Synopsis: A severe drought is at present (September 2019) affecting the eastern region of Australia, particularly the inland region of northern New South Wales (NSW) and southern Queensland. An analysis has been made of the 138 years of rainfall records since 1881 at Wellington in north-eastern NSW. Droughts (and floods) are a feature of the Australian climate and eleven other 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) may also affect drought severity.
Wellington is a town in northeast of NSW, located to the west of the coastal ranges. It is a rich agricultural area (mainly sheep, cattle, crops) dependent on local rainfall, and there are also extensive irrigated areas (mainly cotton) in the region that are dependent on reservoir storages. Burrendong Reservoir, a major storage on the Macquarie River upstream from Wellington, is down to below 5% storage capacity at the moment (see photo below).
Burrendong Reservoir 2 August 2019 (at 5% of capacity)
Monthly rainfall data are available at Wellington since November 1881 from the website of the Australian Bureau of Meteorology http://www.bom.gov.au The latest available data used were for August 2019. There were missing data in only four months: October 2000, August and October 2011 and May 2014. For the missing months data from a nearby station (Arthurville) was used.
The mean annual rainfall at Wellington over the period 1881-2019 was 617 mm, with annual (Jan-Dec) totals ranging from 286 mm (in 1944) to 1,386 mm (in 1950). The plot of annual rainfalls indicates a possible climatic shift in 1947. The mean monthly rainfalls in the period January to April were about 35% greater since 1947, with the mean annual rainfall about 15% greater. This effect has been observed elsewhere in eastern Australia.
Rainfall totals were extracted for seven durations (ranging from 6 months to 60 months) continuously since 1881. These totals were ranked to determine the 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 138 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.
The regular pattern of droughts at Wellington is demonstrated in the plots for durations of 12 to 60 months. These plots are of the minimum rainfall totals for the nominated durations for each year since 1881. It is seen that severe droughts occur, on average, about every 10-15 years.
Twelve major droughts were identified over the period since 1881, at an average interval of about 10-15 years. The period 1917-1930 is complicated in that there are overlapping dates of droughts of different duration. 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 nine most severe droughts varied from 701 mm to 809 mm. This compares with the long-term 24-month rainfall of 1,234 mm.
The severity of the present (2012-2019) 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 Wellington.
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Plots of the monthly rainfalls at Wellington for each of the identified drought periods are:
Following a telephone interview, the following item was published in “The Wellington Times” on 26 September 2019:
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.