[ UPDATED 16 FEBRUARY 2020 ]
Synopsis: A severe drought is at present (February 2020) 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 Balranald in south-western New South Wales. Droughts (and floods) are a feature of the Australian climate and five major droughts were identified that were of similar or greater severity 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.
Balranald is a town (pop 1,100) located on the Murrumbidgee River in south-western New South Wales. The region experiences a semi-arid climate with hot summers and mild winters.
Data Used
Unbroken monthly rainfall data are available at Balranald 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 December 2019.
Annual Rainfall
The mean annual rainfall at Balranald over the period 1879-2019 was 323 mm, with annual (Jan-Dec) totals ranging from 122 mm (in 1967) to 692 mm (in 1973). The 2019 rainfall total was 150 mm. On average the annual rainfall occurs fairly uniformly throughout the year.
Drought Intensity
Rainfall totals were extracted for five periods (12, 24, 36, 48 and 60 months) continuously since January 1879. These totals were ranked to determine the dates with minimum (drought) rainfalls. Note that the rainfalls identified are for the period up to the specified date. For example the lowest 24-month rainfall recorded was 264 mm over the 24-months, December 1942 to November 1944. An example of the results, for a period of 36 months, is as follows:
Based on similar plots for the other periods the five most intense droughts were identified for each period. The present drought (to 2019) was added if it was not already included in the five most intense droughts. The results are:
Drought Duration
In many cases the most serious drought indicator is its duration. This was analysed for the same five periods (12 months to 60 months). It was found, for example, that over the period1934-1947 there were 142 months when the 36-month minimum rainfall was less than 890 mm. Months were considered to be in drought if x-month rainfall (ie the total rainfall for the preceding x months (x = 12, 24, 36, 48 or 60) was less than the mean minimum value. An example of the results, for a period of 36 months, is as follows:
Based on similar plots for the other durations the five longest droughts were identified for each period. The present drought (to 2019) was added if it was not already included in the five longest droughts. The results are:
Severe Drought Graphic
The relative severity of individual droughts varies with the situation of the observer. Some droughts (eg 1896-1917) are very long, while others (eg 1882-1889) are intense over shorter periods. An example of one of the more severe droughts at Balranald is that of 1934-1949 and this is shown graphically (for 12, 36 and 60 month periods) as follows:
Drought Trends
The long-term trends of the minimum drought rainfalls indicate that since the 1950s the intensity, duration and the frequency of droughts at Balranald were less than during preceding decades. The following plots show, for the five periods, the minimum rainfall total and the total number of drought months for each decade. The time scale of the minimum rainfall graphic is plotted at the date of the conclusion of the individual drought period.
Comments
Five major droughts were identified over the 141 years since 1879, at an average interval of about 25 years. The duration of the droughts varied depending on the period (12-60 months) being analysed, but the enveloping dates of the five major droughts identified are: 1882-1889, 1896-1917, 1922-1933, 1934-1949 and 1997-2006.
There was a sudden trend change in about 1950 with lower intensity, duration and frequency of droughts since then.
The intensity of many of the five identified major droughts, for periods between 12 months and 60 months, are similar. For example, for a 24-month period, the minimum rainfalls during the five most severe droughts varied from 264 mm to 340 mm. This compares with the long-term average 24-month minimum rainfall of 558 mm.
Many of the droughts extended over many years. These were particularly so for the droughts of 1896-1917, 1922-1933, 1934-1949 and 1934-1949.
The current drought, which commenced in 2018, is still continuing but up to December 2019 had not yet attained the severity of many historical droughts.
The analysis involves the study of only 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. Even with rainfall, the drought effect will be different depending on the seasonal distribution and intensity of rainfall.
The severity of a drought has different significance for different situations. For example, the different impact of a given drought on dry-land livestock farmers, irrigation farmers, town water supplies, horticulturists or environmental interests.
The severity of the present drought in the Balranald region is fully recognised. The purpose of this analysis is to compare the current drought with those that have been experienced over the past 141 years.
Please let me know what you think of my analysis.
brigun (at) westnet.com.au
The Author:
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.
Climate Trends 