Studies show that a combination of factors has weakened the Indian monsoon. If deforestation continues at the current rate, matters will get much worse.
Large-scale conversion of forests to crop lands is disrupting India’s rainfall, a new study has found.
Around 80 percent of India’s annual rainfall comes from the Indian summer monsoon, spanning from June to September. But deforestation over the past few decades has caused summer monsoon to weaken, resulting in a considerable decline in rainfall, concludes the study published in Scientific Reports.
“Monsoon is believed to be a product of large scale atmospheric circulation,” co-author Subimal Ghosh, associate professor at the Department of Civil Engineering in IIT Bombay, told Mongabay. “But our study found that there are local factors such as changes in land use and land cover that lead to changes in monsoon rainfall. These local changes are in our hands, and because of them there has been a significant reduction in rainfall over two major regions, the Ganga basin and northeast India. That is really alarming.”
Ghosh and his colleagues found that both forest cover and monsoon rainfall declined considerably between the years 1980-1990 and 2000-2010. The reduction in rainfall was especially pronounced in north and northeast India. The study’s models also showed that the large-scale conversion of forests to crop lands had resulted in a decline in evapotranspiration, a process by which moisture is transferred from soil and plants to the atmosphere.
“When there is a huge conversion of forest to cropland, there is a reduction in leaf area index,” Ghosh explained. “Moreover in forests, the vegetation is deep-rooted, and can easily extract soil water. But crops have shallow roots and are unable to extract water that easily. So deep rooted forest vegetation tends to have higher evapotranspiration.”
Decline in evapotranspiration in turn reduces recycled precipitation, or rainfall resulting from evapotranspiration. In a previous study, Ghosh’s team found that recycled precipitation plays an especially important role in north and northeast India, accounting for around 25 percent of the rainfall during late monsoon (August and September).
This is because the Himalayas cause monsoonal winds to circulate internally, and the moisture generated by evapotranspiration ends up residing over these two regions, resulting in rainfall, Ghosh said. In contrast, in a region like the Western Ghats in southern India, which has also lost considerable forest cover, the major source of moisture is the Arabian Sea, so evapotranspiration does not play that big a role.
Recent studies have shown that other factors, such as the warming of the Western Indian Ocean, have weakened the Indian monsoon. So if deforestation continues at the present rate, summer monsoon rainfall will reduce further, the researchers warn.
“For Indian monsoon, impacts of land use-land cover change is critical and needs to be considered for regional projections and planning,” Ghosh said in a statement.
This article was originally published on Mongabay.