Irrigation management
Flood irrigation is the oldest and still the most common type of irrigation. Flood irrigation has two advantages. It is easy to manage (low technology) and most of, or the entire root zone, of the plant is watered. However flood irrigation is wasteful of water and is the major cause of environmental damage in the Murray-Darling Basin (the main irrigation areas of Australia are within the MDB). The reason for the wastefulness and damage caused by flood irrigation is the same. Unless the irrigation bays are very short and the delivery time for water is also very short (i.e. there is very good pressure) the water at the start of the irrigation bay will have infiltrated below the main root zone of the plant (and therefore be effectively wasted, or unrecoverable by the crop) before water has reached the end of the bay. Flood irrigation can be slightly improved by delivering water in surges, so that the wetting front has time to cause soil to expand and pore spaces seal, thereby minimising soakage at the start of the bay.
Sprinkler systems can be much more efficient, depending on irrigation timing and management. Short irrigation cycles, especially during the hot part of the day, result in 80% of water being lost to evaporation or by blowing away on the wind. It is probable that 40% of all water applied to lawns in the Adelaide suburbs is lost to evaporation, even accounting for nighttime irrigation. Very long irrigations cause water to soak past the root zone. Remember too, that even after the system is turned off, water will continue to soak through the soil profile.
Drip irrigation is now considered much more efficient than the systems mentioned above. One significant advantage is restriction of water to weeds in the inter-row area. The main limitation on efficiency of drippers is the narrow zone of distribution of water, which results in plants having very restricted root zones. There may also be increased salinity at the margin of the wetted rim. Drippers are also sometimes mismanaged, by having a long duration of operation and inadequate frequency of irrigation.
Subsurface irrigation is increasing in popularity and has several theoretical and practical advantages, especially now that new pipe designs have been developed, with holes that do not easily become blocked. It is possible to operate subsurface irrigation for very short intervals, because the pipe is already located within the root zone, so effective delivery of water starts immediately. Other advantages are: there are no surface pipes to tangle equipment, there are no evaporation losses, no water is present at the surface to encourage weeds and irrigation can occur simultaneously with other management activities on the surface (eg pruning, harvesting etc). The main disadvantage of this system is that it is prone to mismanagement, as it occurs almost entirely out of view of the operator. Water is most effectively delivered in subsurface irrigation if it is delivered at high flow rates, in short bursts, to encourage lateral distribution rather than deep penetration.
