Is There Any Drought Now?

No. In Manilla just now, there is no drought of any kind: not a short drought, a medium-length drought, or a long drought; not an extreme drought, a severe drought, or even just a serious drought.

A new comprehensive graph of the severity of drought at one site.

In this graph, each line of data points is for one particular month. The middle line, joining the red squares, shows the whole rainfall drought situation for last month: September 2016.
This is a new kind of graph. (See Note 1 below.) It can show how severe a drought is, not only during the last month or two, but during the last year, and during the last many years. That is a lot of information.

How to read the graph

A month of extreme drought would have data points very low down on the graph. The scale on the left side is amount of rainfall. It must be a “percentile” value. For example: if the amount of rain that fell is just more than has been seen in the driest 5% of all months, it has a value in the 5th percentile. (See Note 2 below.)

Along the top and bottom of the graph I have plotted a number of months.
The number does not show time passing. It shows the number of months I included in a calculation. For each month on record I did many calculations. I added up the total rainfall for:
* the month itself;
* two months including the previous month;
* three months including the month before that;
* … and so on.
I found the totals for larger groups of months extending back as far as 360 months (30 years).
Using all these rainfall totals, I calculated percentile values to plot on the graph. For example, for groups of 12 months, all groups of 12 consecutive months are compared with each other, to find the percentile value of the 12-month period ending in a given month. (See Note 3 below.)

Which months had the most drought and least drought?

The worst drought there could ever have been would be one with data points along the bottom line of the graph. In such a disastrous month, all the rainfall totals would be the lowest on record, not just the one-month total, but also the two-month total and so on up to the 360-month total. Every one of them would be the lowest total on record. It has never been as bad as that.
The “best” time, in terms of being free of drought, would be a month with all its data points along the top edge of the graph. For that month, every rainfall total, for a short period or a long period, would be the wettest on record.
From the Manilla rainfall record, I have chosen to display the most drought and the least drought that actually occurred.

The most drought: August 1946

The month of August 1946 had no rain. Of course, that was the lowest rainfall for any August month (One among 13 months on record that had no rain.). As a result, the percentile rank for that month’s rainfall is zero. Most totals for groups of any number of months ending in August 1946 are also on the “zero-th” percentile, that is, the lowest on record. Thus, it was an extreme drought in the short term, medium term and long term.
For this month, percentile values that are above the third percentile occur in the totals for 48, 60, and 72 months, as shown. These figures, while not extremely low, were still well below normal (Normal is the 50th percentile.). They occur because these totals include some wet months in 1940, 1941, and 1942.

The least drought: March 1894

March 1894, with 295 mm of rain, was one of the the wettest months ever, ensuring a 100th percentile value. The rainfall totals for groups of months ending in that month included six other “wettest ever” values, and all other groups of months were also very wet. No group of months was below the 95th percentile. (See Note 4 below.)

Current drought situation (September 2016)

This month’s rainfall total of 122.4 mm puts it in the 92nd percentile of all monthly rainfall values, far above the median value marked as “normal” on the graph. The 2-month rainfall total (203 mm), and the 4-month rainfall total (350 mm) are almost as high, each in the 90th percentile.
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Manilla’s Droughts, 1884 to 1916

Graphical log of droughts, 1884 to 1916

The catastrophic droughts in 1902 and 1912-16 were quite different.

In the years before 1917 shown here, Manilla had several times of extreme drought. They came in 1888, 1895, 1902, and in a cluster that began in 1912.
(1.) The 1888 extreme droughts were of 2-, 3-, 4-, 5-, 6- and 9-month duration. The 2-month event was in August, and other events came later as they became longer, until the 9-month event came in December (having begun in April).
(2.) In 1895, drought was extreme only for durations of 5-months (June) and 6-months (July and August). Although droughts of 2-, 3-, 4-, and 9-month duration also occurred, they were not extreme, but merely “severe”.
(3.) Manilla’s 1902 (“Federation”) drought was phenomenal. Extreme droughts of nearly all durations from 2 months through to 96 months occurred (and ended) at practically the same time. The 2-month event plots at May 1902. The 96-month extreme drought plots at February-March 1903. None of the drought events around 1902 extended far into 1903; all ceased abruptly. The rainfall shortages began earlier according to a simple pattern; the longer the duration of the extreme event, the earlier it began. The 1902 extreme 1-year drought began in September 1901, and the extreme 8-year drought began in 1895.
(4.) The cluster of drought events extending through 1912 and 1916 was as bad as the events of 1902, but quite different. Merely “severe” short-duration events began in April 1911. Events of increasing duration came at later dates, forming a smooth curve on the graph. Beyond 12-month duration, and up to 72-month duration, there were extreme events at nearly all classes of duration. By the 72-month duration, the date of plotting had drifted forward in time to January-July 1916. The beginning of these 72-month events would have been during Continue reading

The 2002 rainfall shortages at Manilla

Graph of monthly percentile rainfall in a drought

In 2002, Manilla had a 6-month drought with one of the most extreme rainfall shortages on record. In nearly fifty years since 1966 there have been no other shortages like it.

I have discussed this drought in two posts: “Profile of an Extreme Drought”, and 3-year trends to August 2004 (An extreme 1-year drought).

This post is about the rainfall record only. It compares the percentile values of rainfall totals for groups of months: one month, two months, and so on. The graph shows how the drought began, developed and faded. Other droughts may go through similar stages. I have plotted the pattern of rainfall shortages month by month, showing only even-numbered months. I have plotted them in different colours, with matching “Call-out” labels.

