by Gennaro Vellotti
As water becomes less accessible, Australians will need to look at more efficient ways to irrigate. The current worldwide trend towards the use of recycled water in major urban applications shows great promise, and is increasing as the cost of potable water climbs and treated water becomes more accessible. Large open space areas, like sports fields and ovals, lend themselves to using recycled water.
In recent years, subsurface drip irrigation (SSDI), a practice that first gained its popularity in the agricultural industry, has presented councils and local government with an opportunity to gain water savings and to effectively reuse stormwater, Class B and Class C treated water. SSDI involves the application of water to plants or turf via pipes and driplines that are installed entirely below ground, delivering water directly to the root zone and providing significant benefits over conventional, above-ground irrigation.
SSDI can be used instead of, or as a replacement for, a conventional sprinkler system. Despite the higher initial costs of purchase and installation, there are potential advantages such as reduced maintenance costs and improved water efficiency. Water savings of up to 30 percent are possible when compared to a conventional sprinkler system, by minimising losses from evaporation, overspray and wind drift. A highly desirable feature of drip irrigation is the excellent uniformity of water application. With poor irrigation uniformity, it is necessary to over-irrigate in order to ensure proper coverage of drier zones. The uniformity offered by SSDI reduces the need for this, as conventional sprinkler systems tend to be less efficient in application, due to acceptable industry design tolerances in distribution uniformity, nozzle wear, moving mechanical part wear and vandalism. When comparing both types of systems, with optimum management and operation, a minimum of 10 percent in water savings is achievable through SSDI.
The advantages and disadvantages of SSDI, as illustrated in Table 1, should be considered before selecting an irrigation system.
Table 1: Advantages and disadvantages of SSDI systems
When considering a SSDI system, it is important that it be surveyed, designed, specified and installed by a suitably qualified irrigation professional. A list of designers, installers and retailers can be found on the Irrigation Association Limited (IAL) website ( www.irrigation.org.au ).
Successful SSDI is dependent on the collection of site-specific information to ensure proper system selection and good design. Several factors will guide this decision, including:
The next step is to design the system, taking into consideration important factors such as:
Finally, a dripline with ’anti-siphon’ characteristics can ensure air enters the system through the vacuum breakers and not the drippers.
While this approach may seem rigorous or overly conservative, an SSDI system can be less forgiving and more difficult to deal with if issues arise. Investing additional time into the initial planning and design phase will ensure the system layout is simple and easy to operate and maintain.
When considering an SSDI system, it is important to incorporate an injection system for acid, hydrogen peroxide and/or chlorine to clean the system. Chemical treatment should also be used to control and avoid root intrusion. It is best practice to benchmark a system after installation by recording pressure and flow during operation against the actual design of:
While it is common for these to vary slightly from design, large variations should be addressed. Benchmarking will help in regular checking of the system during maintenance and for future system diagnostics.
With SSDI, it is important to have a good maintenance regimen that includes regular, adequate flushing. As such, flushing manifolds should be designed on each zone, grouping as many dripline laterals together within design guidelines to make flushing quicker and easier. Additionally, dripline run lengths should be based upon their ability to be flushed.
Tea Tree Gully Council and Alexandrina Council in South Australia, and Glen Eira Council in Victoria have all successfully used SSDI.
The City of Tea Tree Gully has used SSDI in many of their ovals and has experienced water savings, reduction in weed growth, reduction in lawn mowing frequency, reduction in line marking (lines do not wash away with SSDI), reduction in system maintenance due to reduced vandalism, risk reduction and extended playing time on the fields.
Alexandrina Council in Port Elliot installed SSDI in a football oval in 2008. The system utilises Class B recycled water from the nearby Port Elliot Wastewater Treatment Plant. Benefits realised have included water savings, use of recycled water, reduced vandalism, reduced risk and extended playing time on fields.
Glen Eira Council installed SSDI at Marlborough Reserve soccer ground in 2007. As a result of the success of this installation, the Council installed another SSDI system at Murrumbeena Park Oval 1 in 2008, with capacity designed into the system to utilise rainwater in the near future.
The comparison below considers a 10,000-square-metre (1 hectare) area over a 10-year period, and utilises climate data from the Bureau of Meteorology in Adelaide.
Annual rainfall is 546 millimetres, with annual evaporation of 1,496 millimetres; leaving an annual deficit of 950 millimetres. For kikuyu turf with good appearance and strong growth, the crop factor would be 0.7, resulting in an annual irrigation water requirement of 665 millimetres. This equates to 665 litres per square-metre, or 6,650 kilolitres annually for the test area.
Comparing the following two systems:
The following assumptions have been made for the comparison:
($ over 10 years)
|Pop-up sprinkler ($ over 10 years)|
|Survey, design and specification of irrigation system||$2,500||$2,500|
|Supply and install irrigation system (10,000 square metres or 1 hectare)||$65,000||$45,000|
|Maintenance and flushing for SSDI/maintenance for pop-up sprinkler head adjustment||$7,000||$3,000|
|Sprinkler repairs from vandalism and wear (10 percent per annum)||N/A||$6,000|
|Annual water cost ($2.98 per kilolitre)||*$208,600||$247,900|
|Cost of litigation due to injury||Minimal||**High|
|Total cost over 10 years||$283,100||$304,400|
|Total cost per square-metre||$28.50||$30.50|
*It is important to remember that recycled water available for use would more than likely suit a SSDI system without affecting access to the field or park, reducing the annual water cost.
**The cost from litigation and/or injury due to exposed sprinkler heads is always a risk; this should be seriously considered, though putting a price on it is difficult.
As evidenced by the success of those councils and governments that have ventured into SSDI, and the cost comparison above, this technology shows true promise for councils that are working to become more efficient with both dollars and resources.
Gennaro Vellotti is the South Australia-based Water Resource Design Manager for MWH. Gennaro is a Certified Irrigation Designer (2570 – Landscape Turf Commercial; Agricultural Drip Micro; and Agricultural Sprinkler) with extensive experience in SSDI applications. This article first appeared in the February/March 2011 (Vol.4, No. 4) issue of Sustainability Matters (http://www.sustainabilitymatters.net.au/).