Published: 12 Aug 2022

The Tasmanian Institute of Agriculture (TIA) and the Tasmanian Agricultural Productivity Group (TAPG) will join forces to investigate improvements to centre pivot irrigation application on a new project that continues a history of the two organisations partnering on projects of importance to Tasmanian agriculture.

The new project will evaluate the opportunity and capacity to develop a means to retrofit existing centre pivots  to improve their effectiveness, efficiency, and resilience during drought.

Centre pivot irrigation is the most common form of irrigation in Australia, and the fastest growing type of irrigation globally.

It is estimated that in Tasmania alone around 3500-4000 centre pivot irrigators have been installed in the last 25 years .

However, centre pivot irrigators are not well suited to soils with low infiltration rates (<50 mm/hr). In low-infiltration-rate soils the rate at which water is discharged on outer spans exceeds the soil infiltration rate causing runoff and limited infiltration.

The solution is to increase the wetted footprint from the sprinklers, or increase the time that water is applied at any point around the pivot so sprinkler output can be reduced.

The project received funding through the Australian Government’s Future Drought Fund: Drought Resilience Innovation Grants Program and will be coordinated by Dr Marcus Hardie, University of Tasmania, soil hydrologist and AgTech leader, TIA, and operated by Dr John McPhee, agricultural engineer, contracted through TAPG.

Dr Marcus Hardie (left) and Dr John McPhee.

Dr Hardie has extensive R&D experience in soil-water science, including in relation to infiltration processes, irrigation efficiency and soil water modelling.

Dr John McPhee has 30 years of experience working in the irrigated vegetable industry in Tasmania across a range of production environments and landscapes.

“Many of Tasmania’s agricultural soils have low surface infiltration rates, due to a combination of inherent properties, carbon loss, water repellence, crusting and surface compaction,” Dr Hardie said.

“This results in irrigation application rates which often exceed the infiltration rate of the soil, especially on spans longer than 300 – 400 metres and when the time available for irrigation is constrained, such as when using night electricity tariffs.

“This results in runoff, erosion, waterlogging in low lying areas, and drought in elevated areas.

“Growers are only able to apply between 5-8 mm irrigation a day, which is barely able to keep up with daily evapotranspiration over summer, resulting in what dairy farmers have coined the ‘green drought’, when irrigation can keep pasture green but not supply sufficient water to maximise growth.”

One concept being considered is retrofitting additional trailing spans to a centre pivot, creating a ‘Y-span’ design. Modelling indicates that increasing the wetted footprint through such a design has the potential to reduce runoff from around 35-45% to less than 1 per cent of irrigation and increase the amount of water stored in the soil from 5.5 mm to 9.5 mm per day.

“Such reductions in runoff and improvements in soil water storage would make a huge difference to growers who have problems in achieving adequate infiltration under the outer spans of pivots,” Dr McPhee said.

Importantly, the ability to store more water in the soil with each pass increases drought resilience as it enables growers to “catch up” if the start of the season is late or there is a prolonged dry period, or if irrigation falls behind because of breakdown.

The grant will allow the team to evaluate the potential of developing retro-fit centre pivot options as well as develop the knowledge and partnerships required to enable submission of an Innovation Grant to design, build and evaluate potential solutions, such as the Y-span design.

Much of the engineering required will involve retrofitting existing components, developing sub-main connections, regulating flow rate and pressure, and re-orientation of sub-main wheel tracking.

“The likelihood of success for this project is quite high,” Dr Hardie said.

“The basic problem is known - instantaneous application rates under centre pivots are higher than soil infiltration rates, leading to increased runoff and poor irrigation efficacy.

“Not only does this waste water and energy, but also results in further soil damage, and sub-optimal crop production, leading to low water-use efficiency in what should be highly productive irrigation enterprises.”

The project will build on existing modelling of irrigation responses and on-ground experience to identify irrigation design and management options that can reduce the impact of poor infiltration.

It will allow for the development of a pathway to move from problem definition to design concepts, then to modelling and participant evaluation to determine which options are most likely to have success in a commercial operating environment.

TIA is a joint venture between the University of Tasmania and the Tasmanian Government.