Minisprinkler microspray irrigation for orchards
Note Number: AG0296
Published: September 2000
Updated: January 2010
Minisprinkler and microspray irrigation provides a method to apply water to trees in an orchard with a high level of precision making these systems more efficient than traditional flood or sprinkler irrigation. A basic understanding of minisprinkler and microspray irrigation is important so that systems are appropriately designed, installed, operated and maintained.
What are minisprinklers and microsprays?
Minisprinkler and microspray systems deliver water under pressure to a tree by an emitter that is connected to a lateral pipe. There is generally one lateral pipe per row and these are connected to a submain. Emitters available are commonly known as microsprinklers, microjets, minijets, minisprays and sprayjets. Such terms are best understood by realising that a sprinkler uses a moving part to distribute the water while a jet or spray has no moving part but rather interferes with the water stream to cause it to be distributed over an area.
The prefixes, micro and mini, signify that such emitters have discharge rates of less than 200 litre per hour. The properties of such emitters as they relate to orchard irrigation are as follows.
- Operating pressure. Almost all the above emitters will operate from pressures as low as 75 kPa up to 400 kPa. Because of the cost of pumping it is usual to operate them between 100 kPa and 150 kPa.
- Discharge rates. Commercially available emitters discharge water at rates from 20 litre per hour to 200 litre per hour. This wide range of rates allows the selection of an emitter to suit the particular requirements of irrigation frequency and soil type. The lowest rates would generally require two or three irrigations per week.
- Water distribution. There are as many patterns of water distribution as there are emitters. For this reason it is essential that the person designing the irrigation system is familiar with the various types of emitters.
Most of the spray or jet emitters apply water over small areas (diameters of 2 metres) within which are localised areas receiving much heavier applications. Consequently, it is important that the areas receiving the bulk of the water are in the vicinity of the plant roots.
Minisprinklers generally apply water over a larger area (diameters from 4 – 10 metres) and, apart from an area near the emitter which receives more water, the water is uniformly distributed. Such emitters can be used for a wide range of crops and planting distances.
- Application rates. Rates are calculated from the discharge rate and the wetted surface area. Again there is a wide choice. Rates can vary from 2 mm per hour to 50 mm per hour. Many minisprinklers apply water at an average rate of 2 to 5 mm per hour.
- Lateral sizes. Because these emitters discharge water at a greater rate than most drip emitters, the piping has to be larger. Furthermore, most minisprinkler and microspray emitters do not compensate flow rate in response to variation in pressure. This leads to increased cost of the system.
By increasing the pressure the variation in flow rate can be reduced and systems can be designed with smaller pipe sizes, minimising the capital cost, however the running cost will increase proportionally with the increase in pressure. There have been instances of growers successfully changing from drip irrigation to minisprinkler irrigation by merely changing the emitters and raising the operating pressure slightly. This depends, however, very much on the particular orchard and the existing submain and lateral sizes and layout.
- Filtration. Because minisprinkler and microspray emitters use orifices of diameters of 1 mm or more, blockages caused by silt sized particles or algae should not occur as frequently as with drip irrigation.
However, filtration is still required since plant fibre in the water is very often of 1 mm size and can become lodged in the orifice. Mesh filters finer than 30 – 40 mesh should be satisfactory, and chlorination should still be carried out occasionally as a safeguard.
- Period of operation. The maximum length of time that the irrigation system is operated depends upon the soil type (the water holding capacity of the soil), the irrigated root depth, the soil dryness when an irrigation is due, the emitter discharge rate and the number of emitters per tree.
The irrigation interval depends on how quickly the plants use up the irrigation water applied. That is, the orchard water use. Orchard water use is calculated from pan evaporation (or reference crop evapotranspiration - ETo), crop factors (or crop coefficients - Kc) and the planting square.
Soil moisture should also be measured to help determine when an irrigation is due and to avoid over or under irrigation. Crop factors may need adjustment due to local conditions and measuring soil moisture will aid the adjustment.
Comparison with drip systems
The main difference between minisprinkler/ microspray systems and drip systems of irrigation is the wetting of a larger soil volume by the spray or jet emitters. This occurs by virtue of the water being distributed over a larger area of soil; drip systems apply water to the one point and rely on the soil properties for distribution of the water.
The wetting of a larger surface of soil is important on sandy soils where little lateral movement occurs within the soil, and on some clay soils where cracking of the soil is severe.
The wetting of a larger soil volume should result in bigger trees but not necessary more productive trees. Wetting a larger soil volume makes for a safer system because the interval between irrigations is longer and hence, there is less risk from excessive soil dryness if, for example, the pump breaks down.
Comparison with conventional sprinklers
All forms of micro-irrigation offer advantages over conventional sprinklers in that micro-irrigation systems apply water to each tree individually along the tree-line where the concentration of roots is the highest. Compared with sprinklers there is considerably less evaporation from the understorey because less of the soil surface is wetted.
A system with one mini-sprinkler per tree (or two trees in the case of close plantings), plus hilled-up tree-lines for maximum root growth and surface drainage, appears to offer many advantages as a system of orchard management. However, because of the great range and cost of the emitters available, and the conditions under which they operate, it is most important that the irrigation system be designed by someone who is familiar with the particular irrigation method. This ensures efficiency of operation and minimises the overall cost of the system.
This Agnote was developed by Ian Goodwin, Future Farming Systems Research in September 2000.
It was reviewed by Ian Goodwin, Future Farming Systems Research in January 2010.
Published and Authorised by:
Department of Environment and Primary Industries
1 Spring Street
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