Is irrigation adversely affecting the monsoon in India and delaying its onset? Maybe, according to a report on climate change and land brought out by the United Nation’s Intergovernmental Panel on Climate Change (IPCC) last week.
“Irrigation in India occurs prior to the start of the monsoon season and the resulting land cooling decreases the land-sea temperature contrast. This can delay the onset of the monsoon and decrease its intensity,” the report said, quoting studies carried out in the past.
Beneficial for some
Irrigating farms in India may be having an unfavourable outcome within its national boundaries but it’s probably having some beneficial impact in faraway places.
According to a global modelling study published in 2016, irrigation in India is increasing rainfall in the Horn of Africa.
This IPCC special report, authored by over 100 scientists, looks mainly at how human use of land is driving climate change. According to the report, the use by humans is directly affecting about 70 per cent ice-free land surface on the planet.
One of the chapters in the report dealing with interactions between land and climate says the effects of irrigation on local, regional and global climate are very well studied. According to scientists, it is well established that irrigation increases total evapo-transpiration, increases the total amount of water vapour in the atmosphere, and decreases mean surface daytime temperature within the irrigated area and during the time of irrigation.
Among the studies the report relied on was one that appeared about seven years ago, by a team of French meteorologists from the University of Paris. Appearing in the journal Climate Dynamics , it argued that high levels of irrigation in winter and spring months in the Indian peninsula are delaying the monsoon onset by an average six days. This is also leading to a significant decrease in precipitation during May and July, it argued.
Another study in 2010, to which scientists from the Space Application Centre in Ahmedabad contributed, said that the increase in vegetation, primarily due to agriculture, is weakening rainfall in early monsoon months, particularly in South and North Indian regions.
Then, a team of German researchers led by Philipp de Vrese at Max Planck Institute for Meteorology in Hamburg said in a paper published in the journal Geophysical Research Letters that irrigation in India brings about changes in the atmospheric transport of water vapour in parts of eastern Africa.
“At the onset of boreal spring (February to March) evapo-transpiration is already large over irrigated crops (in India) and the resulting excess moisture in the atmosphere is transported south-westward by the low-level winds.
This results in increases in precipitation as large as a 1millimetre per day in the Horn of Africa,” the scientists said.
According to the Hamburg scientists, if India were to reduce watering its crops, rainfall can decrease in eastern Africa where the consequences of drought are already disastrous.
The IPCC report also said that human-induced climate change is leading to unpredictable changes, particularly in precipitation. It said that the heavy precipitation events have increased in frequency and intensity since 1950.
Extreme rain events
To buttress the point, it cited the example of central India, which has been a seeing a three-fold increase in widespread extreme rain events between 1950 and 2015.
This has led not just to soil erosion, but influenced several land degradation processes, the IPCC report said.