At this time last month, large parts of India were facing drought, with pre-monsoon rainfall at the second-lowest level in 65 years as the monsoon itself ran behind schedule.

This week, more than 10 million people are stranded in floods in the east of the country. Over a hundred people have died in Assam and Bihar.

Assam is historically flood-prone, partly because of the volatility of the Brahmaputra river and partly because of topography. The river, which originates in Tibet, hurtles down the mountainous landscape of Arunachal Pradesh before it reaches Assam. In a valley hemmed in by hills, it slows down and unloads its heavy burden of sediment, causing destruction.

This year, the destruction has come early in the monsoon season.

An archive of daily updates by the Assam Disaster Management Authority shows the floods began to build up around July 8, leaving 12,631 people marooned in five districts. By July 10, this number had gone up to 2.07 lakh people across 11 districts. Over the course of the next week, the inundation intensified, spreading over nearly the entire state.

On July 18, as many as 54 lakh people in 28 of the state’s 33 districts were coping with floods, with 2.25 lakh people living in relief camps.

This dramatic shift can be seen in satellite images of Guwahati. On July 1, the Brahmaputra river, flowing through the city, was a narrow sliver. By July 16, it had become a swollen mass.

ESA Sentinel Hub

Further downstream, in Barpeta district, the change in the river is even more dramatic.

ESA Sentinel Hub

A similar expansion took place in the Ganga basin in Bihar. On July 11, there was heavy rainfall in Nepal, where many tributaries of the river originate. Eight of them – Ghaghra, Gandak, Burhi Gandak, Bagmati, Kamala, Bhutahi Balan, Kosi and Mahanand – flow into North Bihar before merging into the Ganga.

Not only did these rivers swell up at their point of origin, heavy rainfall in North Bihar itself added to the volume of water. On July 12, three districts – East Champaran, Sitamarhi and Muzaffarpur – received the highest daily rainfall in 54 years.

According to the latest flood update by Bihar government, 17 lakh people, or half the population of Sitamarhi district, are stranded in floods. Across 12 districts of the state, more than 55 lakh people are affected, with 1.13 lakh people living in relief camps. Seventy-eight people have died.

Satellite images of Sitamarhi show how Bagmati river expanded between July 5 and July 17.

ESA Sentinel Hub

The satellite images, gathered by ESA Sentinel Hub, have been put through the Normalised Difference Water Index, which uses a calculation to map the presence of surface water.

Satellites use remote sensing to measure solar radiation reflected off the earth’s surface. Based on what kind of radiation is reflected, it is possible to estimate topography.

Since water absorbs infrared radiation as well as visible light, the Normalised Difference Water Index uses green and near-infrared bands of the remote sensing images to estimate the location and boundaries of water bodies. It is important to note that built-up areas also absorb similar radiation, therefore the index might also mark a built-up area as a water body.

Also read:

Can Assam’s annual flood misery be blamed on government red-tape?

Due to an ‘accounting error’, Assam has not received any special Central flood aid since 2014