Issued 3 May 2007
Short term relief but long term drought persists
With the demise of the 2006 El Niño event, 2007 has seen a general improvement in rainfall across Australia. Above average rainfall has been widespread across southern Australia for the four month period January to April 2007, with near to average rainfall across much of the Murray Darling Basin. April saw particularly good falls across southern Western Australia much of South Australia, and parts of western Victoria and New South Wales. While follow-up falls are clearly needed, this rainfall will provide short term relief to many agricultural systems.
History shows that the breakdown of past El Niño droughts has usually been associated with a shift to above-normal rainfall across much of eastern Australia. For example, of the 20 past El Niño-related droughts in the Murray-Darling Basin, in all 20 cases, a period of sustained above-normal rainfall occurred no later than the following winter (June-August). In 60% of cases this had occurred by the February-April period, whilst in the remaining 40% of cases it was later than that, most commonly in the May-July period. There is no historical precedent in 107 years of records for dry conditions to continue unbroken through the winter following an El Niño event.
An important consideration in the recovery from this drought event is the different rate at which systems respond to drought. At the current time, many catchments in eastern Australia are excessively dry from a very protracted period of below average rainfall and above average temperatures. This means that it will take above average rainfall just to produce average runoff, and very considerable rainfall to make a material difference to water storages.
Long term deficiencies remain in parts of southwestern and eastern Australia
Above average rainfall during April in Western Australia to the north of Carnarvon and to the south of Perth, eased areas of lowest on record rainfall for the period, but severe deficiencies still remain. In contrast below average rainfall for April in the area around Melbourne and in northern Tasmania caused an expansion of an area of lowest on record rainfall. Dry conditions were eased in parts of southern South Australia, which received above average rainfall for the month, but for the past twelve months, rainfall deficiencies remain evident in western WA. southeastern South Australia and in a band stretching across much of the region from Tasmania to southeast Queensland.
12-month rainfall deficiencies
For the 12-month period from May 2006 to April 2007, there were serious to severe rainfall deficiencies over southern and eastern Australia in a broad arc extending across southeastern SA, much of Victoria, much of southeastern NSW west of the Great Divide, and a large part of southeast Queensland. Northern and eastern parts of Tasmania were also affected as was WA west of a line from around Exmouth to Bremer Bay in the south. Record low 12-month falls were recorded between Dalby and Goondiwindi in southern Queensland, along much of WA’s southwest coast and Tasmania’s north coast, and in a large area around Melbourne.
The worst of the long-term deficiencies are likely to remain for some time. For them to be removed by the end of July, for example, falls over the next three months would need to be in the highest 10% of the historical record in many areas, especially in Victoria and southeast Queensland.
In comparison with the situation for the twelve months to March, areas of lowest rainfall on record shrank parts of western WA, particularly around Carnarvon and expanded in southern Victoria around Melbourne.
The deficiencies discussed above have occurred against a backdrop of multi-year rainfall deficits that have severely stressed water supplies in the east and southwest of the country.