The 2002 drought contour chart

Contour chart 2002 drought at Manilla NSW

The 2002 drought at Manilla was a failure of winter rainfall. [See Note below: “Manilla’s rainfall seasonality”.]

The top line of this contour chart shows that monthly rainfall shortages occurred in all the six months of winter rainfall dominance (April to September) of 2002. Shortages in May and July were severe, below the 5th percentile. In the summer rainfall months (October to March) that preceded and followed, rainfall was near or above normal. [See Note below: “Classes of rainfall shortage”.]

Lower down the contour chart, rainfall shortages of longer duration are shown. For droughts of 3 months duration, the rainfall shortage was extreme (1st percentile) by July 2002, as it included the serious shortage of May as well as that of July. In the same way, one sees extreme 6-month shortages in September and October, as all the monthly rainfall shortages since March added up.

By November 2002, one sees extreme droughts of 9 months and 12 months duration. The 9-month drought incorporated the consecutive months of below-normal rainfall from March to November. The extreme 12-month drought (307 mm) that was evident in November 2002 began earlier, with below-normal rainfalls in December 2001 and January 2002. That was the fourth driest 12-month period on record, after October 1965 (288 mm), August 1946 (302 mm) and November 1965 (304 mm).

The 2002 drought had no extreme rainfall shortages longer than 12 months. There were, however, some severe shortages of 18 months duration and some serious shortages of 24 months duration, due to some low rainfalls in the previous winter (2001).

By April 2003, hardly any serious rainfall shortages due to the 2002 drought remained. [See Note below: “Limitations of this analysis”]

More about the 2002 drought

Graph of monthly percentile rainfall in a droughtAnother approach to describing this 2002 drought is in the post “The 2002 rainfall shortages at Manilla”. That post has a graph showing selected monthly profiles of percentile values. It also links to two earlier posts with graphs of smoothed values of climatic anomalies.

The 2018 Drought

Drought 2018 contour chartA similar contour plot for the drought of 2018 reveals similarities and differences.

Note: Classes of rainfall shortage

I have adopted two classes of rainfall shortage from the classes of “Rainfall deficiency” defined by the Bureau of Meteorology in their Climate Glossary as follows:

“Serious rainfall deficiency: rainfall lies above the lowest five per cent of recorded rainfall but below the lowest ten per cent (decile range 1) for the period in question,
“Severe rainfall deficiency: rainfall is among the lowest five per cent for the period in question.”

The Manilla rainfall record allows me to go beyond these two classes. Because the record extends back 134 years, it includes more than 1200 cumulative monthly rainfall values. I can identify percentile ranks even below the 0.1th percentile.
To the Bureau’s two classes of deficiency I add a third:

“Extreme deficiency (or extreme shortage): rainfall lies below the lowest one percent for the period in question.”

Rainfall rate versus percentile rank

Classes of rainfall deficiency based on percentile rank (rarity of occurrence) are practical, while classes based on percent of normal rainfall are not. However, some features of graphs showing percentile values (such as this) may be puzzling if one point is not understood. For any class, such as “Extreme shortage”, the rainfall rate in millimetres per month is not the same for droughts of shorter and longer duration. The longer the duration considered, the higher the rainfall rate in that class of shortage.
For example, consider an extreme shortage, below the 1st percentile. A 3-month period at Manilla would be in extreme shortage only if the rainfall was below 8 mm per month (18% of normal), but a 36-month period would be in extreme shortage so long as the rainfall was below 39 mm per month (89% of normal). See the post “Short Droughts are Worst”.

Note: Manilla’s rainfall seasonality

Model of seasonal rainfall, ManillaAt Manilla, rainfall is distributed through the year in a remarkably regular way. Details are in the post “A Seasonal Rainfall Model for Manilla, NSW”.
There are two distinct modes, each with a peak a few days after the solstice of summer or winter. The two modes, each forming a bell-shaped curve of normal distribution, account for almost all of the rainfall in the year. The curves have similar width in months, but the summer (monsoonal) mode brings 64% of the rain and the winter (westerly) mode brings only 36%. In simple terms, the summer mode extends through the six months October to March, and the winter mode through the six months April to September.

Note: Limitations of this analysis

Monthly rainfalls form a single population

For each month of record, the observed rainfall has been tabulated, and added to those of preceding months to form n-month totals. Then percentile ranks have been calculated using the whole data record, in which the median monthly rainfall is 44 mm. No distinction has been made between calendar months. As a result, January months, which generally have the highest rainfall, (median value 75 mm) will be ranked here as rather less drought-prone than other months.
This limitation will not affect those n-month totals that comprise whole years, such as 24 months. When I inspect the main graph here, I do not see any discrepancy between the drought trajectories for whole-year durations (12, 24, and 36 months) as compared to other durations.
Since adjusting the analysis to account for the differing rainfall in calendar months is difficult, I am letting this simpler analysis stand.

Observations are not retrospective

When a current month’s data shows (for example) an extreme rainfall shortage for a duration of 15 months, this graph (or the related line graph) will plot that 15-month duration at the current month. If the previous month did not have an extreme rainfall shortage of that duration, it will not have been recorded there. Logically, that 15-month extreme shortage must have commenced 14 months earlier. It began then, but the evidence for it has appeared only now.
The data for each current month must always be provisional concerning the drought status at durations longer than one month. I am also preparing graphs that project shortages back to the date that they began.

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