Deepti Singh, lead author of the study, said rainfall extremes during the months of the monsoon season can be as important as how much total water is received.

For example, during critical crop growth stages, too many days without rain can reduce yields or lead to crop failure, which can reverberate throughout India's agriculture-dependent economy.

At the same time, short periods of very heavy rainfall can create humanitarian disasters, such as in 2005, when massive flooding killed thousands of people in Mumbai, she noted.

For this study, senior author Noah Diffenbaugh, associate professor of Environmental Earth System Science and a senior fellow at the Stanford Woods Institute for the Environment, and graduate student Singh collaborated with Bala Rajaratnam, assistant professor of Statistics and of Environmental Earth System Science, and Michael Tsiang, a graduate student in Rajaratnam's research group.

The team compared rainfall data gathered by the Indian Meteorological Department and other sources over a 60-year period. They used rigorous statistical methods to compare peak monsoon rainfall patterns during two time periods: from 1951 to 1980, and from 1981 to 2011.

The team looked specifically at rainfall during the months of July and August, which is the peak of the South Asian summer monsoon.
The analysis focused on central India, which is the core of the monsoon region and has extremely high population densities, a media note said.

The team's findings match stories told by Indian farmers in recent decades, said Singh, whose family lives in the region of the country affected by the monsoon.
The team also found changes in the atmosphere – such as winds and moisture –that are likely responsible for the changes in wet and dry spells.

Diffenbaugh said that the next step is to investigate what might be causing the changes in the atmosphere.

"There are many predictions that global warming should cause heavier downpours and more frequent dry spells," he said.
"That is what we have found here, but India is a complex region, so we want to be sure before we point the finger at global warming or any other cause," he added.


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