Rainfall's Origin Reveals a Hidden Driver Behind Drought Risks for Farmers

By Christine Clark

A new University of California San Diego study uncovers a hidden driver of global crop vulnerability: the origin of rainfall itself.

The paper, "Crop water origins and hydroclimate vulnerability of global croplands," was published in Nature Sustainability.

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

Ocean-sourced moisture travels long distances on global winds, often through large-scale weather systems such as atmospheric rivers, monsoons, and tropical storms.

In contrast, land-sourced moisture often called recycled rainfall comes from water that evaporates from nearby soils and vegetation, feeding local storms. The study finds that this balance between oceanic and terrestrial (land) sources strongly influences a region's drought risk and crop productivity.

"Our work reframes drought risk it's not just about how much it rains, but where that rain comes from," said Yan Jiang, the study's lead author and postdoctoral scholar at UC San Diego with a joint appointment at the School of Global Policy and Strategy and Scripps Institution of Oceanography.

"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."

A new way to forecast drought risk

Using nearly two decades of satellite data, Jiang and co-author Jennifer Burney of Stanford University measured how much of the world's rainfall comes from land-based evaporation.

They discovered that when more than about one-third of rainfall originates from land, croplands are significantly more vulnerable to drought, soil moisture loss and yield declines—likely because ocean-sourced systems tend to deliver heavier rainfall, while land-sourced systems tend to deliver less reliable showers, increasing the chance of water deficits during critical crop growth stages.

This insight provides a new way for farmers and policymakers to identify which regions are most at risk and to plan accordingly.

"For farmers in areas that rely heavily on land-originating moisture like parts of the Midwest or eastern Africa local water availability becomes the deciding factor for crop success," Jiang explained. "Changes in soil moisture or deforestation can have immediate, cascading impacts on yields."

Two global hotspots: The U.S. Midwest and East Africa

The study highlights two striking hotspots of vulnerability: the U.S. Midwest and tropical East Africa.

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

"Our findings suggest that the Midwest's high reliance on land-sourced moisture, from surrounding soil and vegetation, could amplify droughts through what we call 'rainfall feedback loops,'" Jiang said. "When the land dries out, it reduces evaporation, which in turn reduces future rainfall creating a self-reinforcing drought cycle."

Because this region is also a major supplier to global grain markets, disruptions there have ripple effects far beyond U.S. borders. Jiang suggests that Midwestern producers may need to pay closer attention to soil moisture management, irrigation efficiency and timing of planting to avoid compounding drought stress.