Update (14/8/2017): India’s meteorological department forecast a normal monsoon in 2017, but excess rain over short periods has flooded some of the country’s driest regions (like Rajasthan and Gujarat) this year. Most recently, eastern states of Assam and Bihar have been hit by fresh floods and army has been called for help. In Assam, 21 of the 22 districts have been affected. Some 80 per cent of the Kaziranga National Park, home to India’s one-horned rhinos, has been flooded. Four districts in Bihar have been flooded as Tapti and Mahanadi rivers swell.
Floods are a regular in India. But there is something peculiar about them this year: they not only came early, but have stayed longer. And worryingly, they have not been proportionate to the rainfall. Madhya Pradesh and Rajasthan were in a drought-like situation when floods occurred in July-August, while Bihar and Assam—two states that are facing their worst floods in almost three decades—have had deficit rainfall this monsoon. These patterns seemed to have escaped attention.
Nobody took notice when floods hit Assam this April. A new government was just sworn in and the media did not pay much heed to the unusually high pre-monsoon rain. Everyone thought the rising waters of the Burhidehing and Desang rivers would subside. It did not happen. The flood continued till the end of August, covering more than 90 per cent of the state and affecting over four million people.
“The pre-monsoon rainfall this year has been very strong, both in terms of the absolute rainfall and its geographical coverage,” says D C Goswami, former head of the Department of Environmental Science, Guwahati University. According to the National Remote Sensing Centre, Hyderabad, heavy rains in the fourth week of April caused the first “wave” of floods in Assam. On April 25, the Burhidehing and Desang crossed the danger mark and caused floods in six districts—Dibrugarh, Sivasagar, Jorhat, Tinsukia, Cachar and Charaideo. Experts say the heavy pre-monsoon showers have provided the clinching evidence of the increasing frequency of western disturbances that bring the rain. According to the Regional Meteorological Centre, Guwahati, rainfall in the state was 21 per cent above normal in March-April.
What is frightening is the fact that such unusual rain will become more common with climate change. “It is the pre-monsoon showers, rather than the monsoon, which will increase because of global warming,” says Subashisa Dutta, professor at the Department of Civil Engineering, IIT-Guwahati. In 2012, Dutta studied the impact of climate change on the hazard of floods in the middle Brahmaputra reaches and found that the peak discharges from the river, the average duration of a flood wave and the intensity of pre-monsoon showers are likely to rise during 2071-2100.
It is quite ironic that Assam witnessed floods when its total monsoon rainfall between June 1 and August 16 was 25 per cent below normal. So, what caused the flood? Data suggests that most floods were preceded by extreme rainfall events (see ‘Under water’).
The India Meteorological Department (IMD) divides extreme rainfall events into two categories: rainfall of 124.5-244.4 mm in 24 hours is “very heavy” and rainfall more than that is “extremely heavy”. In July alone Assam recorded six “very heavy” rainfall days. By July 25, the Central Water Commission (CWC) had declared moderate floods in three districts of the state where the Beki, Sankosh and Brahmaputra rivers were flowing above the danger mark.
Arup Kumar Sarma, professor at the Department of Civil Engineering, IIT-Guwahati, says the floods this year were unusual in another way: they seem to have caused more damage. “This is because the direction of the storm and the direction of flow of the river was the same. In such a situation, rains directly contribute to the rise in water levels,” he explains.
However, there is not enough long-term data to claim that extreme rainfall events have increased over the years. For instance, in Assam, no significant trends in 24-hour rain were observed between 1951 and 2010, says the state’s action plan on climate change. The report, however, states that extreme rainfall events in Assam may increase by 5-38 per cent and incidences of floods by 25 per cent during 2021-2050.