Man-made changes to landscape affecting Indian monsoon rains

In a new study, a scientist from Purdue University, US, has shown that man-made changes to the landscape have affected Indian monsoon rains, suggesting that land-use decisions play an important role in climate change.

According to Dev Niyogi, an associate professor of agronomy and earth and atmospheric sciences, monsoon rainfall has decreased over the last 50 years in rural areas where irrigation has been used to increase agriculture in northern India.

At the same time, heavily urban areas are seeing an increase in heavy rainfall.

"In the rural areas, we're seeing premonsoon greening occurring two weeks earlier than what it did 20 years back as the demand for agricultural intensification to feed India's people increases," Niyogi said.

"The landscape has also moved in some places from what was once a traditionally rural setting to large urban sprawls. Both of these phenomena have affected monsoon rains," he added.

Niyogi used more than 50 years of rainfall data - spanning back to 1951 - collected by 1,803 recording stations monitored by the India Meteorological Department to determine different regions' average yearly monsoon rain totals.

While the mean monsoon rainfall for the entire country remained stable, Niyogi found that rainfall averages in India's northwest region decreased by 35% to 40% from the historical mean during the past 50 years.

Analysis of soil moisture showed that before monsoon rains came, the northwest region had become as much as 300% wetter in recent years relative to the past 30 years, which has been attributed to irrigation from groundwater to sustain intensified agricultural production.

This wetter surface causes cooling that weakens the strength of low pressure necessary for monsoons to progress into northern India.

Satellite data showed that northern India is greening sooner than it had in the past.

That greening is creating a barrier for monsoons, which provide much-needed rain to replenish groundwater reserves being used for irrigation.

"In this case, you need a warm, dry surface to advance the monsoon," said Niyogi. "Because of increased irrigation, you now have a wet, green area, which does not allow the monsoon to reach far enough north," he added.

Since that rain isn't reaching the region, more irrigation is needed to sustain agriculture there.

"Unless this is checked and controlled, the problem is going to become more and more severe," Niyogi said.

"With more irrigation, we will have less monsoon rain. With less monsoon rain, you will need more irrigation, and the cycle will continue," he explained.