Micro-irrigation systems are used when water that is sprayed directly onto the fruit of plants can cause damage, to irrigate under trees in orchards, or to save water by providing only the water needed by individual plants, without also irrigating non-productive areas in the field.

When dealing with drippers or simple micro-sprayer systems, the pattern of water distribution on the soil surface is no longer important. Irrigation involves placing water in the root zone of an individual plant. Here a wetted root volume is managed. All the water and nutrients needed by the plant are provided in this zone.

 Drip irrigation

Drip irrigation is most often used on water sensitive crops and on high value crops when water is scarce. Irrigation efficiency is good as water is placed directly into the root zone of individual plants. Surface runoff and evaporation from the soil surface are reduced to a minimum. All active hair roots of the plant develop inside the root manage­ment zone (the wetted bulb formed by water released from the dripper.) The plant searches for all the water and nu­trients it needs in this zone. No water is wasted by irrigating areas between plants or plant rows.

Drip irrigation systems can be adapted to suit a wide variety of crops and growing conditions. Drippers allow for the management of water applica­tion to small areas, and drip irrigation is normally very efficient. The use of drip irrigation is limited mainly by the cost of the dripper lines and the additional cost of filters and control systems required to properly manage the system. A wide variety of drip irrigation systems are used.

Very low-pressure drip irrigation sys­tems supplied by buckets at the start of dripper rows, can be an extremely effective system for small garden plots.  Drippers are also used to irrigate large orchards. Each tree served by the system can be provided with a number of drippers that can supply the plant water requirements at a suitable point of application.

 If the fruit of a high value crop can be damaged by water droplets, or the tree is more susceptible to disease if the trunk gets wet frequently, then drip irrigation is almost always the system of choice.  The distribution of water in the soil below a dripper will vary significantly, depending on soil type and structure.

A number of different types of drip­pers are available on the market. They can be fitted externally to pipelines, be inserted into pipes, or be formed as an integral part of the pipe wall. Drippers can have a normal, open flow path, or contain pressure control mechanisms. Multi-point off-takes can feed micro-tubes that supply individual pen drip­pers inserted into the ground.

The way in which a dripper works to control the flow at an emitter does, however, remain constant, regardless of the type or make of dripper used.

The principles used in drippers can be described as follows:

The dripper is designed to lose pressure, through friction losses, between the inlet and outlet of a long flow path. Pressure at the dripper out­let is very low. Pressure compensated drippers are fitted with an elastic membrane that distorts under pressure to reduce the flow path. This ensures a constant flow rate through the dripper across a range of pressures.

Types of dripper:

Micro-sprayers

Micro-sprayers are used instead of drippers where sandy soils produce long, narrow wetted bulbs. Plants do not easily find a wet column and the volume of water stored in the root management zone becomes too small to manage effectively. A micro-sprayer spreads water above the soil surface in a manner designed to simulate a group of closely spaced drippers operating on the soil surface (in a restricted area). Spreading the water above the soil surface helps to improve water  distribution in the root zone of the plant. Evaporative losses in the air and from the soil surface can, however, be significant.

As with drippers, micro-sprayer sys­tems normally operate at low pressure. The design layout is similar to that of a solid-set sprinkler system. The only labour needed is a person to walk around to check that individual spray­ers are working correctly and to deal with problems.

Micro-sprayers are assembled using a range of components. A hole is punched into the wall of a supply pipe.

A pipe coupling (usually barbed) connects to a small riser pipe.

A spray head is mounted on the top of the riser pipe.

The complete assembly makes up the micro-spray unit that is used to irrigate crops in the field (normally trees in an orchard). Each component could need to be repaired or replaced.

Many manufacturers offer extensive ranges of micro-sprayers. It is not always easy to classify the difference between a micro-sprayer and a mini-sprayer or mini-sprinkler. In principle a micro-sprayer should be designed to operate as a series of drippers, to increase the root management zone. The design principle remains that the sprayer provides only the water needed at a single point in the field, and the effects of overlapping wetting patterns are not significant. An accepted definition of the term micro-sprayer is a riser pipe smaller than 5 mm in dia-meter and outlet nozzles smaller than 2 mm diameter.

Micro-sprayers are loosely classified according to the structure of the nozzle and spreader housing. Common types include:

A-frame sprayers

These sprayers are normally mounted on a rigid riser pipe and operate as free-standing units. Nozzle sizes vary from 0,8 to 2,0 mm in diameter. A range of static or rotating spreader mechanisms can be mounted at the top of the frame. Both nozzles and spreader attachments are normally colour coded according to size and function. This allows for quick and easy identification in the field. Riser pipes are usually of 4 mm diametre HDPE and up to 500 mm in length.

G-type sprayer

This type of sprayer is mounted either on a rigid or flexible riser pipe. Where a flexible LDPE pipe is used, the sprayer is held in position by a pen-type plastic stake. Both configurations are commonly used. The open frame allows for an all-round distribution of water. The spreader mechanism can be fixed or rotating. A rotating spreader (spinner) allows the sprayer to act as would a mini-sprinkler and spray water more evenly across a large area.

Frameless micro-sprayers

Many of the earliest micro-sprayers were simple nozzles without a support­ing frame. Some frameless micro-sprayers are still among the best sprayers on the market for certain applications. These sprayers are most often mounted directly onto the supply line, but they can also be fitted to rigid riser pipes.

The spray pattern of micro-sprayers will vary depending on the type of spreader mechanism used.

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.