The Dry

The Detail

The Bureau of Meteorology (BOM) website has some good annual rainfall data/maps available going back to 1900 and we have commented in the past on the normal gap, in years, seen between wet years to dry years. Across the entire continent, previous analysis undertaken has shown that the average number of years between moving from a wet to dry pattern is between 2-4 years. However, it is often the case that above and below average rainfall can impact parts of the country at the same time.

For example, we are currently in the midst of a wetter than normal pattern for the south eastern corner of Australia (NSW, Victoria and SA). Meanwhile, parts of the NT, QLD and Tasmania are experiencing much drier than normal conditions. The rainfall decile animation below, of the whole of Australia, shows the last significant wet event in 2016 and the three year transition to widespread drought in 2019.

The second animation highlights the move from wet to dry in Queensland, seen from 2010 to 2013. Instead of a focus on the entire country, analysis was undertaken at a state level to look at the gap seen in years between wet to dry seasons from 1900 onwards. The gap years were tallied and three statistical measures were taken to measure how long it usually is between wet to dry seasons within each state.

The mean, or average, number of years was the initial measure of the gap, in years, between wet to dry. However, we also took a look at the median and mode. The median is the middle value in a number sequence and the mode is the most frequently occurring value. If you consider the number sequence 3,5,2,1,1,4,3,2,1 representing years between wet to dry seasons, the average is calculated at 2.4 years. To obtain the median value the sequence is re-ordered from lowest to highest and the middle value is taken, in this example (1,1,1,2,2,3,3,4,5) the median is 2. Whereas, the mode, which is the most frequently occurring, would be 1.

Analysis of the gap years between wet to dry rainfall cycles across each state shows little variation between the mean, median and mode in Queensland, of about 2 years. Most other states demonstrated mean levels pretty close to 2 years and medians/modes closer to a 1 year gap. Tasmania was the outlier, with median/mode at 3 years gap between wet and dry seasons.

The analysis was re-run with a focus on seasons where successive wet years had occurred. Taking a look at each state we measured how long after two or more successive wet seasons did it take before a dry season returned. For most states the mode remained at either 1 or 2 years gap between wet to dry. The median gap ranged between 1 to 4 years, meanwhile the mean (average) gap was between 1.5 to 5 years.

This suggests, that no matter where you are situated in Australia, if you’ve had a pretty good season in the last year or two then you are probably due a dry spell in the next few years, assuming the historic cycles between wet to dry seasons continue to hold.

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