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Updated: Jun 14, 2020 21:28 IST

In August 2018, Kerala faced one of the worst floods in its history although this isn’t reflected in the annual rainfall data for the state. According to the Centre for Monitoring Indian Economy (CMIE), which has annual rainfall data from 1902, total rainfall in Kerala in 2018-19 was 3164.1 millimetres (mm). This is the 34th highest annual rainfall in the state, nowhere near extraordinary. So, why did the floods happen?

A shorter time window presents a clearer picture.

Again, according to CMIE, which has weekly and monthly rainfall data from 1989, there was nothing out of the ordinary about the rainfall in June, July, and September (three of the four months that are considered the monsoon season in many parts of India; August is the other month) in the state. The rainfall in June, July, and September 2018 in the state was the sixth, seventh, and 26th highest since 1989.

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August 2018 was different. It was the wettest since 1989 in Kerala.

Further disaggregation is even more illuminating.

In the two weeks ending 22 August, 2018, Kerala received 652.4 mm of rain, the highest in two weeks since 1 January 1989. It was this burst of rain, rather than a large increase in annual rainfall, which resulted in large-scale flooding in the state.

Rainfall patterns vary. That is why they are measured in terms of long-period averages rather than one year values. However, events such as the 2018 floods in Kerala have drawn attention to the trend of increasing skewness in rainfall patterns. Total rain in a year might not change significantly, but is it the case that increasingly, large parts of the monsoon season are dry and there is a deluge in a short time-span?

This is not a matter of mere statistical interest. No rain during large parts of the monsoon can delay and derail sowing and growth of crops. Sudden bursts of rain can lead to flooding and replenish less ground water than what spread-out and regular rainfall would do.

HT has analysed granular rainfall data from the India Meteorological Department (IMD) to check whether this is happening.

The short answer is that it is.

This is clear in the difference between the mean and median rainfall, which has been increasing in the first two decades of this century as compared to the last five decades of the last century.

A quick digression on the significance of the measure. Mean rainfall is the simple average of total rainfall divided by the number of rainy days. Median rainfall is the middle value of rainfall during the monsoon period when daily rainfall is arranged in increasing order.

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If much of the season is dry and much of the rain happens in a short intense burst, then the difference between the mean and the median will be high.

HT’s analysis shows that this value is higher in the last decade and this (2001-2010 and 2011-) than in any decade between 1951 and 2000.

This suggests that rainfall has become more skewed in the last two decades.

To be sure, this method is an approximation. This is because rainfall varies drastically across India. The India Meteorological Department or IMD provides rainfall data across grids (basically a small square of area). there are 4964 grids in India , which is almost eight times the number of districts (640) in the country at the time of the 2011 census.

Doing this exercise from 1950 to 2018 yields a plot with at least 342,000 points. The plot is shown in chart 2.

An upward shift in the number of points with time suggests that rainfall is getting more skewed across India, as the difference between mean and median rainfall is rising. The chart does show such a shift to some extent, which is in keeping with the summary trends shown in Chart 1.

If these trends intensify, we should prepare ourselves for a monsoon season, which has fewer rainy days, but more rain than what we’re used to.