Regions more vulnerable to drought when one-third of moisture for rainfall comes from land: Study

New Delhi, Nov 4 (PTI) A new study shows that croplands are significantly more vulnerable to drought when moisture from land constitutes over one-third of a region's rainfall.

Moisture in the air that contributes to rainfall can be traced to whether it evaporated from the ocean or land surfaces, such as from soil, lakes or forests. Heat from the Sun turns water on these surfaces into vapour, which rises in the atmosphere and falls later as rain.

Researchers from the universities of California, San Diego, and Stanford explained that moisture sourced from oceans travels long distances on global winds, often through large-scale weather systems such as monsoons and tropical storms.

However, land-sourced moisture comes from water evaporating from nearby soils and vegetation, feeding local storms, they said.

Ocean-sourced systems tend to deliver heavier rainfall, while land-sourced systems tend to deliver less reliable showers, increasing chances of water shortage during critical stages of crop growth.

The team looked at nearly two decades of satellite data to measure how much of the world's rainfall could be traced to moisture originating from land.

"Our analysis shows that the fraction of rainwater originating from land varies both geographically and seasonally, with an important threshold at 36 per cent," the authors wrote in the study published in the journal Nature Sustainability.

"Regions with higher (fraction of rainwater originating from land), that is, more dependent on land-originating water, are more prone to insufficient rainwater supply and soil moisture deficits during the main growing season.

Lead author Yan Jiang, a postdoctoral scholar at the University of California, San Diego, said, "Our work reframes drought risk -- it's not just about how much it rains, but where that rain comes from." "Understanding the origin of rainfall and whether it comes from oceanic or land sources, gives policymakers and farmers a new tool to predict and mitigate drought stress before it happens," Jiang said.

The researchers also found that more than 40 per cent of the world's maize and 60 per cent of wheat planted in autumn is grown in regions where rainfall depends heavily on moisture sourced from land.

The result underscores how critical staple crops could be vulnerable to stress linked to a strain in water resources due to a drought, flood or an extreme weather event, they said.

The study highlights the importance of managing local land sources of moisture and reveals how targeting water management strategies would be most expected to enhance agricultural resilience, the team said. PTI KRS KRS KSS KSS