Issued 4 October 2007
Rainfall deficits worsen following a dry September
Rainfall deficits have expanded and intensified over the southern half of
the country as a result of widespread below to very much below average
September rainfall. Averaged over the Murray-Darling Basin (MDB), it was
the driest September in the historical record back to 1900. Unless October
and November are wetter than average, 2006-07 will be one of the very few
instances in history where the MDB has suffered a winter-spring drought in
successive years. The other years when this has occurred were 1918-19 (the
latter was extremely dry), and 1940-41, with 1940 being very dry. In each
of the years 1927, 1928, and 1929, large parts of the MDB had a winter-spring
rainfall total in decile 3 or lower, although individually none of the years
was extremely dry.
See September rainfall pattern
4-month rainfall deficiencies
Since the start of winter, South Australia has been the worst hit state, with the four-month average being the lowest on record at just 26 mm. At the yearly time-scale, rainfall deficiencies are evident in all states and territories, although the NT has only a few small patches. As September 2007 had generally similar rainfall to September 2006 in the areas with twelve-month deficiencies, there was little change to their pattern compared with the situation at the end of August.
For the 3-month period from June to September 2007, an area of serious to severe rainfall deficiencies covered most of SA, southwest Queensland, western NSW, northern Victoria, regions in the southern NT, scattered parts of WA and a small patch in northeast Tasmania. Several regions had record low totals, most notably in SA, Queensland and New South Wales.
5-month rainfall deficiencies
Rainfall deficiencies for the 5-month period from May to September 2007 were widespread across the southern half of Western Australia. SA was also affected, but to a much lesser extent than for the four-month period.
12-month rainfall deficiencies
For the 12-month period from October 2006 to September 2007, zones or regions of rainfall deficits existed near the southwest and west coasts of WA (generally south of Shark Bay), along parts of coastal SA, in northern and eastern Tasmania, in a band from south-central Victoria to the tablelands and western slopes in southeastern NSW, and in southeast Queensland and adjacent parts of far northern NSW. Record low falls for this particular 12-month period were recorded just to the east of Melbourne, and along WA’s west coast between Shark Bay and Geraldton.
The deficiencies discussed above have occurred against a backdrop of multi-year rainfall deficits and record high temperatures that have severely stressed water supplies in the east and southwest of the country. Several years of above average rainfall are required to remove the very long-term deficits. Furthermore, the combination of heat and drought during the past five to ten years over the MDB and southeastern Australia, is outside the typical range of variability experienced during the previous 100 years.
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.
Product Code IDCKGD0AR0