IMD proposes to revise quantum downward from the long period average of 89 cm by 2%

The India Meteorological Department (IMD) is likely to revise downward by 2% the quantity of ‘normal’ monsoonal rain, two independent sources confirmed to The Hindu.

Currently 89 cm of monsoon rain from June to September is considered ‘normal’, more technically, the long period average (LPA), and this is derived from the average rainfall that the country got from 1960-2010.

Surge in drought

Were this change to happen, the definition of ‘normal’ rainfall could dip below 88 cm, unprecedented since 1950. The dip in average rainfall is largely due to a surge in drought and depressed rainfall since 2000; 13 out of the last 18 years have seen ‘below normal’ rains (where rainfall is less than 95% of the normal) in India and there have been droughts (where rainfall was less than 90% of the normal) in 2002, 2004, 2009, 2014, 2015, 2016 and 2017.

The IMD routinely updates ‘normal’ rain every 10-12 years but the changes can take nearly five years to reflect in the agency’s official forecasts. This time the change could be reflected by 2021 after taking into account the monsoon rainfall from 1970-2020, a senior meteorologist associated with the IMD's forecast process told The Hindu.

This year the IMD has forecast monsoon rains to be 96% of the LPA, just shy of ‘below normal’ rainfall.

Research into the monsoon has observed a rough 30-40 year cycle of a rise and fall in average rainfall when computed since 1900. Since the 1990s India is in a ‘low epoch’ meaning that average rainfall has been below normal and hasn’t recovered. The average temperature over India has also increased by half a degree in the last year and, as climate scientists repeatedly point out, global warming has generally made Indian rainfall more erratic and prone to bursts of intense rainfall or long periods of drought. “The several droughts in recent years have depressed averages and there could be climate-change link to it,” another meteorologist connected to the IMD said.

This year the monsoon has been 16% short over India so far, largely due to a shortfall in June. It has improved in July and the IMD has forecast a pick-up in the coming days due to a 'monsoon pulse' in Southern India. Kerala, the Konkan coast, Odisha and Madhya Pradesh will see heavy rainfall.

Separately, the IMD is also examining an internal report on whether the monsoon withdrawal and onset dates in parts of central India and north India need to be changed. This is because records over nearly two decades show that the monsoon rainfall after entering Kerala on June 1 has in recent decades been stalling and sluggishly advancing into parts of central India, eastern Uttar Pradesh, Rajasthan and also withdrawing late.

“These changes are needed to update agricultural advisories so that farmers can make better decisions on sowing their crop,” said a senior IMD meteorologist. However there would be no changes to the dates of the onset and withdrawal of the monsoon over India, which remain June 1 (Kerala) and September 30 (Rajasthan) respectively.

IMD Director-General K.J. Ramesh said the IMD was still in the process of studying these recommendations and no decision had yet been taken on changes of dates or on the definition of ‘normal.’

“This is a long-drawn process and we are studying the changes. For a change to be made, it has to be statistically significant in say a 30-year time frame. A delayed monsoon in four to five years wouldn’t necessarily mean a change in dates,” he told The Hindu.