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.
Nearly all data points for this month, including the data point for the 30 years back to September 1986, show rainfall above normal.
The only data points showing rainfall below normal are in a cluster around the 42-month total, and also the 180-month total. The cluster reflects dry times around 2013, and the 180-month total the legacy of the extreme 6-month drought of 2002.
The lowest rainfall total for September 2016, the 42-month total (1986 mm) is in the 22nd percentile. That is not a drought. For it to be called a “42-month drought”, even a merely “serious” one, it would have to be in the 9th percentile at the highest.
Simply by looking at the graph, you can see that the most droughty data points for the current month are like the least droughty points for August 1946.
So, No, we are not having a drought of any sort.

Note 1: an earlier graph (2002).

I have presented a graph like this in an earlier post.

That one was about the progress of the extreme drought at Manilla in 2002. I tried to answer a question about it in a comment on the post.

Note 2: Drought severity

The classes of drought severity adopted are:
* 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.

Note 3: Long droughts vs. acute droughts

Droughts cannot be 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 more useful, 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 gets higher as the drought lasts longer. 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.

Note 4: March 1894 long-term values missing

For March 1894, no values can be calculated for groups of months greater than 120 months because the record began only in 1883.

2 thoughts on “Is There Any Drought Now?

  1. So is rainfall the only factor that defines a drought? I should have thought that other factors such as initial soil moisture content, and evaporation rates would be very relevant especially for short-term droughts like 1 month. If a very wet period (like we have experienced up until the end of September) was followed by a dry but very cool and cloudy month with low evaporation, would that still qualify as a 1-month drought? The soil might still be wet at the end of it!

  2. Thanks for the question, Allan.
    Indeed there are other factors than rainfall but, as you say, they are
    “…relevant especially for short-term droughts like 1 month.”
    That limits their importance in a couple of ways.
    First, the factors tend to converge within three or four months. It is one of the things I have found out, as I have said in various posts, and the series of 3-year climate trend graphs. Our climate swings between between two types: droughts, and moist non-droughts. The droughts are marked not only by by low rainfall, but also by low humidity, cloudless skies, high daily temperature range, high daily maximum temperature, and high soil temperature. As a drought goes on, generally all of these symptoms appear. In particular, major droughts have always had hot sunny days as well as shortage of rain.
    Second, the very short droughts where these mismatches are likely, are not even called droughts officially. I put one-month and two-month droughts on my graph just so people wouldn’t wonder why I had left those numbers out.
    It has been long-standing policy of the Bureau of Meteorology not to publish drought maps for durations shorter than three months.
    The current Australian Government scheme of Drought Concessional Loans demands evidence of a drought of no less than 12 months duration. If my calculations correspond with those of the Bureau of Meteorology (as I think they do), the 2002 drought at Manilla would just barely qualify. It was an “extreme” 6-month drought, but only a “severe” 12-month drought, if that.

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