Oxygation : optimising delivery and benefits of aerated irrigation water.
Increasing competition on supply of fresh water for irrigation by agricultural, domestic, sports and industrial users demands water use efficient irrigation methods and compliancewith environmental regulations. Drip irrigation (DI) and subsurface drip irrigation (SDI) areadvocated for improvements in water use efficiency (WUE) and are increasingly beingadopted by horticultural industries in Australia and overseas. Greater flexibility for automation and versatility of application of drip irrigation technology encourage wider-scale adoption by these industries. However, the higher initial investment for installation and lack of significant yield gains due to drip irrigation compared to conventional irrigation methods are somehow deterrents for broad-scale adoption. Ways to optimise the use of DI and SDI will have multiplier effects on water savings for irrigation in agricultural and other industries and minimize environmental impacts associated with traditional irrigation methods. One of the significant areas where greater optimization of DI and SDI is realized is through the use of aerated water for irrigation (oxygation). Sustained wetting fronts around emitters associated with DI/SSDI impose hypoxia in the rhizosphere. This impedes root respiration leading to sub-optimal plant performance. As irrigation water exits an emitter, it purges soil pores of soil air (containing up to 20% by volume of oxygen) with water that contains less than 10 ppm oxygen, aquantity we have shown is used up quickly by roots and soil microbes. Rising soil temperatures, salinity, and soil compaction will exacerbate this effect, as may disease such as Phytophthora of pineapple. Plant roots and soil microbes require oxygen for respiration. In soils with inadequate aeration the lack of oxygen results in reduced plant growth and diminished productivity for many reasons, including: reduced root growth and root size; reduced root ability to absorb minerals and water; reduced photosynthesis and plant growth due to stomatal closure; loss of soil N due to the in-activity of microbes; adverse changes in soil chemistry; increased susceptibility to disease, and an alteration of the balance and supply of plant growth regulators. Aeration of the irrigation stream, a process termed ‘oxygation’, overcomes this constraint. Oxygation is a new innovation in irrigation technologies. Aerated DI and SDI by different methods, such as venturi for air injection, allows for the simultaneous application of water, air and other agro-chemicals directly to the crop root zone. Therefore, it can potentially improve crop yield and water use efficiency. Conventional irrigation methods such as flood irrigation have large inefficiencies due to run-off, drainage and evaporative loss. SDI cansignificantly improve the WUE over that of flood irrigation, and oxygation can significantly improve WUE of SDI. Oxygation involves mixing air with water using a venturi and delivering via a surface orsubsurface drip irrigation system. An oxygation system can be installed as part of a new SDI system or may be retrospectively fitted to any existing SDI system. A venturi air injector is installed within the pipeline and draws air directly into the water stream. A single venturi can be installed immediately after the pump outlet and the air distributed through the main line to sub mains and lateral drip lines, or a single injector may be fitted to the beginning of each drip line. The amount of air ingress depends on the pressure differential across the venturi and the motive flow through the venturi.