News

Researchers predict more frequent floods in Brahmaputra

K. S. Jayaraman

doi:10.1038/nindia.2021.7 Published online 13 January 2021

Higher frequency flooding of Brahmaputra could jeopardise the lives of millions of people living in downstream countries India and Bangladesh.

© S. Priyadarshini

While China's plan to build a hydropower dam on Brahmaputra has raised concern among downstream countries India and Bangladesh, a new study precipitates these worries with a warning of higher frequency flooding in south Asia’s mighty river.

Researchers led by Mukund Palat Rao of Columbia University's Lamont-Doherty Earth Observatory urge that countries should be prepared for a “higher frequency of flooding in the Brahmaputra than current predictions”.

Low-lying Bangladesh is hardest hit due to floods in Brahmaputra. In 1998, as much as 70 per cent of the country went under water following incessant flooding.

Existing projections of flooding of the Brahmaputra are based on observations of past rainfall patterns, but they rely on discharge-gauge records that date back to the 1950s. The new warning is based on an examination of the tree rings of ancient trees in and around the river's watershed which provided a picture of rainfall patterns going back seven centuries.  

As trees grow, they incorporate information about the environmental conditions in their annual rings. “They put on wide rings in wet monsoon years and narrow rings during droughts thus indirectly reflecting rainfall and resulting river runoff,” Rao told Nature India. By taking a pencil thin tree-core from these trees and measuring their rings under a microscope, the team learnt more about rainfall patterns going back centuries before instrumental and historical records.

The researchers "reconstructed" the stream-flow of the Brahmaputra from 1309 to 2004 by assembling a 696-year chronology of the tree rings – the oldest tree dating back to year 449. Comparing data from the tree rings with official records from a river-flow gauge in Bangladesh, they could see that the widest rings lined up neatly with known major flood years. This in turn allowed them to extrapolate yearly river discharge in the centuries preceding modern records.  This showed there have been much wetter periods in the past, driven by natural oscillations and that the post-1950s period was actually one of the driest. 

Climate models predict an intensified monsoon in the Brahmaputra valley with attendant increased flood risk but "anyone relying only on modern discharge record to estimate future flood hazard due to climate change would be underestimating the danger by 24 to 38 percent”,   their report concludes.

"It reinforces that we need to continue to invest in flood preparedness,” Rao said.  

Short duration instrumental records are by themselves inadequate to accurately characterize future flood risk in this region – both in terms of climate change and its impact on the natural variability of river discharge, says geologist Sampat Kumar Tandon, a professor emeritus at Delhi University. "Only when these are integrated with long-term tree-ring based reconstruction can meaningful inferences be made of the flood hazard,” he said.

Such proxy data from paleoclimate archives must be integrated with instrumental data and climate model projections in drawing up regional flood management measures, Tandon emphasised.


References

1. Rao, M. P. et al. Seven centuries of reconstructed Brahmaputra river discharge demonstrate underestimated high discharge and flood hazard frequency. Nat. Comm. 2020 doi: 10.1038/s41467-020-19795-6