Rainfalls: 8 in the 0.1th percentile

Rainfall status October November 2019

Normal rainfall in November

Rainfall in November 2019 (40.2 mm) was near normal. That reduced shortages at durations from 2-months to 5-months. Otherwise it had little effect: fifteen of twenty longer durations remained extreme shortages.

Values plotted in the 0.1th percentile

For simplicity, the bottom line is labelled with the 0.1th percentile value, and percentile values below 0.1% are plotted on the line. As there are 1600 months of record, both the 2nd-driest month (percentile value 0.063%) and the driest month, (percentile value 0.000%, by convention), which would plot below the line, are plotted on it.
Driest records have again been broken at durations of 12-months (270 mm), 24-months (611 mm), 30-months (834 mm), 72-months (2927 mm), and 96-months (4205 mm). The 96-month record had stood at 4405 mm since November 1919.
A value that equals an earlier record occurs at 84-months (3555 mm).
Values that are 2nd-driest occur at 36-months (1129 mm) and at 240-months (11816 mm). The 240-month (20-year) total is now only 50 mm more than the lowest-ever value of 11766 mm set in February 1931.

How to read the graph

This graph shows all the present rainfall shortages at Manilla, short term and long term, as percentile values. The latest values, as at the end of October 2019, are shown by a thick black line with large circles. Those from one month earlier are shown by a thinner line with small diamonds. [The method is described in “Further Explanation” below.]


Further Explanation

The following notes explain aspects of this work under these listed headings:

Data analysis

Cumulative rainfall totals
Percentile values
Severity of rainfall shortages

Limitations of this analysis

Monthly rainfalls form a single population
Observations are not retrospective
The rain gauge failed

Data analysis

Continue reading

How much rainfall is now lost?

Rainfall deficits in mm at Manilla NSW, Oct 2019

This graph shows the total rainfall shortage in this drought at Manilla NSW. It uses the same rainfall figures as in the October Rainfall Status graph, that is, the rainfall totals for up to 360 months. In this case, I have subtracted from them the normal rainfall totals. That shows the rainfall deficit: how much less rainfall we have had than we normally would.

Generally a 1-year deficit

In many cases, the shortage now is about 650 mm, or one year of normal rainfall. This is true when calculated for 24-months and for several other durations including 360-months. In broad terms, this drought has left Manilla about one year short of rain.

Deficits are smaller at short durations.

A one-year deficit could occur at a duration of 12-months only if there were no rain. A curve on the graph shows “No rain at all”. A second curve is labelled “50% of normal Rain”. These October data show that the rainfall totals for durations below 24-months are around 30% of normal.

Deficits larger than 650 mm occur at some long durations

There are three dips in the curve where deficits are greater than one year of rainfall.

  1. The 3-year deficit is now 820 mm, due to very low rainfall in the last six months. Such values may yet appear at other durations.
  2. The 7-year deficit is now 920 mm. This deficit is due to the inclusion of severe rainfall shortages of 12-month to 30-month duration in 2013 and 2014. These appear on this drought duration graph, but were scarcely noticed at the time.
  3. The 20-year deficit is now 1100 mm. The 650 mm deficit due to the current drought is supplemented by deficits of some 200 mm carried forward from both the 2013-14 shortages (above) and the extreme 12-month shortage of 2002.

What the deficits mean

In times of normal or excess rainfall, the whole terrain is kept supplied with water. It goes to aquifers deep underground, to shallow aquifers near streams, to stream-flow, and to the soil and plants. In general, the longer the duration of normal or excess rainfall, the larger the reservoirs, both underground and on the surface, that can be filled.
The present deficit of one year of rainfall must have drawn down all the reserves of water in the terrain. The empty state of the Keepit Dam (0.8%) is an obvious result, but all other reserves must have fallen.

21stC Rain Shortage Oct ’19

Record of rainfall shortages Jan 2000 Oct 2019

Since the graph for May, the drought at Manilla has become more severe at all durations, from two months up to 240 months.

[For explanation of this graph, see below: “About drought duration graphs”.]

Shortages Jan 2000 to Oct 2019

Seasonal rain shortages recorded to October 2019 are extreme (1st percentile) in the winters of both 2018 and 2019.
For durations of 9 months up to 96 months, extreme shortages now occur at all of the durations shown . This is much worse than in May. At that time, shortages had not been “extreme” (red), but merely “severe” (grey) at the durations of 36-, 42-, 48-, 60-, and 96-months.
By May, “severe” shortages had already appeared at the very long durations of 120-, 150-, and 240-months. Now, there are also severe shortages at 108-months and 180-months.
The shortage at 240-months has now also become extreme (red). Very low rainfalls since May have dragged down the 240-month total to make it the sixth driest on record at Manilla. This links the short extreme drought of 2002  to become a part of the current drought. That would have seemed unlikely during later deluges, as in summer 2011-12.

