects and spiders can occupy the nozzle between waterings. Frequent checking for blocked nozzles is recommended, as it is for sprinklers failing to rotate or not standing in the vertical position. Sand and silt in die water may cause nozzle wear, but most high quality sprinklers are designed to resist damage to bearings and seals caused by die presence of sand. Plastic sprinkler parts may become brittle with rime and therefore more susceptible to damage.
High pressure rain guns.A single high pressure (400-700 kPa) giant sprinkler is used on travelling irrigators and hose reel machines. It operates on a similar principle to a medium pressure type, but nozzle size and water pressure are much higher, giving a much larger wetted diameter and sprinkler discharge.
Low pressure sprinklers.Rotary knocker type sprinklers require a certain water pressure to drive me striker arm. Low pressure (100-200 kPa) sprinklers use other means to accomplish droplet generation. In many centre pivot and lateral move irrigators, water is directed onto a stationary "anvil", shaped to break die water stream into a fan type spray. Omer devices include spinning discs and spinning arms, often used with microirrigation systems.
Others.Many horticultural and ornamental applications use a reaction type sprinkler. In diis type, two nozzles pointing in opposite directions are carried on die ends of a relatively wide arm. Water leaving die nozzles causes die arm to rotate with die angle of trajectory of die nozzles variable to adjust die rate of rotation. Often, this same mechanism is used to provide power to a cable winch to propel die machine along.
Various types of pop-up and quick coupling sprinklers are available, mainly for turf and grounds watering.
Figure 6.16 Sprinkler irrigation of roses. Many horticultural crops prefer not to have wetted foliage.