Origin of precipitation reveals hidden factor behind drought risks for farmers

A new study from the University of California, San Diego reveals a hidden factor in the vulnerability of global crops: the origin of precipitation itself.

The paper titled “Crop water origins and hydroclimatic vulnerability of global croplands” was published in Sustainability of nature.

The research traces atmospheric moisture back to its source, whether it evaporates from the ocean or from land surfaces such as soil, lakes and forests. When the sun heats these surfaces, the water turns into vapor, rises into the atmosphere, and then falls as rain.

Moisture of ocean origin travels long distances on global winds, often through large-scale weather systems such as atmospheric rivers, monsoons, and tropical storms.

In contrast, land-based moisture – often called recycled rain – comes from water that evaporates from nearby soils and vegetation, fueling local storms. The study reveals that this balance between ocean and terrestrial (terrestrial) sources strongly influences a region’s drought risk and crop productivity.

“Our work reframes drought risk: It’s not just about the amount of rain, but also where that rain is coming from,” said Yan Jiang, lead author of the study and a postdoctoral researcher at UC San Diego with a joint appointment in the School of Global Policy and Strategy and the Scripps Institution of Oceanography.

“Understanding where precipitation comes from and whether it comes from ocean or land sources gives policymakers and farmers a new tool to predict and mitigate drought stress before it happens.”

A new way to predict drought risk

Using nearly two decades of satellite data, Jiang and co-author Jennifer Burney of Stanford University measured the share of global precipitation that comes from terrestrial evaporation.

They found that when more than a third of precipitation comes from land, croplands are significantly more vulnerable to drought, soil moisture loss and reduced yields – likely because ocean-derived systems tend to provide more abundant precipitation, while land-based systems tend to provide less reliable downpours, increasing the risk of water deficits during critical stages of crop growth.

This information offers farmers and policymakers a new way to identify regions most at risk and plan accordingly.

“For farmers in regions that rely heavily on land-based moisture, such as parts of the Midwest or East Africa, local water availability becomes the deciding factor for crop success,” Jiang explained. “Changes in soil moisture or deforestation can have immediate and cascading impacts on yields.”

Two global hotspots: the US Midwest and East Africa

The study highlights two striking vulnerability hotspots: the US Midwest and tropical East Africa.

In the Midwest, Jiang notes, droughts have become more frequent and more intense in recent years, even in one of the world’s most productive and technologically advanced agricultural regions.

“Our results suggest that the Midwest’s heavy reliance on land-derived moisture, from surrounding soil and vegetation, could amplify droughts through what we call ‘precipitation feedback loops,'” Jiang said. “When the land dries, it reduces evaporation, which in turn reduces future precipitation, creating a self-reinforcing cycle of drought.”

As this region is also a major supplier to global grain markets, disruptions there have repercussions well beyond America’s borders. Jiang suggests that Midwest growers may need to pay more attention to soil moisture management, irrigation efficiency and planting timing to avoid worsening drought stress.

On the other hand, East Africa faces a more precarious but nevertheless reversible situation. Rapid expansion of croplands and loss of surrounding rainforests threaten to undermine the moisture sources that support precipitation in the region.

“This creates a dangerous conflict,” Jiang said. “Farmers are clearing forests to grow more crops, but these forests help generate the precipitation that crops depend on. If this source of moisture disappears, local food security will be more at risk.”

However, Jiang sees opportunities as well as risks:

“East Africa is on the front lines of change, but there is still time to act. Smarter land management, such as forest conservation and vegetation restoration, can protect rainfall and support agricultural growth.”

Forests, makers of rain

Research highlights that forests and natural ecosystems are crucial allies for agriculture. Forests release large amounts of water vapor into the atmosphere through evaporation and transpiration (when plants produce moisture), thereby seeding the clouds that bring rain to nearby croplands.

“Upland forests are like natural rainmakers,” Jiang said. “Protecting these ecosystems is not just about biodiversity, it’s also about supporting agriculture.”

A tool for smarter land and water management

Jiang’s research provides a new scientific framework linking land management, precipitation regimes and crop planning, a relationship that could become central in future drought resilience strategies.

The study’s new satellite mapping technique could help governments and farmers identify where to invest in irrigation infrastructure, soil water storage and forest conservation to maintain reliable rainfall.