April 2002 (Red): no drought yet.
In April, the monthly rainfall was slightly below average: in the 40th percentile. In this month, nearly all rainfall totals up to the 42-month total were also below average. Only the 6-month total was above average. This set up the conditions for a drought. Notice that rainfall totals for periods longer than 42 months were all well above average. This hardly changed at all in this year. There had been a lot of rain in previous decades.

June 2002 (Orange): 2, 3, and 4-month droughts.
When May rainfall was in the 1st percentile and June rainfall in the 25th percentile, the June 2, 3, and 4-month totals became serious or severe shortages (below the 10th percentile).

August 2002 (Green): 2, 3, 4, 5, 6, and 9-month droughts.
With July rainfall again in the 1st percentile, and August rainfall in the 26th, the drought became extreme. The 4, 5, and 6-month totals were in the 1st percentile: few months had ever had such low figures.

October 2002 (Blue): 3, 4, 5, 6, 9, 12, 15, and 18-month droughts.
September and October both had rainfall in the 18th percentile. That relieved the short-term shortages somewhat, but not those in the medium term. Shortages in the 4, 5, and 9-month totals were in the 1st percentile, but the 6-month total was very much worse. At 76 mm, it was the third driest on record, beaten only by August 1888 (43 mm) and September 1888 (69 mm).

December 2002 (Purple): only 9- and 12-month droughts remain.
November rainfall that was near average (40th percentile) and high December rainfall (84th percentile) broke the drought. Only some longer-term effects persisted as severe rainfall shortages in 9- and 12-month totals.

3-year trends to January 2015

Parametric plots of smoothed climate variables at Manilla
“January 2015: rainy cool days”

Trends to January 2015

 

January raw anomaly data (orange)

Anomalies for all variables except subsoil temperature moved across the graphs, from “droughts” in November to “flooding rains” in January. Subsoil temperature had been normal from February to December (11 months!), then became cooler than normal in January.
Most raw anomaly values for January were close to the fully-smoothed anomaly values of the La Niña-affected cool summer of 2012. This month’s daily temperature range was even narrower, and the subsoil temperature lower, but the daily minimum temperature was not so low.

Fully smoothed data (red)

The latest fully-smoothed data anomalies (July 2014) were near normal. (Dew point, like most recent values of that variable, was 3° lower than normal.)


Note:

Fully smoothed data – Gaussian smoothing with half-width 6 months – are plotted in red, partly smoothed data uncoloured, and raw data for the last data point in orange. January data points are marked by squares.
Blue diamonds and the dashed blue rectangle show the extreme values in the fully smoothed data record since September 1999.

Normal values are based on averages for the decade from March 1999.* They appear on these graphs as a turquoise (turquoise) circle at the origin (0,0). A range of anomalies called “normal” is shown by a dashed rectangle in aqua (aqua). For values in degrees, the assigned normal range is +/-0.7°; for cloudiness, +/-7%; for monthly rainfall, +/-14 mm.

 * Normal values for rainfall are based on averages for the 125 years beginning 1883.

Rainfall Deficiencies IV: 120-months Duration

Log of severe and extreme rainfall deficiency of 120-month duration at Manilla This is the fourth 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 fourth graph includes those periods of severe or extreme rainfall deficiency that last one-hundred-and-twenty months. They are rainfall droughts that affect about ten successive years.

(As I note below, in this series, a time of severe rainfall deficiency is one that is drier than the 5th percentile of cases, and a time of extreme rainfall deficiency is one that is drier than the 1st percentile of cases.)

In Manilla’s climate, a time of severe 120-month rainfall deficiency has a rainfall total less than 5860 mm, when it normally would be 6390 mm: that is, through the 10-year period, there is as an average a rainfall deficit of 53 mm each year. A time of extreme 120-month rainfall deficiency has a rainfall total less than 5670 mm: that is, through the 10-year period, there is as an average a rainfall deficit of 72 mm each year.

Even more than three-year droughts, these ten-year droughts have quite different effects to those that are shorter. The importance of severe and extreme rainfall deficiencies of 120-month duration is that even very large surface and sub-surface reservoirs may not be adequate to supply demand through to the end of the drought.

The graph shows that such ten-year droughts hardly occurred earlier than 1915 or later than 1955, but were confined to that 40-year interval. While deficiencies of this duration were twice as common as normal (5%) in the decades around 1920 and 1925, it was the decades around 1945 and 1950 that were extraordinary: 26% of all months had a severe rainfall deficiency of this duration.

Again, extreme 120-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 III: 36-months Duration

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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3-year trends to December 2014

Parametric plots of smoothed climate variables at Manilla
“December 2014 rain again”

Trends to December 2014

December data (orange)

From November to December, the raw anomaly value for daily maximum temperature returned to normal from extremely high. Rainfall anomaly moved from very dry to rather wet. Most other anomalies came back to near zero, where they had been (as smoothed values) in June 2014, six months earlier.
Temperature range became low while daily minimum temperature remained high. (Which causes which?)

Fully smoothed data (red)

The latest fully-smoothed data (June 2014) mainly show a renewed movement towards drought.


Note:

Fully smoothed data – Gaussian smoothing with half-width 6 months – are plotted in red, partly smoothed data uncoloured, and raw data for the last data point in orange. January data points are marked by squares.
Blue diamonds and the dashed blue rectangle show the extreme values in the fully smoothed data record since September 1999.

Normal values are based on averages for the decade from March 1999.* They appear on these graphs as a turquoise (turquoise) circle at the origin (0,0). A range of anomalies called “normal” is shown by a dashed rectangle in aqua (aqua). For values in degrees, the assigned normal range is +/-0.7°; for cloudiness, +/-7%; for monthly rainfall, +/-14 mm.

 * Normal values for rainfall are based on averages for the 125 years beginning 1883.