More than 7,000 people have been relocated in Gujarat, and schools in two districts of Rajasthan shut after a third consecutive day of heavy rainfall battered parts of Saurashtra, northern Gujarat and southern Rajasthan. The India Meterological Department has also issued warnings for intense rain in parts of Odisha, West Bengal, Jharkhand and eastern Chhattisgarh at least until Tuesday. Six districts in Odisha have been at risk of floods and bus services suspended in Midnapore in West Bengal.

This heavy rain has developed because of an unusual formation of the monsoon trough, an area of low pressure that generates intense rainfall. Low pressure regions develop frequently over the Bay of Bengal and move West or North West, drawing rain to the northern regions of the country. This year, the pattern has been different, with two separate depressions present at the Arabian Sea and Bay of Bengal at the same time.

“This time, a depression formed over the Bay of Bengal and simultaneously also at the Arabian Sea,” said Mahesh Palawat, chief meteorologist at Skymet Advisory, a private weather forecasting company. “The monsoon trough is now in a dumbbell-like position, which is rare.”

This is the second such weather system to have developed over Gujarat in this month, Palawat said. In the second week of July, a similar depression developed over South East Uttar Pradesh and travelled to Gujarat across Madhya Pradesh, leading to heavy rains and floods at that time.

The present system developed around Odisha around July 21 and moved to Gujarat via Andhra Pradesh, Telangana and Madhya Pradesh. This system will now move towards South West Rajasthan over the next two days before it reduces and dissipates.

“This depression or low pressure area is formed if there is warming over the ocean,” Palawat said. “Normally, it travels North West, but for the last four to five years, we have begun to see this pattern of the depression travelling in a westerly trend.”

Before this, he said, the normal monsoon pattern was that depressions that developed in the Bay of Bengal would travel North West instead of West. This, he added, is the reason central India has had higher rainfall in this period and North India less rain each year.

In all, India has had 3% more than normal rainfall so far, said Charan Singh, a scientist with the National Weather Forecasting Centre at the India Meterological Department, with parts of Gujarat and Jharkhand receiving up to 200 millimetres of rain on Sunday. However, three of 36 subdivisions have received highly deficient rains. These are in Karnataka, Kerala and Tamil Nadu.

The red and yellow colours on the map shows places with deficient rainfall, while the blue shows excess rainfall.

(Image credit: India Meteorological Department).

River basins and reservoirs full

While flooding is common in eastern parts of India in July, this remains a relatively limited occurrence in the West. And heavy rains explain only one part of why parts of Gujarat and Rajasthan are facing such severe floods. Other reasons include their soil composition and drainage systems.

“Most rivers in Gujarat outfall into the Arabian Sea,” said VD Roy, director of flood forecasting and management at the Central Water Commission in Delhi, which has also been issuing flood advisories. “River flow is also very mild in Saurashtra and Kutch and there was a four metre high tide in the sea. That is why water was not draining.”

Now that the tide has come down, he added, floods would recede in these parts of Gujarat.

Roy said that the problem was different in Rajasthan, where the soil composition affects water drainage. Rivers such as Luni in Barmer drain westward, but because of the sandy soil, they does not have well-defined channels.

“Whatever rainfall occurs on the ground, it will get accumulated in low lying areas and doesn’t get drained out into the Arabian Sea,” Roy said. “And the morphological structure of that area is such that there is an impervious layer below the top soil, which is why water is not getting percolated below.”

A look at the river basin rainfall map of India shows just how unusual the rainfall is in this part of the country. The Luni river basin has received, as of July 23, 72% more rain than normal. The Sabarmati and Mahi basins have received 42% and 26% more than normal.

(Image credit: India Meteorological Department).

A third reason for flooding lies in reservoirs getting full. In Rajasthan, the soil might prevent too much permeation or runoff towards basins, but in Gujarat, 30 of 133 reservoirs are now 100% full, Roy said and 12 between 90% to 100% full. The Central Water Commission identifies such reservoirs as causes for high alert as states are likely to release excess water from these to prevent damage to the structures.

Agriculture affected

Flooding in these states will definitely impact agriculture in these areas, said Anand Sharma, scientist with the Agromet Advisory division of the India Meterological Department.

“Cotton and groundnut will be affected in parts of Gujarat where there has been more than 30 cm of rain and where there is waterlogging,” Sharma said. “Farmers in parts of Gujarat and Rajasthan may have to undertake resowing.”

Gujarat, he said, already had formed contingency plans for planting gram, sunflower and castor oil in the place of damaged crops after the inundation in early July. Large tracts of paddy were also destroyed in parts of Assam, he added. Nearly 60,000 people in nine districts have been affected by floods in Assam.