Explainer: New normal rainfall based on long period average introduced by IMD

The India Meteorological Department (IMD) on Thursday introduced the new rainfall normal based on rainfall data from 1971-2020 for the southwest monsoon season, replacing the current rainfall normal that was based on data from 1961-2010.

Called the Long Period Average (LPA), it will be 87 cm now, replacing the 88 cm from earlier 1961-2010 data.

"The current rainfall normal that was being used was introduced in 2019 and was based on rainfall data from 1961-2010. The new normal is based on data from 1971-2020 and it will be effective from June 1, 2022," IMD Director General, Meteorology, Mrutyunjay Mohapatra said.

Also Read: Southwest monsoon likely to be normal across India, says IMD

With the change from 88 cm (880.6 mm) based on 1961-2010 to 87 cm (868.6 mm) in all-India rainfall normal based on based on 1971-2020, the new all-India annual rainfall normal based on 1971-2020) will be 116 cm (1,160.1 mm) compared to the earlier normal of 117 cm (1,176.9 mm) based on 1961-2010, again showing a slight decrease in the rainfall.

It was 89 cm for duration 1951-2000 and was introduced with a delay of 7-8 years. The next one would be introduced after a decade.

"This decrease is part of natural multi-decadal epochal variability of dry and wet epochs of all India rainfall. Presently it is in a dry epoch which started in the decade 1971-80. The dry decade peak is over and now it is reviving. The decadal average of all India SW monsoon rainfall for the decade 2011-20 is -3.8 per cent below the long-term mean. The next decade i.e. 2021-30 will come close to neutral and will likely to enter in the wet epoch from the decade 2031-40," he said.

Explaining the reasons for the change, Mohapatra said: "As per the international practice, we go for change every 10 years to take into consideration various aspects. One of them is the climate variability that changes over the period of time. Second, the number of rain gauge stations increases over the period of time giving us more data that is uniformly distributed. So, it (forecast) becomes more realistic and also caters to requirements of smaller regions and specific locations."

Compared to the new normal introduced in 2019 (based on 1961-2010 data), the next change - the current one - was introduced relatively faster. "That is because, within a period of two years, we could update our normals; it took a long time earlier and now we have introduced automated processes of data reception, data delegation and calibration of instruments."

"The new rainfall normal has been computed using rainfall data from 4,132 rain gauge stations well distributed over the country representing 703 districts of India," he said.

South-west monsoon rainfall contributes 74.9 per cent of annual rainfall with June, July, August, and September contributing 19.1 per cent, 32.3 per cent, 29.4 per cent, and 19.3 per cent respectively to the total SW monsoon rainfall. Also, the all India pre-monsoon rainfall (130.6 mm) is around 11 per cent of annual rainfall and while north-east monsoon contributes around 10 per cent to the annual rainfall of India.

"The proportion of rainfall from the monthly value to seasonal value does not change while we go for changing rainfall normal. Similarly, the amount of rainfall contributed by the SW monsoon season to the annual rainfall also does not change," Mohapatra said.

Gujarat region receives maximum rainfall i.e. 96 per cent of its annual rainfall in south-west monsoon season while Tamil Nadu receives maximum rainfall i.e. 48 per cent of annual rainfall in post-monsoon season (October-December) followed by 36 per cent of annual rainfall in southwest monsoon season.

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