Kolkata: The famed Indus Valley civilisation remained under severe drought for about 900 years around 4,350 years ago, which led to the people to abandon their settlements and migrate to south and and eastward regions of India, according to a study by IIT Kharagpur.

The civilisation was the most widespread among the ancient civilisations, covering an area about 1.5 million sq km - now comprising modern India, Pakistan, Baluchistan and Afghanistan.

It had a well-developed infrastructure, architecture, metallurgy, besides having trade relations and cultural ties with other concurrent civilizations across the world.

A team led by Professor Anil K Gupta of Department of Geology and Geophysics at the Indian Institute of Technology Kharagpur has observed that the drought-like phase for the period of 900 years led to reduction in water supply because of the "very weak" Indian summer monsoon, which was under the influence of strong El Nino activity, the climate cycle in the Pacific ocean.

This decreased the moisture transport and in turn the snow deposition in northwest Himalaya, which have been a major source of water supply in the Indus river and its tributaries thus affecting agriculture production, an IIT Kharagpur statement said on Monday.

Gupta, along with members from the Wadia Institute of Himalayan Geology, Dehradun, and Institute of Estuarine and Coastal Research, Shanghai, China, carried out the study in the Tso Moriri Lake in the Ladakh region of Jammu and Kashmir.

A paper on the study has been accepted by Elsevier's Quaternary International Journal, a highly respected scientific journal.

According to the study, drought, however, did not mean absence of rainfall or river water supply but a reduction in water flow thereof gradually turning the region into an arid zone,

Such phases of climate change could be cyclical over a period of time stretching over millennia, it adds.

"They (people) might have tried to adapt to the situation but this arid phase continued for more than 900 years. Therefore, in the search of better water availability for their agriculture and animal husbandry, which were the major occupation for people of Indus Valley Civilisation, they had to migrate to south and eastward regions in India, which were under more influence of the Indian summer monsoon," the statement quoting Gupta as saying.

It said the study gives a fair idea on long-term effects of climate change on human settlements.

Around 4,200 years ago, the population of the Indus Valley Civilisation abandoned its major villages and urban settlements near Indus river, including Harappa and Mohenjo-daro, and started migrating to the Ganga-Yamuna plains.

Diverse theories have been developed and debated over decades for the possible cause of displacement, such as droughts, destruction by major floods and foreign invasions, he statement said.

"No good explanation responsible for the migration of such an advanced society was available until now," it said.