Number Theory: How rainfall pattern is changing significantly

Total monsoon rainfall in 2020, as of 8.30am on Sunday, is the 11th highest for this period since 1901. However, 2020 seems to be an outlier when read with long-term rainfall statistics. Monsoon rainfall in India has been gradually declining since the 1950s. This trend has been accompanied by rains becoming more skewed; most of the rains fall over a smaller period of time. Rains are also getting more intense, meaning there are fewer episodes of rain, but when it rains, it pours. In 2019, heavy and extreme intensity rainfall was the highest since 1901.

Total monsoon rainfall in the country this year, as of 8.30am on Sunday, has been 771.1 mmTotal monsoon rainfall in the country this year, as of 8.30am on Sunday, has been 771.1 mm, according to the India Meteorological Department (IMD)’s gridded rainfall data set. This is the 11th highest monsoon rainfall for the June 1-August 30 period since 1901. That makes 2020 an outlier already. Decadal averages of monsoon rainfall up to August 30 suggest that total monsoon rainfall in India has been declining. The period from June 1 to September 30 is considered the monsoon season.

Not only are rains declining, they are also getting skewed

This decrease in total rainfall has accompanied another trend: the decrease in the number of days on which at least half the rain falls. In the last decade (2020 has been excluded to see the pattern over the entire monsoon season), 50% of the rainfall over the 122-day monsoon was seen on an average of 40.4 days. This was the fastest in the 12 decades since 1901. To be sure, the corresponding time for 75% of rainfall and 90% was only the fourth highest (70.6 days and 94.3 days respectively), although the long-term trend for those higher shares of rainfall is also the same: it takes fewer days now.

Regional variations in the number of days it takes for rainfall to take place

The all-India numbers hide a significant regional variation in this pattern. For example, Delhi received 50% of its total monsoon rainfall in 2011-19 in just 83 days. This number was 30 for Karnataka.

When it rains, it pours

Skewed rainfall could mean an increase in heavy and extreme rainfall. And this has indeed happened for India as a whole since the second half of the last century. Combined heavy and extreme intensity rainfall was the third highest in this decade. In 2019, heavy and extreme intensity rainfall was the highest since 1901. A 24-hour rainfall between 35.5 mm and 244.5 mm in a grid - this analysis uses IMD’s gridded rainfall data set - is considered heavy intensity rainfall and that above 244.5 mm, extreme rainfall. A grid is a box bound by two latitudes and longitudes 0.25 degrees apart.

Skewed rainfall does not guarantee heavy rainfall

The two trends - days taken for a certain proportion of monsoon rain to fall, and its intensity - can diverge when the total monsoon rainfall is decreasing. This is exactly what has happened over north-central India, the lndo-Gangetic plains and the northeastern states. With declining total rainfall, the amount of heavy intensity rainfall has decreased although the number of days it takes for 50% of monsoon rainfall to fall has reduced.