Graph of Rainfall Shortages
This graph shows all the present rainfall shortages at Manilla, short term and long term, in terms of percentile values. The latest values, as at the end of August, are shown by a black line with black circles. Those from one month earlier, at the end of July, are shown by a thinner line with smaller white circles.
The classes of rainfall shortage are:
• Serious shortage: below the 10th percentile;
• Severe shortage: below the 5th percentile;
• Extreme shortage: below the 1st percentile. [See note below on my usage “Extreme shortage”.]
Changes this month
Rain late in August raised the one month and two month rainfall totals out of the class of “serious shortage”. The total for August (28.2 mm) is at the 40th percentile for the month, and the total for July and August is at the 10th percentile. The three-month total, which had been an extreme shortage, fell to become only a severe shortage.
Extreme shortages, seen less than 1% of the time since 1883, are now seen for the durations of 4, 5, 6, and 15 months. Without last week’s rain, the 6 month total would have been one of the lowest ever recorded.
There is an extreme shortage at 15 months, due to low rainfall in mid-2017, in the months of July (13.2 mm), August (13.8 mm), and September (5.5 mm).
The 6-year rainfall total for August (3252 mm) is a severe shortage, only slightly above that of July (3234 mm). Both these values are lower than any 6-year rainfall totals since 1962. When rainfall shortages of such long duration persist, rainfall does not maintain the groundwater levels or river flows required for irrigation or town supply.
[A graph showing rainfall shortages to the end of September 2018 is in the later post: “Record 15-month Drought in 2018”.]
Note: The term “Extreme shortage”
I have adopted 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.
“Areas where the rainfall is lowest on record for the given time period are also shown.”
The Manilla rainfall record allows me to be more exact than the Bureau. 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.”