Issued 6 January 2009

Short-term rainfall deficiencies ease over much of Australia

Above to very much above average rainfall over most parts of Australia during November and December 2008 has resulted in a significant reduction in the total area of short-term rainfall deficiencies. Drought periods of less than 12 months generally show only small and isolated areas with rainfall totals in the lowest 10% of historical records.

12-month rainfall deficiencies

For the 12-month period from January to December 2008, above average rainfall was recorded across the Top End, eastern Queensland, northeast NSW and far west parts of WA. Rainfall was average to below average across the remainder of the country. For this period, areas of serious to severe rainfall deficiencies are evident in central and western Victoria, eastern Tasmania, near Hobart, parts of western Queensland and in areas across the central NT.

The La Niña conditions that developed during 2007 resulted in a wet start to 2008 across the Top End and most of eastern Australia. However, during autumn the La Niña weakened, coinciding with the failure of autumn rains in many areas. When averaged across the southern half of the country it was the second driest autumn since records began in 1900. Winter rains were mixed, while spring started with well below average rains across most of the southeast.

Low annual rainfall over the southern Murray Darling Basin further exacerbated the long dry spell in this region and a drier than normal autumn and spring for southeast Australia severely stressed farming activity and water supplies in these areas. A dry autumn for southern WA also significantly impacted on farming practices and water supplies in this region.

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18-month rainfall deficiencies

The rainfall deficiencies map for the 18-month period from July 2007 to December 2008 shows serious to severe deficiencies in areas across southeast Australia, western Queensland and central Australia. A small area of lowest on record is evident near Hobart. Over the 18-month period the Top End and much of eastern Queensland and northeast NSW had some benefit from above average rainfall associated with the 2007/08 La Niña. However, in southeastern Australia, central Australia and western Queensland, apart from wet November and December periods in both 2007 and 2008, rainfall was generally below to very much below average.

The Indian Ocean Dipole (IOD) is a pattern of sea surface variability across the Indian Ocean that is associated with lower than normal rainfall over central and southern Australia when it is in a positive phase (see IOD). Both 2007 and 2008 were consecutive years of sustained positive IOD, which partly explains rainfall deficits through the 18-month period in these areas. The IOD usually has its greatest impacts between June and November.

The deficiencies discussed above have occurred against a backdrop of decade-long rainfall deficits and record high temperatures that have severely stressed water supplies in the east and southwest of the country. The combination of record heat and widespread drought during the past five to ten years over large parts of southern and eastern Australia is without historical precedent and is, at least partly, a result of climate change.

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Definitions

Lowest on record - lowest in the historical analysis, which runs from 1900.
Severe deficiency - rainfalls in the lowest 5% of historical totals.
Serious deficiency - rainfalls in the lowest 10% of historical totals, but not in the lowest 5%.

Very much below average - rainfalls in the lowest 10% of historical totals.
Below average - rainfalls in the lowest 30% of historical totals, but not in the lowest 10%.
Average - rainfalls in the middle 40% of historical totals.
Above average - rainfalls in the highest 30% of historical totals, but not in the highest 10%.
Very much above average - rainfalls in the highest 10% of historical totals.

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