Complete Manilla drought record to Oct 2019

Compete record updated to October

When the graph of the complete record of months of rainfall shortage at Manilla is updated to October 2019 it is clear that the current drought is one of the greatest droughts in history.
Now that an extreme rainfall shortage has appeared at 240-month (20-year) duration, along with a complete suite of extreme shortages from 2-months to 96-months, only the droughts of 1912 and 1946 are comparable. The Federation drought of 1902 may also have had a 20-year extreme shortage, but data for that duration are incomplete.


About drought duration graphs

These graphs show the onset, persistence, and breaking of episodes of extreme and severe rainfall shortage (droughts) at Manilla. The Continue reading

October: 20-year extreme shortage

Rainfall status Manilla Sep-Oct 2019

Most rainfall shortages are now extreme

The Rainfall Status graph for October 2019 at Manilla shows extreme rainfall shortages (below the 1st percentile) at most durations.
Of the 25 durations shown, 16 are now extreme shortages, and 7 are severe shortages (below the 5th percentile).
That leaves only 2 that are not far below normal: the October 1-month rainfall (21.4 mm) at the 16th percentile, and the 360-month (30-year) total (18696 mm) at the 14th percentile. Even this 30-year total is lower than any seen here since 1952.

A long-duration extreme shortage

An extreme shortage has now appeared at the very long duration of 240 months (20 years). In the 20 years since November 1999 the total rainfall was only 11893 mm – the 6th driest in history. Drier 20-year periods occurred only in the 1940’s and 1950’s.
Given that the median 20-year rainfall at Manilla is 13010 mm, this is a shortfall of 1117 mm, which is nearly two years of rainfall lost.

Only three new records this month

In this drought, rainfall totals in the 1- to 7-year range have broken records for low rainfall repeatedly. This month only three records have been broken: 24-months (615 mm), 30 months (850 mm), and 36-months (1111 mm).

How to read the graph

This graph shows all the present rainfall shortages at Manilla, short term and long term, as percentile values. The latest values, as at the end of October 2019, are shown by a thick black line with large circles. Those from one month earlier are shown by a thinner line with small diamonds. [The method is described in “Further Explanation” below.]


Further Explanation

The following notes explain aspects of this work under these listed headings:

Data analysis

Cumulative rainfall totals
Percentile values
Severity of rainfall shortages

Limitations of this analysis

Monthly rainfalls form a single population
Observations are not retrospective
The rain gauge failed

Data analysis

Continue reading

More drought records in September 2019

Rainfall status Aug-Sep 2019 Manilla

Graph of Rainfall Shortages

This graph shows all the present rainfall shortages at Manilla, short term and long term, as percentile values. The latest values, as at the end of September, are shown by a thick black line with large circles. Those from one month earlier are shown by a thinner line with small diamonds. [The method is described in “Further Explanation” below.]

Record low rainfalls for 15-months to 36-months

With only 1.2 mm of rain falling in September 2019, most of the rainfall totals that had been record low values in August fell further to become new records. Some of these rainfall totals had been getting lower in each of the last four months. Until 2019, these records for low rainfall had not been broken in half a century, some having been set in the great drought of 1966.
Some of the current rainfall totals are very much lower than the previous long-standing records. For example:

  • 18-month total to Sep 2019: 384 mm; to Apr 1966: 514 mm; now lower by 130 mm.
  • 30-month total to Sep 2019: 853 mm; to Oct 1966: 1078 mm; now lower by 225 mm.
  • 36-month total to Sep 2019: 1161 mm; to Jan 1947: 1333 mm; lower by 172 mm.

Record low rainfalls for 72-months and 84-months

Record low rainfalls for 72-months and 84-months, which had appeared during 2019, got steadily lower. The earlier records had stood for more than a century, (Feb, Mar 1903) but are now beaten by about 100 mm.

The latest four months

September’s low rainfall has dragged down the totals for 2-, 3-, and 4-months. The 4-month total of 22 mm is the 2nd lowest ever.

The pattern of this drought

Two features of this drought are now clear from this data:

  • It is an extreme drought of two to three-year duration: one of Manilla’s six great droughts.
  • Record-breaking rainfall shortages at 72-month and 84-month duration show that the summer droughts of 2012-13 and 2013-14 still have an effect, not compensated by the wet winter of 2016.

Table of lowest-ever rainfalls

In a post of July 2018, I tabulated the lowest-ever rainfall for selected durations up to 360 months.

