Sub-surface drip irrigation and other novel irrigation management tools
What is SDI?
Sub-surface Drip Irrigation or SDI is the irrigation of crops through buried plastic tubes containing embedded emitters located at regular spacings. Above ground drip irrigation (CDI) has been widely used in vineyards for improved water-use efficiency, greater control of wine grape quality and ease of management, however, with rising costs of water and reduced water availability, irrigators are now considering sub-surface systems as a means of saving water.
Why use SDI instead of CDI?
There are a number of claimed benefits of SDI, however many of these benefits are those derived from annual cropping systems where the SDI may only be in place for one season, unlike perennial plantings where the SDI must remain effective for a number of years. Some of the issues grape growers have identified with the long-term use of SDI include termites; foxes; root intrusion; drip tube compression due to soil compaction (reduced flow); salinity build up near the soil surface, displaced movement of soil nutrients over time and soil degradation.
SDI setup
- Necessary elements of an SDI system SDI systems re quire additional elements to the basic design of conventional drip systems. As well as a pump, water filter, chemical injection, mainline, sub-mains and drip-line laterals, an SDI system needs to incorporate additional monitoring/control equipment to track system performance (see Figure 1.) because much of the equipment is buried underground. Figure 1. Necessary elements of an SDI system
Early SDI systems had dripline laterals buried at approximately 10cm. This is suitable for sandy soils, however in clay soils greater capillary movement will bring water to the surface and may result in significant losses from surface evaporation. Also, if saline irrigation water is used this may result in salt being brought to the surface. Burying dripline laterals at a shallow depth also risks damage from cultural operations. A suggested depth range is 10cm for light sandy soils to 50cm for heavy clay soils. Lateral tubing should have thicker walls to prevent tubing collapse through soil compaction. This will also ensure an extended life of lateral tubing, up to about 10 years.
- The filter is the heart of an SDI system High quality water filtration is particularly important with SDI, toensure removal of any soil particles from the water. Once in the system soil particles are not displaced by the next irrigation, but cause blocked emitters that are time-consuming and costly to locate and replace. Experience has shown that there must be adequate air and vacuum release valves to minimize “suck-back” of soil particles when irrigation ceases. Higher flushing volumes are required compared to CDI to ensure drip lines remain clean. A number of design features are incorporated into recent SDI systems to minimize physical clogging, in addition to improved water filtration and regular flushing of the system.
•Mainline and sub-mains are usually buried deeper than the dripline laterals to prevent them from draining when the system is turned off. This will accelerate re-pressurising of the laterals when the system is turned on again, and assist in preventing clogging of emitters.
•Emitters should be pressure compensating, and have an anti-syphoning and self-flushing function to reduce the risk of clogging by soil particles. During installation the dripline laterals should be orientated so that emitters are on top to minimize clogging from any particulate matter that accumulates along the bottom of the lateral.
- Cost of installing SDI vs CDI As an example of the set-up costs of SDI the following table (Figure 2.) shows costs incurred by Brown Brothers Mystic Park vineyard in the Swan Hill district of Victoria. In this case, the SDI required mounding in the vine row to improve water infiltration, and the CDI system was above ground requiring attachment of the drip-line lateral to a wire. SDI establishment cost was almost 30% cheaper, however, this difference may be less in other situations. In general, it would be expected that SDI set-up costs would be lower over an extended period of time. In addition the dripline lateral tubing is laid more quickly, easily and cheaply with specialist machinery.
Figure 2. SDI costs compared to CDI costs
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Wetting Patterns of SDI and CDI
The wetting pattern under SDI allows efficient wetting of the root-zone, as there is minimal surface wetting and capillary rise. High soil moisture is maintained adjacent to plant roots for much longer, and infiltration rates in poorly structured soils may be improved. This could reduce plant moisture stress at critical stages, and remove the need for any over-watering of poorer soils in the vineyard to ensure they are adequately irrigated. A direct comparison between SDI and CDI, from Israeli data with pome fruit, shows higher soil water content and a larger wetted area under SDI, from a similar amount of water applied suggesting a higher water use efficiency.
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Does SDI save water/increase yield?
