Rainfall Decreases In North-East But Sudden Downpour Leading To Frequent Flooding


New Delhi: Studies based on India Meteorological Department (IMD) data show that the north-east, especially Assam, has witnessed a “significant decreasing” trend in the average monsoon rainfall since 1870 even as extreme rainfall or sudden downpour days that lead to frequent flooding are on the rise.

Floods, which are an annual phenomenon in Assam, have marooned 30 of the 33 districts of the state affecting around 4.28 million people, according to the state disaster management authority officials. (By Wednesday, the number of districts affected had reduced by one). They, however, claimed deaths were lesser this year because of “government preparedness”. As compared to 30 deaths this year, 45 people died in floods in 2018, 85 in 2017 and 38 people in 2016, according to Assam government data.

The long-term rainfall data for Assam, however, shows a decreasing trend that has accelerated since 1981. The annual average rainfall for Assam is 1,524.6 mm. The average rainfall deficiency between 1871 and 2016 was 0.74 mm per decade, but in the period between 1981-2016, the average rainfall deficiency has been 5.95 mm per decade.

A study conducted by Punebased Indian Institute of Tropical Meteorology (IITM) last year made this observation based on IMD data. In the same period, east Uttar Pradesh, and Meghalaya and sub-Himalayan West Bengal have also shown a decreasing trend in average rainfall, while an increasing trend can be seen in East Rajasthan, Saurashtra, Kutch and Diu, the study found.

The study, based on the monsoon rainfall data between 1871 and 2016 for 306 rain gauge stations across India also showed that the mean monsoon rainfall has also decreased “significantly” — about 10 percentage points — for other north-eastern states of Nagaland, Meghalaya, Manipur, Mizoram and Tripura during the same period.

Thus, the amount of rainfall in this region is falling, and the deficiency is widening. M Rajeevan, who co-authored the study with D R Kothwale, confirmed that the average rainfall pattern in Assam was indeed falling, especially during the monsoon months.


However, a look at the daily rainfall data clearly indicates that days where there have been sudden downpours [or extreme rainfall events] have in fact increased, thus leading to extreme flooding in several parts of the Brahmaputra valley in the state, Rajeevan pointed out.

As one-fourth of the total rainfall during four monsoon months (June to September) happens in July, according to the IITM study, the probability of flooding during July is the highest. Various studies in the past have shown that average monsoon rainfall in India was falling whereas extreme rainfall events have increased by about 5%.

Indeed, this might be one of the reasons why we do not perceive the problem: we assume that since there is flooding, it must mean that the season’s rainfall has been excessive.

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