Irrigation interfering with monsoons, says study
[NEW DELHI] Agricultural intensification and irrigation may decrease the Indian monsoon rains over the long term, a Purdue University and Indian Space Research Organisation study has found — suggesting regional land surface feedbacks play an important role in climate change.
The study analysed Indian Meteorological Department rainfall data of more than 50 years and tracked land use change using satellite imagery. Using a set of statistical tools, the researchers attempted to co-relate and then causally relate the data.
While the average rainfall over the whole of India remained stable, the study found a drastic change over north-western India where average seasonal rainfall has decreased by 35–40 per cent. The largest region with a negative trend in rainfall included portions in the states of Punjab, Haryana and western Uttar Pradesh — which benefited from India's Green Revolution when foodgrain production increased in the 1960s with adoption of high-yielding varieties, fertilisers and pesticides.
The scientists analysed the trend more closely and found further clues in soil moisture data for the country from the National Oceanic and Atmospheric Administration series of satellites. This data suggested a 300 per cent increase in pre-monsoon soil wetness over north-west India between 1988 and 2002.
The study authors argue that the increase in soil moisture is due to irrigation during the pre-monsoon phase.
"You need a warm, dry surface to advance the monsoon," Dev Niyogi, an associate professor of Regional Climatology at Purdue, said. "Because of increased irrigation, you now have a wet, green area, which does not allow the monsoon to reach far enough north."
"With more irrigation, we will have less monsoon rain. With less monsoon rain, you will need more irrigation, and the cycle will continue," Niyogi said.
But "clearly monsoon system and the associated rainfall is a complex, multi-scale process, and irrigation and agricultural land use change is just one component of the complex system", he added.
The findings were published in Water Resources Research last month (30 March).