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.
I have described the data and the method in “Notes” to my post “Manilla’s Record of Droughts”, of 25 November 2014.
More readable graphs
In my post “Extreme Droughts by Decade at Manilla”, dated 23 December 2014, I graphed the occurrence of extreme rainfall droughts of different duration.
That graph suffers from a problem common in showing extreme events: they are so rare it is hard to see any pattern!
This time, I have used three “tricks” to improve the display:
- adding to the few extreme rainfall deficiency events the more numerous severe rainfall deficiency events, and displaying the two together;
- making the resolution finer by overlapping by five years the decades that are used for counting; and
- applying some smoothing ((1:2:1)/4).
At the beginning and end of the record, decades are incomplete. I have assumed that the rate of occurrence for the part-decade would continue for the full decade.
Rather than simply counting the rainfall deficiency events in each decade, I have expressed the result (on the y-axis) as a percentage occurrence. Since there are 120 months in a decade, the percentage is 5/6 of the count of months meeting the condition.