Explained: What new monsoon dates mean

Sowmiya Ashok, Anjali Marar

The four-month southwest monsoon season, which brings as much as 70 per cent of the country’s annual rainfall, officially begins on June 1, with the onset over Kerala, and ends on September 30. (File)

Earlier this week, Earth Sciences Secretary M Rajeevan announced that the India Meteorological Department (IMD) had decided to revise the normal onset and withdrawal dates for the monsoon in some parts of the country from this year.

The four-month southwest monsoon season, which brings as much as 70 per cent of the country’s annual rainfall, officially begins on June 1, with the onset over Kerala, and ends on September 30. It takes about a month and half after onset on the Kerala coast to cover the entire country; and about a month, beginning from the northwestern parts of the country on September 1, to withdraw completely.

Although the June 1 date for the onset of the monsoon on the Kerala coast is unlikely to be changed, the dates for onset in many other parts of the country are expected to be revised. Mumbai, for example, expects to start getting rain from June 10 — the revision is likely to push this date back by a few days. Adjustments are likely to be made for many other parts of the country as well. Similar changes are expected in the withdrawal dates.

Effectively, the monsoon is now expected to have later arrival and withdrawal dates in most parts of the country.

Why was this revision needed?

The main reason for the revision in the normal dates is the changes in precipitation patterns that have been taking place over the last many years. In the last 13 years, for example, only once has the onset over the Kerala coast happened on June 1. While two or three days of earlier or later onset falls within the yearly variability, in several years the onset happened five to seven days late.

Similarly, the commencement of withdrawal has happened in the first week of September only twice during this period, and last year, the withdrawal started as late as October 9 — and was completed in around just a week.

Effectively, the monsoon is now expected to have later arrival and withdrawal dates in most parts of the country.

“This change (of dates) was inevitable because things have been different for more than a decade now. A delayed onset seems clear but the withdrawal is never dramatic, and it may be conflating with the northeast monsoon (winter monsoon, which begins in October) in some places. So we have to watch how the monsoon trough and the monsoon rain itself are related in September and October. Is it still monsoon if the trough has retreated but rainfall is occurring over some parts of India? This question has not been considered seriously so far,” said Raghu Murtugudde of the University of Maryland, College Park, United States.

Within the four-month season too, rainfall has been observed to deviate from normal expected patterns very frequently.

One of the significant changes being noticed is that rainfall is getting increasingly concentrated within a narrow band of days within the monsoon season. So, there are extremely wet days followed by prolonged periods of dry days. A report in this newspaper in September 2018 used IMD data to show that over several previous years, nearly 95 per cent of monsoon precipitation in 22 major cities of the country had happened over a period of just three to 27 days. Delhi, for example, had received almost 95 per cent of its monsoon rainfall over just 99 hours. And half of Mumbai’s monsoon rain had fallen over just 134 hours, or five and a half days, on average.

Patterns of regional variations in rainfall are also changing.

Areas that have traditionally received plenty of rainfall are often remaining dry, while places that are not expected to get a lot of monsoon rain have sometimes been getting flooded. Climate change could be one of the factors driving these changes, but there could be other reasons as well.


What will be the impact of IMD’s move?

The revisions are meant to reflect the changes in precipitation patterns in recent years. They will help the IMD track the monsoon better, and improve what Rajeevan described as its “impact-based” forecasts. But the revisions will have implications beyond IMD’s operations, too.

New dates will likely nudge farmers in some parts of the country to make slight adjustments in the time of sowing their crops. “It would definitely have an impact on our agriculture practices — when to start sowing, when to harvest. Farmers would probably have to make small adjustments in these dates,” Rajeevan said.

Agro-meteorologists, however, agree that more than the onset, it is the information about the spatio-temporal distribution of rainfall that will be more helpful for farmers.

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Farmers sowing paddy after monsoon rains in Dairhi village on the outskirts of Mohali in Punjab. (Express Photo: Sahil Walia)

“A timely onset of rainfall followed by a prolonged break in the rains would mean that the sown seeds would not benefit. So, even if there is a delay in the arrival of monsoon by three to four days over a region, it would not matter much if there is a fairly good rainfall distribution thereafter,” said Kripan Ghosh, head of the agrimeteorology division at IMD, Pune.

Ghosh said crops that need transplantation, such as rice, require advance knowledge about the arrival of rain. “If the rainfall over the rice-growing regions occurs very late, then the transplantation of rice would be affected, which in turn could hit the crop yield. However, sometimes, a slight delay in sowing can save the crop,” Ghosh said.

Rajeevan said the change in dates would affect water management practices as well.

“Water management agencies, for example those managing the dams in the central plains, should now expect more rain only in the latter part of June. Instead of planning only until the start of June, they would now be prompted to preserve and hold on to some water until later in the month. Similar adjustments would need to be made towards the end of the monsoon season as well,” he said.

The planning that goes to beat the heat — several cities execute heat action plans — just ahead of the monsoon would have to factor in the need to be prepared for longer periods of heat.

Rajeevan said many other activities including industrial operations, the power sector, or those using cooling systems, would also need to change their behaviour. The power grid can, for example, have more realistic planning for peak periods of electricity consumption in certain months.

Ultimately, the change in normal dates of the onset and withdrawal of the monsoon would help people understand when to expect rains, and to plan their activities accordingly. The changed dates are expected to be announced in April, when the IMD makes its first forecast for the monsoon.

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