Choosing an orchard irrigation system

• With trickle irrigation there may be excessive losses of water through very permeable soils (if water is applied at night), and through cracking clays if the soil is allowed to dry between applications. In such instances, microjet emitters may be more appropriate.
  In other situations, microjet irrigation is likely to suffer higher losses than trickle irrigation because of wind drift and evaporation. However, these losses would be less than with other conventional forms of irrigation.
• Constant ideal moisture conditions for root growth in a restricted volume of wetted soil result in development of a dense mass of roots which will make the plant drought sensitive in times of water shortage. Under these circumstances the crop may suffer moisture stress if there is equipment failure or some other unavoidable reason for withholding water.
  The problem can be minimised by using a microjet emitter to increase the reservoir of water held in the soil after irrigation and reducing the frequency of application accordingly .
• There is often a fear that if the rootzone is too small the tree may lack stability in high winds and blow over. However, trees immediately following flood or full scale sprinkling would be more unstable in high winds than trickle or microjet irrigated trees since the whole of the rootzone would be located in soft, water saturated soil. Roots of trees under trickle and microjet irrigation are not entirely restricted to the wetted zone.
• Low oxygen concentrations have been measured in the saturated zone of poorly drained soils that are trickle irrigated and, in extreme cases, trees have died because of waterlogging.
  This problem can be avoided by applying water at night and by using tensiometers to match the amount of water applied to the amount of water required. Night application will ensure that the maximum volume of soil will be wetted since plants do not use water at night, and plant use will rapidly reduce the soil water level below saturation when the system is off during the day.
• Blockages can be a major problem with micro-irrigation systems. Microjet emitters have flow paths of comparable diameter to trickle emitters but are much shorter and so the speed of flow is much faster. For this reason they are somewhat less prone to block than trickle emitters.
  Microjets are also more readily observed in operation than trickle emitters and so blockages are easier to detect. However, whatever method of micro-irrigation is used, good filtration practices are required.
• Microjets may be knocked from their correct alignment during pruning and harvesting operations. This may result in water being applied to areas that would not normally receive water and where few active plant roots grow. The use of two half-circle microjets attached to galvanised brackets which are nailed to the tree (one on either side) is one way of overcoming this problem.
  In high density orchards the irrigation lateral and emitter is often raised above the ground, where knocking from the preferred position is minimised.
• Microjets are likely to wet tree butts; this may cause disease problems in certain situations. Experience in the Goulburn Valley suggests that this is not likely to be a major problem.
• With microjet emitters trees usually only need to be irrigated twice a week to maintain water supply at a tensiometer reading not higher than 50kPa. Growers therefore find it an easier method than trickle.
• Microjet irrigation is usually more expensive to install than trickle irrigation because of the larger pipe sizes required, and more expensive to operate because of the higher pressure used.

• cost
• filtration needs (especially on very permeable soils where a large number of outlets, each with a very low flow rate, is required for trickle)
• ability to vary the time of application to night or day depending on whether the soil is poor-draining or free-draining
• willingness to provide a facility to chlorinate the lines
• reliability of the water supply.