While much of Australia has received average to above average rainfall over recent months, parts of Australia such as western Queensland are in the middle of a drought.
Drought has been a feature of the Australian climate throughout its recorded history. But compared with other parts of the world, Australia is not as dry as you might think. None of the drier parts of Australia average less than 100 millimetres of rain a year, while parts of the Sahara and South America’s Atacama desert receive less than 10 millimetres.
Australia is not as consistently dry as some of the world’s great deserts, but year-to-year variation of rainfall in Australia is high by global standards. Many parts of Australia have received more rain in a month than is normal in a year, or gone a year with less than half the normal rainfall, many times. Droughts occur at the dry end of the weather see-saw; the other extreme is often characterised by floods.
Drought can be considered on a number of timescales. The Bureau of Meteorology has typically focused on monitoring short-term rainfall deficiencies for the advice of government and related industry sectors.
Such deficiencies are especially important for dryland agriculture, as they typically last for periods extending from a few months to about a year, linking to annual cropping or pasture growth cycles.
Annual time scales also allow us to compare the relationship with El Niño events. El Niño events greatly increase the risk of drought in eastern Australia, and have typical lifespans of nine to 12 months.
In addition to short-term “agricultural” droughts, we also recognise long-term droughts, spanning periods of several years when rainfall is well below normal.
Long-term droughts are especially significant for large water storages (and the agricultural and urban users who depend on them). This is because they often take the form of a number of individual severe drought years, embedded in a longer span of below-average to near average years, which do not give the storages a chance to recover.
The so-called “Millennium Drought”, which affected much of the Murray-Darling Basin between 2001 and 2009, was of this type, with two severe drought years (2002 and 2006) and the remaining years recording near-to-below-average rainfall. While the individual years in 2002 and 2006 were very dry, it was the failure of recovery during the intervening period which set this event apart from most of the past.
The end of the Millennium Drought
2010 and 2011 were extremely wet years through large parts of Australia, and drought had disappeared from the map of eastern Australia by early 2011. Unfortunately, not all areas shared the drought breaking rainfall, with 2010 an exceptionally severe drought year in the southwest of Western Australia.
Early 2012 was also very wet in many areas, especially the inland southeast which experienced severe flooding, before widespread drier conditions through the middle of 2012 as a La Niña event broke down.
Drought returns to eastern Australia
Over the last 12 months, drought has returned to substantial areas of inland eastern Australia.
The area most substantially affected has been inland Queensland. Most parts of the state that are more than 300 kilometres from the east coast have had rainfall well below average since late 2012. Over large parts of this region, the 12 month period to November 2013 has been in the driest 10% on record. Over most of this region, rainfall for the last 12 months has been 40 to 60% below average, and in a few areas it has been as far as 70% below.
While the bulk of the drought area is in Queensland, it has also extended across borders to neighbouring parts of northern inland New South Wales, the eastern Northern Territory and northeastern South Australia.
The background for the current drought was set during the summer of 2012-13. As a tropical or subtropical region, inland Queensland gets the bulk of its rainfall between December and April; in the south about 60% of the year’s average rainfall occurs during this period, increasing to 80% in the state’s northwest.
While flooding made headlines in Queensland in early 2013, most of this rain was associated with Tropical Cyclone Oswald. Little of the rain which caused that flooding made it west of the Great Dividing Range, and summer rainfall was well below average over most inland regions.
Most parts of western Queensland had less than 50 millimetres during winter and spring, usually a dry time, and some sites in the far west – as well as in the Northern Territory and South Australia – failed to reach double figures for those six months. Moomba, in the far northeast of South Australia, has had only three millimetres since the start of June.
The current drought is meteorologically significant, but 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.
The last 12 months have been the second driest in 47 years of records at Mount Isa, and rank between sixth and tenth driest in 100 years or more of records at locations including Croydon, Tambo, Longreach, Injune and Bourke. In general, the last year that saw a similar level of dryness in western Queensland was 2001-02.
Averaged over western Queensland as a whole, rainfall for the last 12 months has been 48% below average, the sixth lowest since 1900 and the lowest since 2001-02. The region’s worst drought year was in 1901-02.
We will now be watching to see if the 2013-14 summer wet season is more productive than its 2012-13 counterpart. Heavy rains have fallen in the northern tropics in the last two weeks, but have so far only had a marginal impact on the drought areas outside their northern fringe.
The Bureau’s Seasonal Climate Outlook leans towards slightly towards drier-than-normal conditions over the region in summer, although not as strongly as it did in spring.