Microirrigation refers to various systems of watering where attempts are made to supply water to more closely match the plant's daily requirements. Various types of emitters are used, located along polythene laterals, at a spacing matched to the soil type and plant spacing. The system is designed to operate for longer periods of time at low discharge rates. The emitter discharge rate is selected such that the plant's water requirement is replenished at frequent intervals. This constantly maintains the soil moisture content close to field capacity, minimising water stress. Emitters are designed to operate at low pressure, such as 100 kPa (15 psi).
Various types of emitter are available to suit the crop, irrigation schedule and soil characteristics. Most applications require permanent installation, but some systems are installed and removed each season (including drip irrigation of vegetables and cotton).
There are potential advantages of this type of watering:
• Increased yields are possible, since optimum soil moisture levels are maintained.
• Application efficiently is high (over 90 per cent), since the emitter is normally located near or at ground surface, minimising wind and evaporation losses. Laterals can be installed below ground, increasing efficiency even further. Application rates are sufficiently low and controlled that runoff and deep percolation can be eliminated. Consequently, limited water supplies can be utilised most effectively.
• Because most microirrigation systems are permanently installed, labour requirements can be low and the system can be easily automated (even by the use of soil moisture monitors to switch the system on and off).
• Plant foliage is kept dry, reducing fungal problems.
• Interrow spaces are kept dry during irrigation, reducing weed growth and creating minimal interference with mechanization. Consequently, insect, weed and fungal control can be improved.
• Liquid chemicals can be applied through the irrigation water quite easily, with good control over dose rates.
• Microirrigation is suitable for steeper sites, where spray and surface methods are not feasible. Where long laterals are required, or where steeper slopes are involved, pressure-compensated emitters are available to control emitter discharge, and lateral diameters can be selected to give more precise control over water pressures along each lateral.
• In some applications, water which is too saline for spray and surface irrigation methods may be able to be used with microirrigation, as continuous water application can leach accumulated salts to outside the wetted root zone.
There are some limitations to the use of microirrigation:
• Cost is high.
• Most emitters rely on very small diameter orifices or flow paths to give control over discharge rates, and are therefore easily blocked. Filtration of the water supply is essential, and additional measures are necessary, under certain circumstances. Further problems are that algae can grow in the system beyond the filter, clay colloids can be caused to flocculate, or that dissolved elements can pass through the filter and may precipitate due to various chemical processes. Consequently, water quality must be assessed thoroughly prior to installation, so that appropriate strategies can be considered. Emitter selection is very important.
• Poor watering techniques can be introduced. Overwatering is quite possible, even at very low application rates, particularly where sub-soil barriers inhibit water movement. Such problems are often not apparent at the soil surface.