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Opinion

This monsoon, Manipur, not Delhi, has the largest rainfall deficit

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June 1 to September 30 is officially considered as the monsoon season in India. The actual amount of rainfall during this period varies from year to year.

The first three weeks of this year’s monsoon season have been exceptionally good. In terms of cumulative rainfall, the monsoon rain for all of India up to June 22 this year is 144.15mm. This is the twelfth highest accumulated precipitation for this period since 1901, the earliest period for which precipitation data is available from the Indian Meteorological Department (IMD).

Certainly, not all parts of India have received abundant rains. Delhi, which is perhaps going through the worst phase of a prolonged summer season at the moment, is among one of them. But it is not the only region with a rainfall deficit. The northeastern states have the largest rainfall deficit this monsoon season. Here are four charts that summarize the progress of the monsoon so far.

Read also | IMD incorrect monsoon forecast series for Delhi

The windfall in accumulated rainfall appears to be evaporating

Because rainfall varies each year, the standard practice for measuring the variation in rainfall is to compare the rainfall for each year with what is called the Long Period Average (LPA). LPA is defined as the precipitation over a particular region in a given interval averaged over a long period. The long period used by the IMD currently is from 1961 to 2010.

This year’s monsoon started off with a bang. Rainfall on June 1 was 83.7% more than the LPA and the 18th highest in absolute terms since 1901. But it lost momentum in the first week; The accumulated precipitation was 23.75% more than the LPA on June 8. This margin increased to 46.83% on June 20 and was 36.24% on June 22. As noted above, the accumulated precipitation so far is the 12th highest since 1901.

The number of the title on the precipitations that exceed the LPA hides regional variations

Not all regions have received higher than usual rainfall this monsoon season. An HT analysis of the IMD gridded database (each grid is a box covering 0.25 degrees of longitude and latitude on the map) shows this clearly.

Of the 613 census tracts from 2011 (Delhi districts merged as one and 19 districts are too small in area or irregular in shape to be captured by a grid) for which this calculation is possible using the IMD gridded dataset , 359 have received excess to large excess rainfall compared to the LPA, while 254 districts have received normal to large deficient rainfall.

These categories are based on deviation ranges of current precipitation from the LPA value. Most of the districts with a large excess of rainfall so far are in the Ganges plains or in central India. 12 of 13 districts in Uttarakhand, 55 of 71 districts in Uttar Pradesh, 30 of 38 districts in Bihar, 20 of 24 districts in Jharkhand, 8 of 19 districts in West Bengl and 24 of 50 districts in Madhya Pradesh have received a large excess of rain.

At the state level, Manipur has the largest accumulated rainfall deficit compared to the LPA. Many other northeastern states also have a rainfall deficit compared to the LPA. Delhi ranks third among states in terms of rainfall deficit relative to LPA. It should be noted that a higher percentage deficit with the LPA tells us nothing about the actual precipitation in a region, because the LPAs themselves vary drastically. For example, even with a 62% LPA deficit, Manipur’s cumulative rainfall is 79.44mm, much more than the 21.24mm for Delhi, which has a 34% LPA deficit.

The days of heavy rain have not experienced a big jump so far

The nature of the rain matters as much as the amount of rain in a monsoon season. In theory, the accumulated precipitation will be the same whether it rains in some parts throughout the season or it rains heavily in just a few days. Heavy rain events also cause damage such as flash flooding or waterlogging and traffic disruptions.

The IMD classifies rain into light intensity (or 7.5 mm or less in 24 hours on a grid), moderate intensity (7.5 mm-35.5 mm in 24 hours on a grid) and heavy and extreme intensity (greater than 35.5 mm in 24 hours). on a grid). An analysis by HT shows that, in absolute terms, it is light and moderate rain, which has been exceptionally high this year; they occupied the fifth and seventh highest respectively since 1901. Intense and extreme rains, this monsoon so far is only the 24th

Unsurprisingly, there are regional variations here too. The proportion of heavy and extreme rains this year is the sixth highest since 1901 in Uttarakhand, the twelfth highest in Uttar Pradesh and the eighteenth highest in Bihar.

This monsoon, Manipur, not Delhi, has the largest rainfall deficit

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