Category Archives: Rainfall from 1883

Rainfall Deficiencies III: 36-months Duration

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883 by

Log of severe and extreme rainfall deficiency of 36-month duration at Manilla

This is the third of four graphs that show Manilla’s history of rainfall deficiencies (rainfall droughts), for periods of duration 3 months, 12 months, 36 months, and 120 months.

This third graph includes those periods of severe or extreme rainfall deficiency that last thirty-six months. They are rainfall droughts that affect about three successive years.
In Manilla’s climate, a time of severe 36-month rainfall deficiency has a rainfall total less than 1505 mm, when it normally would be 1940 mm. A time of extreme 36-month rainfall deficiency has a rainfall total less than 1380 mm.
Droughts of this duration have quite different effects to those that are much shorter.
While the 3-month drought that just qualifies as “severe” (by having rainfall in the fifth percentile) would have a rainfall total of 50 mm, in the similarly defined 36-month drought, each 3-month period within it would have, on average, a rainfall total of 125 mm (i.e. 1505*3/36). This 3-month rainfall total is only 25 mm less than the normal 150 mm total. It would scarcely be noticed if it did not persist for 36 months.
The importance of severe and extreme rainfall deficiencies of 36-month duration is that they use up the reserves that are held in surface and sub-surface water storage.

In the Manilla rainfall record, such three-year droughts were concentrated in just a few decades. They were very common around 1910-1915 and 1945 (in more than 12% of months) and in 1965 (in 9% of months). They were very rare or absent (less than 2% of months) before 1900, from 1925 to 1935, and in the entire forty years since 1975.
Extreme 36-month droughts generally comprised about one-fifth of the total, as one might expect (unlike the case for one-year droughts).

Areas shown on the graph

Rainfall deficiencies are called “severe” when they are lower than are recorded for five percent of the months. I have called deficiencies “extreme” when they are lower than are recorded for one percent of the months.
In this graph, I have coloured extreme deficiencies in blue. The maroon colour is deficiencies that are severe, but not extreme. The top edge of the maroon area marks the proportion of severe deficiencies including extreme deficiencies. As an average, this line is at five percent.

Data analysis

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Rainfall Deficiencies II: 12-months Duration

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883 by

Log of severe and extreme rainfall deficiency of 12-month duration at Manilla

This is the second of four graphs that show Manilla’s history of rainfall deficiencies (rainfall droughts), for periods of duration 3 months, 12 months, 36 months, and 120 months.

This second graph includes those periods of severe or extreme rainfall deficiency that last twelve months. They are rainfall droughts that affect four successive seasons, sometimes making for two failures a year apart.
In Manilla’s climate, a time of severe 12-month rainfall deficiency has a rainfall total less than 400 mm, when it normally would be 640 mm.
The graph shows that such one-year droughts were very common around 1945-1950 and 1965-1970 (in 8% of months) and also 1905 (in 7% of months). They were not common (only 2% of months) around 1885, 1890, and 1980. Recently, around 2015, there have been none at all.
Remarkably, extreme 12-month rainfall droughts (in blue) were almost as common as severe ones in the long period from 1940 to 1975.

Note added June 2015

I have analysed a remarkable and unexpected relation between days of heavy rainfall and the frequency of year-long droughts at Manilla (as graphed here) in a series of posts:
More droughts After Heavier Rains I.
More droughts After Heavier Rains II.
More droughts After Heavier Rains III.

Areas shown on the graph

Rainfall deficiencies are called “severe” when they are lower than are recorded for five percent of the months. I have called deficiencies “extreme” when they are lower than are recorded for one percent of the months.
In this graph, I have coloured extreme deficiencies in blue. The maroon colour is deficiencies that are severe, but not extreme. The top edge of the maroon area marks the proportion of severe deficiencies including extreme deficiencies. As an average, this line is at five percent.

Data analysis

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Rainfall Deficiencies I: 3-months Duration

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883 by

Log of severe and extreme rainfall deficiency of 3-month duration at Manilla

This is the first of four graphs that show Manilla’s history of rainfall deficiencies (rainfall droughts), for periods of duration 3 months, 12 months, 36 months, and 120 months.

This first graph includes those short periods of severe or extreme rainfall deficiency that last only three months. They are rainfall droughts for one season rather than for a year or more. Crops and pastures may fail.
In Manilla’s climate, a time of severe 3-month rainfall deficiency has a rainfall total less than 50 mm, when it normally would be 150 mm.
The graph shows that such short-term droughts have occurred in every decade, but more often in some than in others. These brief droughts were most common (in 7% of months) around 1915, 1920 and 1970. They were least common (in only 3% of months) around 1895, 1935, 1940, 1955, and (more recently) in the twenty years since 1995.
Extreme short-term droughts (3-month total less than 26 mm) were more common near those times when severe short-term droughts were more common.

Areas shown on the graph

Rainfall deficiencies are called “severe” when they are lower than are recorded for five percent of the months. I have called deficiencies “extreme” when they are lower than are recorded for one percent of the months.
In this graph, I have coloured extreme deficiencies in blue. The maroon colour is deficiencies that are severe, but not extreme. The top edge of the maroon area marks the proportion of severe deficiencies including extreme deficiencies. As an average, this line is at five percent.

Data analysis

Continue reading →

Extreme Droughts by Decade at Manilla

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883, World Climate by

Extreme droughts per decade at Manilla NSW

The record of extreme droughts at Manilla, NSW, relates to the Southern Oscillation only now and then, and relates to global warming not at all.

