Record 15-month Drought in 2018

The 15 months to date is the driest 15 months in the Manilla rainfall record from 1883.

Rainfall shortages August and September 2018

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 September, are shown by a black line with black circles. Those from one month earlier, at the end of August, are shown by a thinner line with smaller white circles.

Changes this month

The September rainfall total of 12.5 mm was at the 18th percentile. That raised short-term totals (2-, 3-, and 4-month totals) compared with those of a month ago. However, totals fell very much lower for 5-, 6-, 9-, 15-, 18-, and 24-months.

Extreme shortages

Extreme shortages, seen less than 1% of the time since 1883, are now seen for the durations of 5, 6, 9, 15 and 18 months. This drought is now worse than the drought of 2002. In that drought the longest duration of extreme shortage was only 12 months: 307 mm (at the 0.2th percentile) from December 2001 to November 2002.

A record rainfall shortage

The 15-month total of 400 mm is the lowest in the 136-year record. It is a rate of 26.7 mm per month, 49% of normal, and at the 0.06th percentile. It beats the previous lowest 15-month total of 404 mm that was set in May 1912.
The last time that records for low rainfall were set at Manilla was nearly 50 years ago. Those records were: only 1 mm rainfall in the two months to April 1971, and only 14 mm in the four months to June 1971.

Long-term shortages

The 6-year rainfall total (3244 mm) is a severe shortage, unchanged for three months. 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 serious 20-year shortage (9.7th percentile) has developed in this month. Such a very long-term shortage has not been seen since 1949. Up to that date, the Namoi River had suffered decades of low flow, which was followed later by much higher flows. Manilla’s mean annual rainfall has been above normal from 1949 until recently.

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.
“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.”

Manilla rainfall records

Manilla Post Office rain gauge, Station 055031, was read daily from 1883 to 26 March 2015. Then, for 15 months there was no official Manilla rain gauge. A Bureau of Meteorology automatic rain gauge was re-located to the museum yard and operated as Station 055031 from 23 May 2016 to 7 October 2016 (4 months). It failed, and did not operate for 5 months. After repair, the gauge was read automatically at 9 am daily as Station 055312: Manilla (Museum). It failed again after 6 months, on 24 September 2017. The web-page for Station 055312 shows that, since its repair on 15 March 2018, the gauge has been unreliable, with readings frequently missed. No readings were recorded in September 2018.
Since April 2015, I have read my rain gauge in Monash Street Manilla daily. I have used these readings when official readings are lacking. The gauge is not precise, the site does not meet specifications, and it is 1 km from the Post Office. However, the daily readings are seldom more than 3 mm higher or lower than available official Manilla readings.

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