The ANU team drought-proofing staple crops to improve Pacific food security

In Tonga, yam or ufi is not just a tasty and nutritious vegetable, it also holds incredible cultural value. These starchy tubers are purchased for special occasions and even gifted to royalty.

Yams, and other root crops such as taro and cassava, form the basis of people’s diets in Tonga, and across the Pacific. Unlike rice and wheat, these staples don’t need to be imported, making them essential for the food security of the region.

 Agriculture in the Pacific relies almost entirely on rainwater, and for centuries this rain has fallen seasonally.

But these cycles are no longer so predictable.

While this decade began with a rare three-year La Niña, it is only a matter of time before the next El Niño event, spelling dry weather.

Dr Harry Myrans is a researcher with the Australian National University’s Institute for Climate, Energy & Disaster Solutions (ICEDS).

“Climate change is causing longer dry seasons, with even less rainfall in Tonga and the Pacific in general,” he says. “Which means that crops will either die or they’ll need to be irrigated more.”

For sensitive tropical plants, even a short delay to the rainy season can cause losses. Farmers who have spent months cultivating their crops may watch them wither just days before the rains arrive.

In 2015, much of the Pacific, including Tonga, was impacted by drought caused by a strong El Niño. Millions of people faced hunger, water shortages and heightened disease risks. The climate crisis is expected to increase the frequency, duration and severity of these dry spells. This is a particular concern where people are reliant on growing their own food.

That’s why researchers at the Australian National University (ANU) are working to find solutions.

Novel desalination

On Tonga’s main island of Tongatapu, near the small village of Vaini, a team of scientists has established a research site filled with rows of growing yam and taro.

The site sits between groves of palm trees on a plateau with rich clay soil. It’s hot and humid with little shade or wind, which makes cultivating these deep soils hard work.

It’s here that researchers from ICEDS – in collaboration with the Tongan Ministry of Agriculture, Food and Forests, and with funding from the Department of Foreign Affairs and Trade – are investigating whether crops can be irrigated with partially desalinated water during acute droughts.

The team has been exposing the crops to varying levels of salinity,  inspired by emerging desalination techniques, such as thermodiffusive desalination (TDD).

TDD relies on the simple principle that when salt water is placed in a channel where one part is hotter than the other, the salt particles will move to the colder side.

Although desalination has been available commercially for decades, it mostly relies on reverse osmosis – passing seawater through synthetic membranes to produce drinking water. The large treatment plants used for this purpose are not only expensive to build and run, but they are also often powered by fossil fuels and can harm plants and animals through discharging hyper-saline brine – extremely salty industrial wastewater – back into the ocean.

Reverse osmosis is also too expensive to use for agriculture, despite the immense need. Agriculture already uses around 70 per cent of the world’s freshwater resources. Global food demand is expected to grow, while climate change further disrupts the water cycle.

ANU engineer Dr Juan Felipe Torres is hopeful TDD will be able to mitigate some of these problems.

“We would be able to address not only water scarcity but food security,” Torres says.

Torres and his team are still developing prototypes. An off-the-shelf technology that could be rolled out in the Pacific is not yet ready. Some other commercial applications are close to ready though.

Encouragingly, Torres’ team has developed a modular design for the TDD technology that can be 3D printed and assembled on location, with the parts costing less than $200.

The cost of buying energy to power the unit could be an economic barrier for desalination applications. To manage this, the team is exploring the possibility of coupling the units with waste energy from industrial processes.

Climate-proof yams

While the engineering team continues their work, ANU plant scientists have been exploring whether partially desalinated water could be used for irrigation during droughts.

The researchers chose to study taro because, while it is highly susceptible to drought stress, it may be salt tolerant. This means it could benefit from irrigation using slightly salty water, especially if only for a short period of time at the end of the dry season.

Yam was selected because of its cultural significance, water sensitivity and potential salt tolerance.

“The price of yam skyrockets when there’s a drought, but people have to buy it anyway because it’s so important,” Myrans says.

The crop trial team started planting midway through the 2024 dry season. They exposed the taro to four different conditions – no irrigation; irrigation with untreated groundwater; irrigation with ground water that included a small amount of salt added to mimic saline drought conditions; and finally a high-salt treatment. Meanwhile, yams were exposed to either a groundwater treatment or a high-salt treatment. They monitored the crops over their whole life cycle.

When the trial crops were ready for harvest nearly a year later, the team harvested, tagged and weighed the taro corms and yam tubers – some of which were the width of a person’s head.

The initial results have been promising – exposing taro to moderate levels of salt resulted in only slightly reduced yields.

“It suggests that this saline irrigation would prevent plant deaths and significant yield losses during drought,” says Myrans. “And the yams fared even better than we expected.”

This is encouraging news for researchers at ANU who are hurrying to find solutions to the accelerating climate crisis.

“The crops we tested are the ones that people can grow in their own gardens,” says Myrans.

The plant science research team is now preparing their results for publication, while the engineering team continues to work on its prototype. Myrans and Torres are also discussing the possibility of testing the TDD prototype with tomatoes in the lab at ANU.

In Tonga, the plant trials continue and the race is on to prepare farmers before the next El Niño.