Converting to Drip Irrigation

Drip irrigation applies water to the rootzone via plastic tubing, either with in line or button drippers. In line drippers are either pressure compensated or non – pressure compensated. Options for dripper type, output, spacing and filtration need to be carefully evaluated. In sandy soils more drippers are needed to wet an adequate volume of soil compared to a heavier clay soil, due to a narrower wetting pattern in sandy soils than in clay soils. Good filtration is vital due to the small size of the outlets – clogging due to precipitation of chemicals and /or organic matter (algae and bacteria) can easily occur, however most drippers these days are designed to be self cleaning.

System maintenance should include annual flushing of the system with chlorine or copper sulphate to avoid clogging problems. Drip systems should be flushed regularly to ensure that sediment or algae does not build up and block drippers. This should be done at least three times during the season. In some areas where water is of very poor quality flushing may need to be more frequent.

Fertigation/fertiliser injection can be an effective way of applying fertiliser. As it is being applied to the rootzone alone, little fertiliser is wasted and can be applied more regularly compared to broadcasting fertiliser. This is especially beneficial to young trees, vines and vegetable crops, which require small amounts of fertiliser regularly.

Advantages Installation and pumping costs lower than with other systems. Water distribution to rootzone uniform. Highest water use efficiency of all above ground systems. Weed growth reduced as less soil is wetted Able to implement Partial Rootzone Drying Foliage is not wetted Suitable for Fertigation Potential for precise control of vine/citrus stress Pressure compensated drip suitable for undulating areas Compaction of soils and access by heavy machinery less problematic due to less soil being wetted Drip lines are easily shifted Disadvantages Small openings prone to blockage Cover crops difficult to establish Need of drippers to be close together in soils with poor lateral spread No frost control Additional cost is PRD used Can cause water logging in heavy, poorly structured soils Constant supply of water needed Precise management and frequent use needed. May require automation On sandy soil it may not be possible to wet a sufficient volume of soil Fertigation only way of fertilising (i.e. not Superphosphate as it is not soluble enough) Filter and maintenance level high Higher standard of filtration required Higher volume of back flush water required

Diagram: Typical wetting pattern under drip irrigation Spac e r Diagram: Wetting pattern difference between sand, clay and loam soils

Subsurface Drippers
Subsurface drippers discharge as low as 1l/h through a pressurised system similar to normal drip. The drip line is buried about 30-60 cm deep, depending on the soil type, structure and rooting depth. Due to root intrusion and pinching by the crop, tube wall thickness needs to be considered, as does the location of the tube. Dripper output needs to be carefully matched to the infiltration rate of the soil in order to achieve the desired lateral spread (which can be increased by pulsing the irrigation). Subsurface drippers can re-use effluent as the nutrients and bacteria applied are not exposed to the open air.

Advantages

• See drip irrigation
• Evaporative losses are minimal

• See drip irrigation
• Blockages can not be detected
• Water ‘chimneying’ to soil surface may occur
• Back siphoning of soil particles into the emitter due to inadequate system design