Record evaporation rates across eastern Australia exacerbate drought
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Record evaporation rates across eastern Australia exacerbate drought

Australia's evaporation levels are running at record rates, especially across eastern states, increasing the misery for drought-hit farmers and raising bushfire risks as the mercury starts to climb.

While rainfall deficiencies have drawn much attention, stronger-than-usual winds, abnormally sunny days and low humidity have combined to push up evaporation levels, Bureau of Meteorology data shows.

Cold can deceive: Just because temperatures are low – such as this empty dam near Oberon in the NSW central west – it doesn't mean moisture loss from evaporation is low.

Cold can deceive: Just because temperatures are low – such as this empty dam near Oberon in the NSW central west – it doesn't mean moisture loss from evaporation is low.Credit:Dean Sewell

Across the nation, evaporation last month averaged 145.21 millimetres, well above the 128.6 mm typical for July, and the most on record for data going back to 1975, said Karl Braganza, head of climate monitoring at the bureau.

The national tally beat the previous record in 2002. On a regional level, the evaporation rate was the highest on record for Victoria, and also smashed previous records for eastern Australia as a whole.

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Temperatures, wind speed and humidity are the key to evaporation, a contributor to drying out "that is often overlooked" as drought starts to bite, Dr Braganza said.

He said the wind chill people feel on cold days is the result of surface moisture being lost from the skin, illustrating how evaporation can still have an impact amid low temperatures.

As it happens, southern Australia is also having its windiest August in at least 10 years, he said.

Drying out

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While parts of drought-hit NSW may get their best rain in weeks in coming days, the falls could quickly be cancelled out by high evaporation rates.

For instance, Sydney – which may collect as much as 30 millimetres over three days from Friday on current forecasts – has been averaging daily evaporative water loss of about 5 millimetres so far this month. Sunshine has been running at 9.4 hours a day.

That compares with just 1.9 millimetres during a typical August day for the city, with 7.1 hours of sunshine. Evaporation loss for Sydney is more than 110 millimetres, with just 1.4 millimetres falling in the rain gauge.

Brisbane has also recorded high evaporation rates. Water loss totalled 13.4 millimetres on Monday, or triple the daily average so far this month, and well above normal for this time of year.

Record smashed

The issue of rapid evaporation is hardly a monthly matter. So far this year, evaporation across eastern Australia has smashed all previous records dating to 1975, reaching 1618 millimetres on average. That easily exceeds any previous January-July period by more than 200 millimetres, the bureau said.

The evaporation drain on available moisture in the landscape will likely get worst with spring's arrival.

"As you get into a drought, soil moisture dries out and temperatures become more and more important for evaporation," Dr Braganza said.

That could be bad news not least because daytime temperatures are already running at a record high nationwide, including for NSW. Australia's January-July maximum temperatures were slightly hotter than the previous record set in 2005, the bureau said.

'Very thirsty'

Tim Duddy, a farmer at Caroona on NSW's Liverpool Plains, said the deep frosts have made things worse this winter. "It pulls moisture out of the soil," he said.

Topsoil down to 1.5 metres has "no moisture whatsoever", he said, adding that even 50-60 millimetres of rain would be rapidly absorbed by "the old black soil that is very, very thirsty".

"There wouldn't be much evidence of that [rain] in about a week's time," Mr Duddy said.

His farm opted not to plant a winter crop and may skip a planting for summer if good rains don't return.

Fire risks

Fire authorities have continued to battle dozens of fires, particularly in NSW, and are aware of the elevated risks linked to the drying out of fuel, particularly forests.

Richard Thornton, chief executive of the Bushfire and Natural Hazards CRC, said NSW had been experiencing dry conditions for the past two years, especially so in the past six months.

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"When preceding conditions have been like this, and the bush and grass is so dry, it doesn’t take much for a fire to get going once the wind is up, regardless of the season," Dr Thornton said.

Firefighting resources are being stretched over time, with evidence bushfire seasons getting longer, he said.

“With fire seasons changing and overlapping across the globe, we need to change and think of new ways of dealing with bushfires, floods, cyclones and heatwaves," Dr Thornton said. "We need to start preparing now for these future risks – we cannot keep doing things the same.”

Peter Hannam is Environment Editor at The Sydney Morning Herald. He covers broad environmental issues ranging from climate change to renewable energy for Fairfax Media.