Beating water losses from evaporation

A study which forms part of the Desert Knowledge CRC’s WaterSmart Pastoralism program looks at the scale of losses from evaporation, with particular reference to the pastoral zone, and compares technologies for reducing it.

“When harvested, water is commonly stored in small storages and dams, but it is estimated that up to half of this may be lost to evaporation. This represents a huge waste of our resource,” study author Dr Ian Craig says. “The price and value of water are increasing dramatically and the scarcity of water is the main limiting factor working against agricultural production in Australia.”

“The study indicates that Australia has over a million farm dams and storages which account for roughly 9% of all the water stored — around 7000 gigalitres (7 billion litres),” says Andy Bubb, leader of DKCRC’s 21st Century Pastoralism project.

With such high levels of evaporation in Australia, only about 5% of the rainfall actually becomes runoff and is able to enter dams. Then, dams in the warmer areas — such as Queensland and northern NSW — typically lose 40% or more of their water volume each year to evaporation.

Even if these dams are only full half the time, the total loss could amount to more than 1400 gigalitres (1.4 trillion litres or nearly 3 Sydney Harbours), the study says.

This loss depends on many factors such as the size and shape of the storage, its depth, location and leakiness. Many approaches have been tried to reduce it, including digging farm dams deeper and using windbreaks, but these have limited value as most water loss is due to solar heating.

The study looks at several of the new methods for lowering evaporation — floating blankets, shade cloth, floating modules and thin layers of a chemical to ‘seal’ the surface. It found that high water savings are possible with these methods on dams of 10 hectares or less in extent.

It found the floating blanket of ‘bubble wrap’ could reduce evaporation losses by around 95% for a cost of $5.50–8.50 per square metre and a breakeven price of water from $302–338 per megalitre.

The shadecloth system, stretched on tensioned wires over a dam, can cut evaporation by 60–80% for a cost of $7–10/m² and a breakeven cost of $296–395 per megalitre.

And the chemical monolayer system could cut evaporation by 5–10% for an operating cost of $826–4050 per hectare a year and a breakeven of $130–1200/ML.

In warm areas where primary producers are losing two metres of water from dams to evaporation every year, such systems could pay for themselves at a price of $300–400 per megalitre saved, the report indicates. The chemical monolayer system, in particular, can be used sparingly — just in the hot months.

The report emphasises that every situation is different, depending on the local climate, the size and shape of the dam and the value of the product grown using the water — and the best solution will depend on these varying circumstances.

However, it points out that covering all of Queensland’s estimated 55,000 hectares of farm storages could save as much as 300 gigalitres in evaporation losses every year — and even if only a tenth of them were covered, it would still save 30 gigalitres.

The DKCRC research has aroused strong interest across the rural sector — but also indicates the urgent need for more research into evaporation and ways to limit or prevent it, including developing a better estimate of the extent of water loss from Australia’s farm dams.

Ian Craig says a major outcome of the project was increased awareness of the potential for evaporation reduction on water storages.

“The project has been highly successful in this regard, with significant interest being shown by landholders, agencies and private companies in the work undertaken. The combination of detailed experimentation at USQ, commercial-scale demonstration sites and wide publicity, through field days, workshops, scientific papers and popular articles, has raised expectations on the potential for cost-effective evaporation control solutions. Already a number of private companies, product suppliers and agency funding bodies are exploring the possibilities for further research, product development and commercialisation in this area,” he says.