This graph shows some of the same data as I presented earlier in the post “Manilla’s Record of Droughts”. The graph there showed precise dates, but it was hard to tell when extreme droughts were more or less frequent. This graph adds up the number of months of extreme drought in each decade. (See Note below: How I count drought months.)

There are separate columns (getting progressively redder) for extreme droughts of duration 3 months, 1 year, 3 years, and 10 years.
Extreme droughts of 10-year duration occurred only in the 1920’s and 1940’s.
Extreme droughts of 3-year duration occurred in the 1910’s, 1940’s, and 1960’s.
Extreme droughts of 1-year duration occurred in the 1900’s, 1940’s, 1960’s and 2000’s.
Extreme droughts of 3-month duration occurred in the 1880’s, 1900’s, 1910’s, 1920’s, 1940’s, 1970’s and 2000’s.
No extreme droughts at all occurred in five of the fourteen decades: the 1890’s, 1930’s, 1980’s, 1990’s, and 2010’s.

[Note added August 2019.

More data for the decade beginning 2010.
This post, dated December 2014, shows no extreme droughts in the decade beginning 2010. Extreme droughts did occurr in 2018 and 2019, as shown in the post “Rain Shortage Jan 2000 – May 2019”.
By August 2019, some months of extreme drought at 3-month and 1-year duration had occurred, and a month at 3-year duration was imminent.]

Relation to the Southern Oscillation Index

I posted this graph of cumulative values of the SOI earlier.

SOI CUSUM plot

The record of the Southern Oscillation Index relates to the Manilla record of extreme rainfall deficiency only now and then. Persistent El Niños from 1911 to 1915 seem to relate to four months in the decade of the 1910’s having extreme 3-year droughts, carrying forward to two months in the 1920’s having extreme 10-year droughts. Similarly, the catastrophic droughts of short to very long duration in the 1940’s relate to El Niños that persisted from 1939 to 1942.
Other major El Niño events did not produce extreme droughts at Manilla: those of 1896, 1982, and 1997.
Long term trends in the Southern Oscillation Index do not predict Manilla’s extreme droughts at all. The 1940’s droughts Continue reading →

Manilla’s Record of Droughts

Posted in Australian Climate, Climate Change, Manilla NSW, Rainfall from 1883 by

Graph of droughts versus time

In terms of rainfall alone, Manilla, NSW, had droughts between 1900 and 1950 that were more severe, and lasted very much longer than those of recent years.

Comparing droughts

It is hard to say how bad one drought is compared to another because some droughts last longer than others. A drought that lasts two months, and has only 10 mm of rain when it would normally have 100 mm, qualifies as “extreme”. In such a very short drought, rainfall as low as 10% of normal just qualifies as extreme. For a drought lasting twelve months, when there is normally 652 mm of rainfall at Manilla, there has never been a case of a twelve-month rainfall as low as 10% of that (65 mm). (The lowest ever was 288 mm, in 1964-65.) Clearly, using 10% of normal rainfall will not do to define longer-term droughts.
I find the severity of each drought, whether it is long or short, by its percentile rank.
The Bureau of Meteorology defines “Rainfall Deficiency” as:

Lowest on record – lowest since at least 1900 when the data analysed begin.
Severe deficiency – rainfalls in the lowest 5% of historical totals.
Serious deficiency – rainfalls in the lowest 10% of historical totals, but not in the lowest 5%.

On the graph, I use this code:

Extreme rainfall shortage: rainfall in the 1st percentile only.
Severe rainfall shortage: rainfall in the 2nd to 4th percentiles.
Serious rainfall shortage: rainfall in the 5th to 9th percentiles.

Major droughts

All of Manilla’s extreme rainfall droughts that lasted for six years or more happened in the first half of the 20th century. Extreme droughts lasting for thirty years ended during 1940, 1941 and 1947.
Since 1950, the longest extreme drought lasted only five years, ending in 1961. The next longest lasted three years, ending in 1968. The last forty-four years have brought only six extreme droughts, all of less than two years duration: 1971, 1974, 1982, 1984 (2 months!), 1994 and 2002. The twelve years since 2002 may be the longest period without an extreme drought in the whole record since 1883.
Extreme droughts had also been few and short in the earliest years, from 1883 to 1902.

Similar, but much improved graphs

[This graph gives the misleading impression that the longer the duration of rainfall shortage, the later it occurs. That is an artefact.

A shortage of a given duration observed in a particular month must have commenced earlier: earlier by the number of months of its duration, less one.
Graphs that remove this defect, and plot correctly the dates of onset, persistence, and breaking of rainfall shortages are “Rainfall Shortage History: Manilla” and “Rainfall Shortage Jan 2000 – Mar 2019”.

Droughts Elsewhere

At Lake George, in the southern highlands of NSW, extreme droughts of long duration were similarly restricted to the first half of the 20th century, as shown by rainfall records and lake levels.
The “Millennial Drought of southeastern Australia” was not a drought of long duration at either Manilla or Lake George.

[Note added May 2016

Graphical log of droughts, 1884 to 1916The post “Manilla’s Droughts, 1884 to 1916” has an enlarged graph showing more detail. Of the catastrophic droughts of 1902 (“Federation Drought”) and 1912-16, the first had a sudden termination but the second had a sudden onset.]

This blog has many more posts about drought (or rainfall deficiency) at Manilla, NSW. Please use the “Search” function at the top right.

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