Increase is most significant in parts of Maharashtra and Southern Gujarat, Karnataka and Andhra Pradesh

The cocktail of drought and heatwaves is engaging several researchers in the country.

Analysing rainfall and temperature data of 50 years, researchers from Indian Institute of Science (IISc) have found that the frequency of heatwaves accompanied by drought has increased not only in magnitude but in area too over the past three decades – particularly in Gujarat and Central India.

While heatwaves (or, a prolonged period when temperatures approach record extremes) and droughts are destructive even when occurring in separate events, their concurrence is far more serious.

“A single extreme event may not be critical, but two extremes occurring at once is much more significant in the distress it causes,” said Pradeep Mujumdar, Professor at the Department of Civil Engineering, IISc who along with his doctoral student Shailza Sharma published the study in the journal Scientific Reports — an online open access journal from Nature — on November 14.

Heating up fast

Researchers calculated the Heatwave Magnitude Index daily (HWMId) — which combines duration and magnitude of heatwaves — and the Standardized Precipitation Index (SPI), which defines meteorological drought from 1951 to 1981.

Using that as the base, they compared it with the HWMId and SPI between 1981 and 2010.

In all combinations of drought (moderate or severe) and heatwaves (3.5 and 10-day events) that were analysed, the percentage increase in frequency was most significant in parts of Maharashtra and Southern Gujarat, Karnataka and Andhra Pradesh.

Even the extreme of extreme scenarios – of 10-day long heatwave where temperatures are above the 95th percentile of range recorded – has been found on the rise in Gujarat and mid-South India. The longest was in 1983 when a heatwave lasted 63 days; while, in 1998, heatwave conditions affected nearly 49% of the country’s area. In both, central India was largely affected.

While the rest of the country, too, showed increases in frequency, a surprising decrease was seen in Rajasthan and West Bengal.

Researchers believe this could be due to the intricate relationship of land surface processes, soil moisture, evapo-transpiration and local climate. (Evapo-transpiration is the sum of evaporation and plant transpiration from the Earth’s land and ocean surface to the atmosphere).

A spatial analysis reveals the area affected by the twin calamities is starkly increasing. The area affected by the ‘extreme of extreme’ incident has gone from almost nothing in 1951, to nearly 4% by 2010. Nearly 18% of the country’s area on average has been facing at least three days of temperatures above 85th percentile.

Ms. Sharma said the next step would be to factor in soil moisture along with the data to develop models that could predict where the extreme events could occur. “With certain accuracy, we can predict these incidents. This could contribute to policy making and ensure preparedness,” she said.

Both phenomena have a serious bearing on water resources, affecting agriculture and human settlements.