Researchers looked into drivers of widespread flooding in seven Indian subcontinental river basins
Narmada basin has the highest probability (59%) of widespread flooding among the rivers studied. Photo: iStock River basins in peninsular India face a higher probability of widespread flooding compared to the Ganga and Brahmaputra, according to a new study published in the American Meteorological Society’s Journal of Hydrometeorology.
Narmada basin has the highest probability (59 per cent) of widespread flooding, followed by Mahanadi (50 per cent), Godavari (42 per cent), Krishna (38 per cent) and Cauvery (19 per cent).
As for transboundary river basins, Ganga and Brahmaputra have a probability of 21 per cent and 18 per cent, respectively.
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Widespread flooding causes enormous losses and damages compared to localised flooding, as it covers a large part of a river basin.
“However, understanding the occurrence and drivers of widespread floods in the Indian subcontinental river basins is limited as the focus has primarily been on localised flooding,” the researchers from the Indian Institute of Technology Gandhinagar wrote in the paper.
The team analysed the occurrence of widespread floods in seven major river basins in the Indian subcontinent, such as Ganga, Brahmaputra, Godavari, Krishna, Mahanadi, Narmada and Cauvery in 1959-2020.
With 40 events in the period, the Mahanadi and Narmada river basins had the highest frequency of widespread floods, the analysis showed. Krishna and Godavari basins witnessed more than 20 widespread floods, while Ganga, Brahmaputra and Cauvery basins saw less than 15 events in the period.
The team also found strong seasonal trends in widespread flood probability in the subcontinental river basins. For example, during the summer monsoon season, all seven river basins, except Cauvery, experienced widespread flooding in August.
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Godavari, Mahanadi and Narmada basins recorded widespread flooding in July, August and September.
The trend of seasonality is also tied to rainfall. India receives around 80 per cent of the total annual precipitation during the summer monsoon season from June-September, the paper highlighted.
Godavari, Mahanadi, and Narmada basins lie in the core monsoon region and receive more rainy days in July to September, the findings showed.
The Cauvery faces flooding in October-December, as most of the river’s subbasins receive rainfall during the northeast monsoon season.
The Brahmaputra river basin experienced widespread floods during June-July as the northeast region receives rainfall earlier compared to north Indian states.
“Therefore, most widespread floods occur during the summer monsoon season and the probability of widespread floods during the summer monsoon season is similar to the annual probability in all basins except for Cauvery,” the paper read.
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The researchers also looked at the drivers of widespread flooding. The 2018 Kerala floods, 2022 Pakistan floods and lower Mississippi river floods in 2008, 2011 and 2015–19 have been linked with atmospheric rivers that usually carry moisture from the tropics to the extratropics. Atmospheric rivers are large sections of the Earth’s atmosphere carrying water vapour through the sky.
Widespread floods in India are associated with large atmospheric circulations that cause precipitation in the river basin, the study noted.
The drivers of widespread floods are expected to alter the timing, occurrence, and probability of widespread floods in a warming climate, the researchers wrote in the paper.
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