This monsoon has seen second highest ‘extreme’ rainfall events since 1901

From Silchar district of Assam in June to vast tracts of Gujarat in July, this year’s monsoon has brought a lot of destruction and misery for people across the breadth of the country.

From Silchar district of Assam in June to vast tracts of Gujarat in July, this year’s monsoon has brought a lot of destruction and misery for people across the breadth of the country. This might appear odd when read with the fact that the overall monsoon performance is in the normal range and there is large-scale rainfall deficiency in states such as Bihar, Jharkhand, West Bengal and Uttar Pradesh.

(HT PHOTO)

An HT analysis of rainfall data shows that the 2022 monsoon has seen more extreme rainfall events than most monsoons. This is in keeping with the pattern of increasing skewness – patches of intense rain with very little rain in between – in monsoon rainfall in recent years, likely a result of the climate crisis. Experts also warn that unplanned development leading to obstruction of natural drainage systems is only going to decrease the rainfall threshold for flooding. Here are three charts which explain this argument in detail.

How does IMD classify rainfall intensity?

According to the India Meteorological Department (IMD), the intensity of daily rainfall(smaller time intervals are not available in the gridded dataset) can be categorized into four broad categories: light for rainfall of up to 7.5 mm in a grid or station; moderate for 7.5 to 35.5 mm rainfall; heavy for 35.5 mm to 244.5 mm rainfall; and extreme for more than 244.5 mm rainfall. As is to be expected, the overall monsoon rainfall is distributed into these four categories. A look at the average distribution of rainfall from 1901 to 2021 shows that extreme rainfall events are very rare.

2022 monsoon has seen 2nd highest extreme rainfall since 1902

According to IMD’s gridded dataset, India has received 317.8 mm rainfall so far this monsoon (from June 1 to July 13). This is the 20th highest since 1901 and 15.8% more than the 1961-2010 average. A deviation up to 19% from the 1961-2010 average rainfall is considered to be in normal range, so this number does not explain the floods in various parts of the country. The puzzle falls into place once the overall rainfall data is broken into the four categories described.

The distribution of the 317.8 mm overall rainfall this monsoon shows that its high rank is because of heavy and extreme rainfall. While light and moderate rain is ranked 55th and 36th since 1901, heavy rain is ranked 18th and extreme rain is ranked second, only behind 1974. That heavy and extreme rain is the outlier this monsoon can also be seen from their respective departures from the 1961-2010 average -- much higher than for light and moderate rainfall.

Extreme rainfall has been increasing in the recent past

The climate crisis has added to the volatility of the monsoon system in India, which also means that extreme rainfall events have increased compared to the past. This is an officially acknowledged fact.

“There has been a shift in the recent period toward more frequent dry spells (27% higher during 1981–2011 relative to 1951–1980) and more intense wet spells during the summer monsoon season,” the earth sciences ministry said in a 2020 report that assessed the impact of climate change over India. This shift can also be seen through the gridded dataset that shows a rising trend in the share of heavy and extreme precipitation in total monsoon season rainfall in the year.

There is also a man-made angle behind the recent floods

While some of the regions which have experienced flooding are facing the crisis for the first time, such rains are not necessarily unprecedented. Only 49 of 4,685 grids in IMD’s dataset have received extreme rainfall during this monsoon. All of them are located in or adjacent to districts that have received such rainfall at least once in the 1961-2010 period. Only six of 2,949 grids that have received heavy rainfall this year did not receive any such rainfall in the 1961-2010 period during this part of the monsoon. This raises questions on lack of preparedness to handle such events, and also brings into play the role of human interference with natural drainage systems which are crucial for discharge of rainwater in case of heavy rainfall events.

Change in land use and land cover due to increased demand for food, shelter, or infrastructure; and the resulting loss of natural drainage paths combined with inefficient artificial drainage system could be one reason for floods even in places that are not necessarily receiving rainfall of higher intensity, said D Sivananda Pai, director of the Institute for Climate Change Studies, an autonomous research institution under the Kerala government. Natural drains being covered for construction could also be creating a flood risk in a similar manner, he said.

At a time when the climate crisis is increasing the risk of extreme or heavy rainfall events, destruction of natural drainage systems is a step in the wrong direction.