In tree rings, warning of Brahmaputra floods


For years, scientists have been looking with concern at the river’s potential for catastrophic flooding in the future, especially as the climate warms. It turns out that this potential has been underestimated so far - even without accounting for a warming climate.

  • Every year, the Brahmaputra floods vast areas in India’s Northeast, particularly Assam, and continues its trail of destruction into Bangladesh, from where it finally flows into the Bay of Bengal. 
  • 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 only to the 1950s.
  • The new study, is based on an examinations of tree rings, which provided a picture of rainfall patterns going back seven centuries.
  • The rings showed that the post-1950s period was actually one of the driest since the1300s — there have been much wetter periods in the past. Using climate models to simulate for future discharge, the researchers found that destructive floods probably will come more frequently than thought.
  • Similarly, climate models suggest that the future will likely be wetter due to our emissions of carbon-dioxide. 
  • Taken together, this suggests that we might be underestimating the current frequency of ‘wet years’ and in turn of flooding.


Why tree rings

  • Tree rings grow wider in years when soil moisture is high. Indirectly, wider rings reflect more rainfall and higher river runoff.
  • As trees grow they incorporate information about the environmental conditions they are living in in their annual growth rings. 
  • Trees in the region grow more and put on wide rings in wet monsoon years.
  • Ancient trees were sampled at 28 sites in Tibet, Myanmar, Nepal and Bhutan, at sites close enough to be affected by the same weather systems as the Brahmaputra watershed. 
  • Analysing the rings, the scientists built a 696-year chronology (1309 to2004).


Findings & takeaways

  • From a river-flow gauge in northern Bangladesh, records showed a median discharge about 41,000 cubic metres per second from 1956 to 1986, and 43,000cumec from 1987 to 2004.
  • The tree rings, in contrast, showed that 1956-1986 was in only the 13th percentile for river discharge, and 1987-2004 in the 22nd. 
  • The rings did show some other relatively dry times - in the 1400s, 1600s and 1800s - but they also showed periods of extreme flooding with no comparable period during 1956-2004. The worst spell lasted from about 1560-1600, 1750-1800 and 1830-1860.
  • The researchers said projecting from the existing discharge record would underestimate future flood hazard by 24-38%, without factoring in climate warming - which would only increase the frequency of future flooding.
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