Study on the South West Monsoon by the IIT Bombay: Key Findings

A team led by Prof. Subimal Ghosh from the Department of Civil Engineering at the Indian Institute of Technology (IIT) Bombay had undertaken a comprehensive study on south west monsoon.

The findings of the study published in the journal Geophysical Research Letters include the following:

• The dense vegetation in the Western Ghats determines the amount of rainfall that Tamil Nadu gets during the summer monsoon.
• Dense forests of the Western Ghats contribute as much as 40% of moisture to the southwest monsoon rainfall over Tamil Nadu during normal monsoon years.
• The average contribution is 25-30%. But during monsoon deficit years, the contribution increases to as high as 50%.
• The study found the forests of Western Ghats contribute as much as 3 mm per day of rainfall during August and September over a “majority of locations” in Tamil Nadu and 1 mm per day during June and July.
• Deforestation of the Ghats led to 0.25 degree C increase in surface temperature across the State.
• To study the role of vegetation cover in the Western Ghats in supplying moisture to the southwest monsoon rainfall, the researchers used models to compare the contribution of Western Ghats with and without the forest cover.
• The researchers found a significant drop in rainfall in the range of 1-2.5 mm per day when the vegetation cover was removed from the Western Ghats. This translates to an average of 25% of the total monsoon rainfall over Tamil Nadu. But only small parts of Kerala get affected by deforestation in Western Ghats.
• The team selected three years (1993, 1999, and 2002) when Tamil Nadu experienced extreme deficit in summer monsoon rainfall. They found that deforestation over Western Ghats reduced rainfall over the State by 40-50% during all the three years.
• The forest cover in the Ghats acts as a capacitor for moisture supply to Tamil Nadu. During the break period during the monsoon season when there is sharp decrease or no rainfall for three consecutive days, the impacts of deforestation in the Ghats on the rainfall over Tamil Nadu is higher compared with the wet spell of the monsoon period.

The researchers also crosschecked the role of vegetation is supply moisture to southwest monsoon rainfall by tracking the source of the moisture. The results of this were consistent with the model with and without vegetation. [Reference / Source]