doi:10.1038/nindia.2016.118 Published online 12 September 2016
Changes in land use and land cover in India are weakening the monsoon rainfall over the Ganga basin and north-east India, according to scientists at the Indian Institute of Technology Bombay, Mumbai, and University of Nebraska, USA1. They came to this conclusion by analysing satellite images taken over 18 years and simulating the Indian monsoon through regional climate models with past and present land use conditions.
The researchers have shown how the conversion of forests into agricultural land has weakened the Indian summer monsoon. The results provide a plausible explanation, if not complete, for the decreasing trend in Indian summer monsoon rainfall since 1950.
In the 1980s, woody savannas covered a significant part of central, peninsular and north-east India. However, by 2005, much of this forest land disappeared, and croplands dominated the landscape. In northeast India, forests were replaced by large swaths of tea plantations.
Interestingly, the reason for a dip in rainfall lies in the leaf area and roots of the plants. Agricultural plants with shorter roots enable lesser water absorption by the soil thereby reducing the amount of water evaporating from the earth’s surface. Decrease in leaf area also results in low transpiration. This vicious cycle results in a reduction recycled precipitation, and ultimately affects the summer monsoon rainfall.
The study shows that by incorporating changes in vegetation, it is possible to fine tune the present climate models for a better future projection. The researchers now plan to study other factors behind the decreasing Indian summer monsoon rainfall, such as warming of the seas and aerosol forcing.