Trial data from the USA suggests water savings from SDI can be up to 50% (conservatively up to 30%) compared to traditional systems. Higher water use efficiency is thought to occur as a result of less water loss from soil evaporation, mist, surface runoff and wind interference. Deep drainage can be potentially eliminated. Unfortunately potential water savings have not been well documented in viticulture. 6 SDI impacts on soil structure In areas where the root-zone is likely to become sodic, SDI may soil structure problems are emerging on clay soils, particularly where poor quality ground-water is used for irrigation, are in some South Australian regions: Barossa and Clare Valleys, McLaren Vale, the Lower Lakes and the Limestone Coast, and in NSW; parts of the Western Slopes. In the future a switch to SDI could be driven by sustainable soil salinity, soil structure and nutrient issues rather than potential savings in water use efficiency.
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Does SDI save water/increase yield?
Preliminary data from McLaren Vale relating to wine quality, suggests that sodium and chloride levels are slightly lower in vine petioles for SDI, while chloride levels in harvested grape juice are lower. This data indicates there may be a role for SDI in regulating nutrient levels in fruit, however this needs to be investigated further. It is important to note that there is an Australian Food Stan
dards requirement that wine, sparkling and fortified must contain no more than 1g/L as Sodium Chloride (606 milligrams of sodium and 304 milligrams of chloride per litre).
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Weed control with SDI
SDI has a potential advantage over CDI in relation to weed growth and disease, due to a drier soil surface and less humid microclimate under the vine row. Soil and foliage are kept dry, reducing fungal diseases caused by surface drippers.Other research has demonstrated the need for reduced herbicide application, e.g. in almonds.
Other potential production advantages of SDI include:
•With a dry soil surface, cultural operations and harvest can take place while the system is operating.
•No sprinkler heads, pipes or surface drip-lines are present that can cause injury, or be subjected to damage by vandalism, animals or harvest activities in the field.
•Fungicides and insecticides are not washed off by irrigation water and direct delivery through the system reduces waste.
However recent heatwaves have raised the issue of possible higher vineyard temperatures with SDI as there is no evaporative cooling from the dry soil surface.
- How do I know if the system is working?
Fitting additional monitoring equipment and using this to make timely management decisions is critical in an SDI system. A flow-meter and pressure valves should be incorporated at various points in the system, to identify leaks or blockages. Global positioning systems (GPS) can be used to locate various buried fittings.
Emitter clogging:
Three types of emitter clogging are of concern with all drip systems, however these issues are accentuated with SDI where the drippers are in direct contact with the soil. Chemical and biological clogging is also a potential hazard which needs to be monitored and managed.
Root intrusion protection for SDI:
Root growth into emitters has been an issue in SDI systems, especially if the root-zone is dry for long periods of time. Emitters should be designed with a physical barrier; however it may be necessary to use chemical control, applied through the irrigation water or incorporated into the emitter itself. Trifluralin is widely used to stop root intrusion into SDI emitters.
- Other novel irrigation management tools There are a number of new and developing technologies that may have a place in irrigation management. Only two are described here:
Thermal imaging for irrigation scheduling
The use of plant based measurements, instead of commonly used soil based measurements, has long been considered for irrigation scheduling. The potential advantage of plant measurements is that they can integrate a wide range of plant, soil and climatic variables.
The development of cheaper, reliable hand-held Infrared (IR) temperature equipment capable of high speed data recording has enabled the canopy temperature of perhaps an entire block of vines to be simply measured from a quad bike. Algorithms (equations) specific to grapevines are in development to convert canopy temperature to stomatal conductance as a indicator of water stress in vines. High leaf temperatures, in relation to ambient temperatures, are generally associated with lower stomatal conductance.
However, estimating a predetermined canopy temperature to trigger irrigation is currently problematic due to the variation observed within the canopy, during the day and between irrigation cycles. This research is still a work-in-progress.
This developing technology may assist in solving one of the major practical issues of SDI i.e. identifying blocked drippers. It may be possible to integrate IR canopy temperature data with GPS technology to pinpoint vines in a block with higher canopy temperature which might be a tell-tale sign of water deficit caused by blocked drippers. Higher canopy temperatures may also be associated with disease, shallow soils etc.
Polyacrylamide (PAM)
Trial work over a wide range of crops has shown that the correct use of PAM can lead to increased water use efficiency, better nutrient use efficiency and improved cleanliness of drip irrigation systems. PAM slows the rate of vertical water movement through the profile in lighter textured soils resulting in increased lateral spread.
There are some commercial experiences from the Riverland where significant yield increases have resulted form the use of PAM on light sandy soils. Even where there has been no improvement in yield, vines have developed a healthier canopy which gave additional protection during the Jan/Feb 2009 heat wave.
Mark Krstic
Grape and Wine Research and Development Corporation
