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

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.

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