Subsurface drip irrigation - an ideal use for recycled water

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 spaces, 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 re-use 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.

Application and benefits

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% 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% 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.

Design considerations

When considering an 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:

• type of application (eg, sport field, oval, park, garden, etc) to determine depth and placement of dripline
• soil type and uniformity across the site to determine dripper flow, emitter spacing and dripline spacing
• frequency and angle of slopes to address pressure and drainage considerations
• climatic data of local area to determine water requirements
• typical uses of the site to consider use of tent pegs, constant heavy machinery or other concerns.

The next step is to design the system, taking into consideration important factors such as:

• flushing velocity to enable proper maintenance and system flushing
• vacuum breakers at high points
• flush valves at low points.

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.

Maintenance and installation considerations

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:

• pumps
• valves
• zones
• driplines.

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 on their ability to be flushed.

Success stories and a cost comparison

Tea Tree Gully Council and Alexandrina Council in South Australia, and Glen Eira Council in Victoria have all successfully used SSDI.

Figure 1: The Marlborough Reserve SSDI installed by Glen Eira Council in Victoria irrigates a soccer ground.

The City of Tea Tree Gully has used SSDI in many of its ovals and has experienced water savings, reduction in weed growth, reduction in lawnmowing 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 uses 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.

Cost comparison

The comparison below considers a 10,000-square-metre (1 hectare) area over a 10-year period, and uses climate data from the Bureau of Meteorology in Adelaide.

*It is important to remember that recycled water available for use would more than likely suit an 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.

Annual rainfall is 546 mm, with annual evaporation of 1496 mm; leaving an annual deficit of 950 mm. 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 mm. This equates to 665 litres per square metre, or 6650 kilolitres annually for the test area.

Comparing the following two systems:

a. SSDI with typical application efficiency of 95% would require application of 700 mm per square metre, or 7000 kilolitres annually.

b. Pop-up sprinkler irrigation with typical application efficiency of 80% would require application of 832 mm per square metre, or 8320 kilolitres annually.

The following assumptions have been made for the comparison:

a. The SSDI system uses a tank and air gap and does not require a reduced pressure zone device.

b. The cost of water of $2.98 per kilolitre for water usage over 130 kilolitres per quarter.

c. The costs of supplying and installing the systems are equal to:

i. SSDI = $6.50 per square metre

ii. Pop-up sprinkler = $4.50 per square metre

d. Both systems incorporate the same level of automation and water monitoring

e. No fertiliser, chemical treatment or site soil conditioning costs have been included as these have been considered to be similar once averaged out.

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.

By Gennaro Vellotti, Water Resource Design Manager for MWH, a global provider of environmental engineering and strategic consulting services. Gennaro is a Certified Irrigation Designer (2570 - Landscape Turf Commercial; Agricultural Drip Micro; and Agricultural Sprinkler) with extensive experience in SSDI applications. For more information, please contact him at Gennaro.Vellotti@mwhglobal.com.