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 1910. In contrast to the 1902 event, these drought events of varying duration all commenced near the same time in 1910-11 and ended at different times (the longer, the later). Meanwhile, extreme 2-month drought events occurred in May 1912 and August 1913, and extreme 2-, 3-, and 4-month events in September 1914. These all coincided with the final month of drought events of much longer-duration (15-month, 30-month, 48-month) that fell on the smooth curve extending from April 1911.
Contrasting drought clusters of 1902 and 1912
These two extreme droughts may each be thought of as clusters of drought elements of different duration. The drought elements in the 1902 cluster all ended near the same date, having commenced at quite different dates, the longer the duration, the earlier the commencement. On the contrary, in the 1912 cluster, the drought elements all began near the same date (1911) and ended at different dates; the dates that are plotted on the graph and form a smooth trailing curve.
Notes on the plotted data.
An enlarged graph
The graph above is enlarged to allow the months of rainfall drought to be seen. It is based on an earlier graph (right) that shows the complete record.
The graph above is one of a series of four enlarged graphs, each spanning 36 years (or less):
* 1884 to 1916;
* 1914 to 1949;
* 1947 to 1982;
* 1980 to 2015.
The classes of drought severity plotted are:
* Extreme rainfall shortage: rainfall in the 1st percentile only.
* Severe rainfall shortage: rainfall in the 2nd to 4th percentiles.
This graph does not include a third class plotted on the original graph:
* Serious rainfall shortage: rainfall in the 5th to 9th percentiles.
Long droughts vs. acute droughts
Droughts are not defined by a given low percentage of normal rainfall, because a given percentage, such as 10%, which might define a short drought, is found never to occur for periods of more than a year. Percentile values are better, and I have used them here.
Despite the use of percentile classes, droughts of a given class, such as “Extreme”, that are defined by a given percentile occurrence (extreme is defined as first percentile only), are less acute when they are long than when they are short. The percentile class limit is constant, but the rainfall rate at that percentile varies with the drought duration. The effect is very large. For example, for an extreme rainfall drought of 3 months duration, the rainfall value at the first percentile is 26 mm, which is only 17% of the mean 3-monthly rainfall of 150 mm; such a shortfall is obvious. For an equally extreme rainfall drought of 120 months duration, the rainfall value at the first percentile is 5670 mm, which is fully 89% of the mean 120 month rainfall of 6390 mm; such a shortfall seems scarcely worth notice.
Due to the way the data were derived, droughts are plotted here on the last month of their duration. The rainfall shortage of a 6-month drought, for example, began five months earlier than the date plotted, and did not extend later than that date.