How severe is the drought in Queensland and New South Wales?
Prime Minister Tony Abbott has announced a $320 million assistance package to support drought-hit farmers in New South Wales and Queensland.
Justifying the package, Mr Abbott said: "Drought of this severity is not the normal course of business. This is not just a once-in-a-decade drought. It's a once-in-a-quarter-century drought in many places. In some places, it's a once-in-a-century drought".
ABC Fact Check investigates whether the current drought really is a "once-in-a-quarter-century" drought or even a "once-in-a-century" drought.
- The claim: Tony Abbott says some places in Queensland and NSW are experiencing a "once-in-a-quarter-century" or even a "once-in-a-century" drought.
- The verdict: The combination of low rainfall and high temperatures has created the most severe drought conditions in parts of Queensland and NSW ever recorded. Mr Abbott's claim checks out.
What defines a drought?
According to the Bureau of Meteorology (BOM), drought is "a prolonged, abnormally dry period when the amount of available water is insufficient to meet our normal use".
Using rainfall as a measure, drought is defined by a period of serious or severe rain deficiency. The Bureau of Meteorology examines rainfall periods of three or more months to see whether an areas falls in the lowest 10 per cent of records. That determines the severity of a drought.
Serious rainfall deficiency is when rainfall measures above the lowest 5 per cent of ever recorded in that area but below the lowest 10 per cent for the period in question. Severe rainfall deficiency is when rainfall is in the lowest 5 per cent for that area over the period in question.
On March 5, 2014, the BOM said: "Serious to severe deficiencies (lowest 10 per cent to 5 per cent of records) persist in areas of western and inland northern Queensland, a large area around the Queensland–South Australia border and smaller areas along the eastern border of South Australia, an area extending from inland southern Queensland through much of New South Wales inland of the coastal ranges and into north-western and north-central Victoria. Deficiencies also persist in an area between Geraldton and Shark Bay on the west coast of Western Australia."
This is shown in the map below.
But rainfall is not the only measure of drought. While meteorologists determine whether or not an area is in drought by looking at rainfall, agriculturalists look at the impact on primary industries and hydrologists compare ground water levels.
A factor that influences the severity of a drought is temperature. The warmer the air, the more water is lost from the soil via evaporation or transpiration via plants, which results in less water remaining available for agriculture and farm animals.
In the past 10 years, the number of extreme daytime heat records in Australia outnumbered extreme cold records by almost three-to-one. 2013 was the warmest year on record since temperature measurements began in 1910.
In its 2014 report, Heatwaves: Hotter, longer, more often, the Climate Council - an independent body that provides expert advice on climate change issues - says climate change can worsen drought conditions.
"Extended periods of heat can increase evaporation rates, leading to increased loss of soil moisture and an intensification of droughts," it said.
"Thus, when droughts occur, the long-term increase in heat in the atmosphere from increasing greenhouse gas concentrations can further intensify the heat that usually co-occurs with drought – one reinforces the other and conditions become hotter and drier."
A 2014 report from the weather bureau says Australian temperatures have warmed by about one degree Celsius, with the majority of warming occurring since 1950.
The State of the Climate 2014 report by the weather bureau and the CSIRO – Australia's national scientific research body - found the frequency of extreme weather in Australia has increased. It projected temperatures would continue to rise, bringing more extremely hot days and fewer extremely cool days.
Soil moisture levels
A recent report by the Federal Department of Agriculture, ABARES, found upper layer soil moisture across summer cropping regions in Queensland and northern New South Wales was predominantly well below average to extremely low. This reflects rainfall received during the 2013-14 summer. The report also found lower layer soil moisture levels at the end of January 2014 in the same cropping regions were largely average to well below average.
Upper layer soil moisture "responds quickly to seasonal conditions and will often show a pattern that reflects rainfall and temperature events of the same month", according to the report. While lower layer soil moisture "is a larger, deeper store that is slower to respond and tends to reflect the accumulated effects of events that have occurred over longer periods of time".
What the experts say
Blair Trewin, a senior climatologist at the weather bureau, says the current drought is "well within the range of historical experience".
"In a broad sense, it can probably be characterised over most of the affected areas as a drought year of an intensity that's typically expected once in 10 to 20 years, although some individual locations have fared more badly," he said.
Dr Trewin says last year was the sixth driest year on record between the Mount Isa region and the New South Wales border. He says it is unusual for so much of Queensland to be very dry at the same time.
This month the Queensland Government declared 79 per cent of the state as in drought - the largest area of state ever recorded to be in drought.
Declared drought areas include Fraser Coast, Gladstone, Goondiwindi, Gympie, Moreton Bay, Toowoomba, and Western Downs. Some areas of Queensland and New South Wales saw record breaking temperatures and dry conditions in 2013.
Professor Albert Van Dijk from the Australian National University agrees with Mr Abbott that parts of Australia are experiencing record drought.
"The Prime Minister is right that it is a more than once-in-a-decade drought for some areas. It may well be true that some areas are experiencing a historic record drought for that particular place," he told Fact Check. But he says other parts of Australia would have experienced similar drought conditions in the past.
Skie Tobin from the weather bureau's climate analysis section says rainfall patterns indicate some drought-hit areas had recently recorded their lowest rainfall totals since records began.
"[There are] some areas where rainfall has been the lowest on record... mostly in western Queensland, inland southern Queensland and adjacent parts of northern New South Wales, rainfall has been in the lowest 5 per cent of records since 1900. Being in the lowest 5 per cent of historical records does mean these conditions are unusual," she said.
Will Steffen, executive director of the Australian National University's Climate Change Institute, says droughts are becoming more severe in terms of their impact because of the increasing hot weather and heatwaves.
He says the current drought is part of a long-term trend towards more severe droughts.
"When droughts do occur (whether this a climate change influence on the drought itself or not), they are likely to become more severe in terms of their impacts because of the increasing heat in the atmosphere," he said.
While other parts of Australia may have experienced similarly severe drought in the past, some of the areas currently in drought are experiencing their driest conditions on record.
The combination of low rainfall and high temperatures has created the most severe drought conditions in parts of Queensland and New South Wales ever recorded.
Mr Abbott's claim that some areas are experiencing a "once-in-a-quarter-century" drought or even a "once-in-a-century" drought checks out.
- Tony Abbott, Lateline, February 26, 2014
- Tony Abbott, media release, February 26, 2014
- Bureau of Meteorology, Rainfall deficiencies
- Bureau of Meteorology, Drought statement archive
- CSIRO, Explainer - What are heatwaves?
- Climate Council, Heatwaves: Hotter, Longer, More Often
- Bureau of Meteorology, Annual climate statement 2013
- CSIRO, State of the Climate - 2014
- ABARES, Australian Crop Report, February 2014
- Queensland Government, media statement, March 7, 2014
- Bureau of Meteorology, New South Wales in 2013: Warmest daytime temperatures on record
- Bureau of Meteorology, Queensland in 2013: Warmest daytime temperatures on record
- CSIRO, Climate Change in Australia - Technical Report 2007
- Trenberth et al, Global warming and changes in drought, 2013