The Hindu : National : Significant decline in monsoon rainfall

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Sunday, Dec 30, 2007
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Significant decline in monsoon rainfall

Divya Gandhi

Bangalore: The southwest monsoon, responsible for 80 per cent of the country’s annual rainfall and the basis of Indian agriculture, has substantially reduced in the last 50 years, shrinking in duration, spatial distribution and quantum.

The quantum of pre-monsoon rainfall (May 15 to 31) has decreased by 59 percent, and post monsoon rainfall (September 15 to October 10) by 39 per cent in the last 50 years, according to a research paper published in Geophysical Research Letters by K. V. Ramesh and P. Goswami from the Centre for Mathematical Modelling and Computer Simulation on December 5.

While the all India monsoon (June to September) itself does not show any significant decline in quantum, there has been a 30 per cent reduction in the spatial coverage of the monsoon. These trends were arrived at through the analysis of daily rainfall data with the Indian Meteorological Department, between 1951 and 2003.

It is difficult to find a single explanation for this phenomenon, because it would mean examining the entire gamut of climate change phenomena – related both to natural climate variability and human-induced climate change, says Dr. Goswami.

Global warming is a highly variable phenomenon, with regional variations, he says. “The difference in temperature over the land and ocean, or the ‘thermal gradient’ is decreasing, as the land is warming at a slower rate than the ocean in this region. This translates into less rainfall for the landmass,” he says.

The paper, Dr. Goswami says, also hopes to highlight the serious socio-economic consequences of a “shifting monsoon” on agro-ecological sustainability. “There are regions that are becoming non-monsoonal, which will soon become unviable for certain crops. Crop choice, sowing schedule and irrigation requirements depend on the quantum of rain and also its temporal distribution.”

The number of rainy days is also decreasing, making the onset and withdrawal of the monsoon unpredictable. For the pre-monsoon period, the number of rainy days (that is days with more than 5mm of rain) has decreased by 47 per cent, while during the post monsoon period it has reduced by 24 per cent, according to the paper.

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