Synopsis: A severe drought is at present (October 2019) affecting the eastern region of Australia, particularly the inland region of northern New South Wales and southern Queensland. An analysis has been made of the 141 years of rainfall records since 1879 at Cunnamulla in south-western Queensland. Droughts (and floods) are a feature of the Australian climate and about twelve droughts have been identified that were of similar magnitude to (and often more severe than) the present drought. The analysis only examined the rainfall records although it is recognised that other factors such as temperature, wind and non-meteorological matters (eg farming practices and the economic situation) may also affect drought severity.
Cunnamulla is a town in south-western Queensland. It is located on the Warrego River and has, over the years, experienced severe flooding as well as severe droughts. The climate of the region is arid and the main industry is wool production from sheep.
Unbroken monthly rainfall data are available at Cunnamulla since January 1879 from the website of the Australian Bureau of Meteorology http://www.bom.gov.au/climate/data/ The latest available data used were for September 2019.
The mean annual rainfall at Cunnamulla over the period 1879-2019 was 374 mm, with annual (Jan-Dec) totals ranging from 123 mm (in 2002) to 925 mm (in 1956). On average about half of the annual rainfall occurs during the hot summer months, December to March.
Rainfall totals were extracted for seven durations (ranging from 6 months to 60 months) continuously since 1881. These totals were ranked to determine the twelve years with minimum (drought) rainfalls. The results, for durations of 6, 12, 18, 24, 36, 48 and 60 months, are as follows:
The long-term trends of the minimum drought rainfalls indicate that there has been little change in either the severity or the frequency of droughts over the past 141 years. The following plot shows, for each of the seven durations, the minimum rainfall total for each decade. The time scale is plotted at the date of the conclusion of the individual drought.
About twelve major droughts were identified over the period since 1879, at an average interval of about 10-15 years. There is no trend towards more or less droughts in recent decades.
The severity of each of these droughts, for durations between 6 months and 60 months, are similar. For example, for a 24-month duration, the rainfalls during the seven most severe droughts varied from 290 mm to 384 mm. This compares with the long-term average 24-month minimum rainfall of 635 mm.
The severity of the present (2014-2019) drought is comparable in both magnitude and duration to many previous major droughts. It is noted that the present drought is still ongoing, so it may eventually be more severe than the present analysis indicates.
The analysis involves only the study of rainfall data. Although rainfall is the primary factor affecting the severity of droughts, other meteorological factors (such as temperature, wind and humidity) and non-meteorological factors (such as farming practices and economic conditions) also affect the severity of droughts on the local population.
The severity of the present drought is fully recognised. The purpose of this analysis is to make a comparison with historical droughts that have affected Cunnamulla.
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This article was written by Brian Gunter of Narooma, NSW, Australia. In his previous life Brian was an engineering hydrologist involved over many years in the analysis of rainfall and river flow data for the planning of water resources projects in Australia, Asia and Africa. In recent years he has been one of the Marine Rescue NSW (previously Royal Volunteer Coastal Patrol) volunteer weather observers who operate the Narooma station for the Bureau of Meteorology.