I commented that such records are rarely broken, and all had stood for at least forty-six years at that date.
The current drought has now broken most of those records for durations between 12-months and 84-months


Further Explanation

The following notes explain aspects of this work under these listed headings:

Data analysis

Cumulative rainfall totals
Percentile values
Severity of rainfall shortages

Limitations of this analysis

Monthly rainfalls form a single population
Observations are not retrospective
The rain gauge failed

Data analysis

Continue reading

New drought records in August 2019

Rainfall status July-August 2019

Graph of Rainfall Shortages

This graph shows all the present rainfall shortages at Manilla, short term and long term, as percentile values. The latest values, as at the end of August, are shown by a thick black line with large circles. Those from one month earlier are shown by a thinner line with small diamonds. [The method is described in “Further Explanation” below.]

Record low rainfall values

August rainfall of only 5.6 mm at Manilla has brought more drought records. The rainfall totals for 18-, 24-, 36-, 72-, and 84-months are new record lows.
The record-low 36-month total is remarkable. Only the great droughts of 1947, 1914, and 1967 had rainfall nearly so low for so long, but the current 36-month total (1283 mm) is more than 50 mm lower than in those great droughts. Given that Manilla’s mean  annual rainfall is 652 mm, 1283 mm represents less than two years of rainfall in three years.

Severe and extreme rainfall shortages

For plotted durations longer than four months, only the very longest (30 years) is not a severe or extreme rainfall shortage. For durations beyond 84 months (including 30 years) every total is lower than has been seen since 1954, 65 years ago.


Further Explanation

The following notes explain aspects of this work under these listed headings:

Data analysis

Cumulative rainfall totals
Percentile values
Severity of rainfall shortages

Limitations of this analysis

Monthly rainfalls form a single population
Observations are not retrospective
The rain gauge failed

Data analysis

Continue reading

21-C Rain ENSO IPO: Scatterplot

The anomaly of monthly rainfall at Manilla, NSW varied with that of ENSO for only a part of the 21st century to date.

Scatterplot Rainfall vs ENSO

This connected scatterplot relates the smoothed anomaly of the El Niño-Southern Oscillation (ENSO) to the smoothed and 2-month-lagged anomaly of monthly rainfall at Manilla, NSW. The earlier data, from September 1999 to September 2011, is plotted in blue, and the later data, from October 2011 to November 2018, in red.

The same data was displayed as a dual-axis line plot in an earlier post titled “21-C Rain-ENSO-IPO: Line graphs”. Data sources are linked there.

The line plot revealed two things: the relationship changed from earlier to later times, and there was a better match when the rainfall data was lagged by two months. To clarify, I prepared various scatterplots with fitted regressions.

Raw data scatterplots

Scatterplots of the raw data values yielded regressions with very low values of the coefficient of determination (R-squared). For the whole population, R-squared was 0.028. I then checked the coefficient when I lagged the rainfall by 1-, 2-, or 3-months. A 1-month lag almost doubled the coefficient to 0.041; a 2-month lag gave 0.055, and a 3-month lag gave 0.041 again.
My observation that Manilla rainfall typically leads ENSO by 2 months is confirmed.

Connected scatterplots of smoothed and lagged data

The smoothing function used in the dual-axis line plot of the earlier post makes a good visual match. That suggests that local rainfall and ENSO are physically related at a periodicity no shorter than 12 months.
Using the smoothed and 2-month-lagged data, I have made the scatterplots shown in the graph above.
The better-matched data from September 1999 to September 2011 (blue) has a satisfactory R-squared of 0.498, nearly 20 times greater than that of the raw data. The very poorly-matched data from October 2011 to November 2018 (red) has an R-squared value of 0.040, no better than the raw data.

Patterns in the sequence of rainfall and ENSO values

In the above graph, I have joined the consecutive smoothed data points to make a connected scatterplot. Because little noise remains, clear patterns appear.

Matched rainfall and ENSO

The pattern up to September 2011 (blue) is mainly a series of ellipses, some clockwise and some anti-clockwise. They are almost parallel to the regression line:

y = -0.047x-0.246

The blue point furthest to the top left is that for September 2002, a time of extreme drought and El Niño.
The blue point furthest to the bottom right is that for December 2010, a time of very high rainfall and La Niña.

Discordant rainfall and ENSO

The pattern from October 2011 (red) swings about wildly and does not repeat. The regression (with a trivial coefficient of determination) is nearly horizontal. Near its ends are the extreme drought of June 2018 and the deluge of January 2012, both at times when ENSO was near neutral. At the top of the graph is the Super El Niño of November 2015, when Manilla rainfall was normal.

Conclusion

Scatterplots, connected scatterplots and regressions confirm that a strong relation between rainfall at Manilla and ENSO failed in 2011 as the IPO was rising from a negative toward a positive regimen.