Subsurface textile irrigation

Geotextiles were used for capillary irrigation not long after World War II. However, it was not until 1995 that extensive research was conducted at the CSIRO Division of Land and Water in Griffith, Australia, by Grain Security Foundation Ltd, that SSTI was established as a serious commercial alternative to drip.

Polyester geotextile of specific thickness and manufacture is required to ensure the system has the appropriate flow characteristics and so that it does not become hydrophobic (repels water).

Studies were done on many forms of geotextile using various dripper rates and configurations to evaluate water flow for the major soil texture types and to determine if SSTI had the same problems experienced by subsurface drip systems (SDI). Specifically, considerable focus was placed on the problem of root intrusion, tunnelling, dripper blockage and insect damage (serious problems associated with SDI).

Research data showed that roots could invade the geotextile but did not thrive nor thicken in the polyester geotextile[2]^:17 and, therefore, caused no damage. The roots were deterred from entering the dripper area because that area is dry relative to the rest of the system. Roots simply went elsewhere.

Tunneling (the process whereby water works its way up to the surface) was nearly eliminated by the narrow reflective impermeable tape above the drip line.[4]

Drippers are often blocked in bare drip systems due to soil lodging in the emitters and/or soil crusting around the emitters when the system is not operating. SSTI eliminates this problem because there is a physical barrier presented by the geotextile and due to the fact that the soil remains moist for much longer than drip systems (i.e. the soil does not form a crust anyway).

Similarly, insects are deterred from damaging SSTI systems because of the geotextile barrier.

Initially solid, thick-walled drip pipes were used in trials at the CSIRO but subsequent trials between 1995 and 1998 demonstrated that drip tapes (thin-walled, flexible pipe) could be used very effectively. The loss of strength from using thin-walled drip tapes was countered by the additional tensile strength of the entire system (base layer, drip tape and geotextile). While significantly reducing the cost of SSTI, the use of drip tapes also permitted the use of large diameter drip lines (of up to 35mm) allowing for run lengths up to one (1) kilometre. The thin, flexible wall of drip tape also meant that master rolls of manufactured product could be much larger (over 600m in length).

The width of SSTI products varies based on the application. However, at the University of Queensland, Australia[3] trials were conducted to discover the optimal width of the SSTI in black cracking clay with alfalfa sown across a single SSTI line. The SSTI line was 20 cm wide and was installed 30 cm below the surface. The alfalfa germinated directly above the SSTI line but covered 75 cm either side (a total of 1.5 m) in uniform lines. As a result of this research, most SSTI systems are between 6 cm and 20 cm wide. However, some products are up to 2 m wide.

In 1997 the first SSTI plow was tested at CSIRO Griffith[5] using a standard three point linkage behind a tractor. This demonstrated that SSTI could be installed at 20–40 cm deep quickly and easily using the plow. SSTI plows are now available to install 1, 2 or 3 lines (more lines are possible).

In 1996 the first ebb and flow mat technology for potted plants was commercialized based on SSTI technology using drip tapes to control the water delivery. This ebb and flow mat form of SSTI proved very effective in producing potted plants, sprouted wheat and barley for animal production and for research purposes in producing seed varieties without the use of any overhead irrigation.[5] It also provided an effective fertilizer delivery platform.

The advantages of SSTI are:

• SSTI is a “permanent” solution if maintained properly. The components are inert and, given that they are situated below the ground, are not subject to the effects of weather, animals, machinery, vandals or other terrestrial conditions.
• Water savings of 50-75% compared with overhead systems[1]
• Low pressure requirement (also means lower power requirements)
• Yields can be improved up to four (4) times in certain crops
• Minimal root intrusion to drippers in the SSTI with a deflective tape on top[3]
• No emitter blockage due to crusting
• Minimal effect of evaporation
• Safe use of recycled or treated water
• Can use the field (for recreation or agriculture) while irrigation is running
• Weed growth is minimized because the water does not reach the surface (saving herbicide cost). Germination of weeds only occurs during rainfall.
• Fertigation can be done directly to the root zone (saving fertilizer cost)
• Efficient distribution of nutrients to the entire root zone
• Broad wetting patterns (moisture covers the entire root zone)
• Water delivery can match the natural capillary rates in soil so saturation is minimized
• Soil moisture can maintained at field capacity (minimised gravitational losses)
• No surface run off
• Soil erosion is minimized
• Fields do not have to be perfectly level
• Fields with irregular shapes can be accommodated
• Distance between lines of SSTI is far greater than drip (lower number of solenoids and other components compared to sprinklers and drip).
• SSTI laterals can be ploughed in behind a tractor on large sites (over 10,000 m per day)
• Foliage remains dry (fungal and bacterial leaf disease is minimized)

The disadvantages of SSTI are:

• Initial capital cost is typically more than overhead irrigation
• Quality installation is critical. If mistakes made, they are difficult to find.
• Installation using correct fittings must be done
• Regular maintenance is required to ensure long life
• Automated control and monitoring systems are preferable (subsurface irrigation does not give any visual indicators to show if it is working or not)
• SSTI is usually not UV-treated so it must be kept out of sunlight until it is installed under the surface
• Temporary overhead watering may be required to establish turf in hot areas
• Germination of some agricultural crops may require overhead watering if insufficient rainfall
• SSTI cannot apply fertilizers or herbicides overhead on the surface
• Rodents may damage the system (although less than drip systems)