Deficit rainfall in some districts, flooding in nearly all: A look at Assam deluge

Assam is facing one of its worst floods in recent memory: Eight of the 27 meteorological districts had deficit rainfall this monsoon, and yet, almost all districts continue to suffer from some level of flooding, according to data from the India Meteorological Department (IMD).

This implies that while overflowing rivers, streams and other water bodies have engulfed most parts of the state, rainfall has not been well distributed.

For instance, Dhemaji district received 2,067 millimetre of absolute rainfall between June 1 and July 22, 2020, which is 136 per cent (highest in the state) more than the normal. Darrang, on the other hand, received 57 mm less rain than normal. Other districts such as Nagaon and Morigaon also had rainfall deficit of 50 per cent and 43 per cent respectively.

Only 12 districts had excess or large-excess rainfall.

The three waves of floods in the state have killed 115 people and affected 56 lakh as on July 23, according to the Assam State Disaster Management Authority. The Union Jal (water) Shakti Ministry has announced a Rs 346-crore relief package for the state.

The first wave of floods occurred in end-May, when the south-west monsoon season had not even started. The second wave occurred at the end of June and the third in July.

Till June 22, Assam had received 309 mm of absolute rainfall, which was a per cent more than the long-term normal for that time of the year, according to IMD data.

The state recorded 34 mm of absolute rainfall in the next two days, which increased the excess rainfall percentage to two per cent. On June 25, during the first wave of intense monsoon rains, the state received 51.5 mm rainfall in a single day, which was 29 times the normal for that day. It brought the seasonal absolute rainfall to 395 mm and the excess rainfall to 13 per cent.

This intense rainfall was the beginning of the second wave of floods in Assam. There has been some respite in the past few days, however.

“It is flooding over Delhi; rains have decreased over North East. This see-saw between the North East and central India is expected,” said Raghu Murtugudde, climate scientist at the University of Maryland.

“The North East experiences rains each year when the core monsoon zone and the Gangetic plain are in a break period,” Murtugudde added. When it is a break period over the North East, it rains over central India or the core monsoon zone. The rains then shifts to gangetic plain.

Heavy rains lash for different reasons in different places of India. Strong south-westerlies produce heavy rains on the west coast, but a combination of south-westerlies and easterlies from the Bay of Bengal produce large-scale heavy rains over the core monsoon zone.

The North East mostly gets its moisture from the Bay of Bengal. But heavy rains alone don’t cause floods: urbanisation, deforestation and mono-cropping are also responsible for causing floods during heavy rains.

Rainfall has been below normal over the west coast and the core monsoon zone this year. For example, 13 of Kerala’s 15 districts have had less rainfall than normal.

The worst hit was Wayanad with a rainfall deficit of 55 per cent. Both Thrissur and Idukki received 42 per cent less than normal rainfall. Similarly, many districts in coastal and interior Karnataka such as Dakshin Kannada, Kodagu, Chikmaglur Shimoga and Hassan are reeling under a dry spell.

“The west coast is dry, which implies that it is not south-westerlies bringing the monsoon, but the Bay of Bengal,” says Murtugudde.

As the moisture gets pumped in from the Bay, the complex mountain territories of the north eastern states squeeze out rain and produce a very patchy distribution over the North East. This is why some places are flooded while others experienced below-normal rainfall, according to Murtugudde.

This has been the story of the north eastern region for quite some time now. In 2019, the region recorded a deficit of 12 per cent while the rest of the country had an excess rainfall of 10 per cent — the highest in the last 25 years.

The North East has been drying up for a while, especially in the last few decades. During 18 of the last 19 years (2001-2019), North East India has received seasonal rainfall less than normal with an exception of 2007 (110 per cent of normal), according to an IMD report from 2019.

“This indicates that the seasonal rainfall over North East India is passing through a below normal epoch like it was during early 1950s to mid-1980s,” according to IMD.

In a research paper published in Current Science journal in 2011, scientists had predicted that climate change will affect agriculture, water availability and forests in the North East. The number of drought weeks during monsoon months in Arunachal Pradesh, parts of Assam, Meghalaya, Mizoram, Tripura and Manipur will increase by 25 per cent in the future, according to scientists.

“I am worried we have below normal rain over the west coast and the core monsoon zone this late into July. But if heavy rains also occur in August, and withdrawal is delayed like it did last year, we may again have a ‘normal’ rain, with a distribution in space and time that is not really ‘normal’,” said Murtugudde.

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