Hidden rainfall pattern could reshape farming
Scientists have uncovered that the source of rainfall—whether it evaporates from oceans or land—plays a powerful, previously overlooked role in determining global crop vulnerability. Credit: Shutterstock
University of California, San Diego — A new study from the University of California, San Diego identifies an unexpected influence on global crop stability: the original source of rainfall.
The research follows atmospheric moisture back to the place where it first evaporated, whether from the ocean or from land surfaces such as soil, lakes and forests.
Ocean-derived moisture can travel across continents within large weather systems including atmospheric rivers, monsoons and tropical storms. Moisture that comes from land, often referred to as recycled rainfall, is created when water evaporates from nearby soils and vegetation, fuelling more localized storms. The study says the ratio of ocean to land moisture strongly shapes regional drought risk and agricultural 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. “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.”
Using nearly 20 years of satellite measurements, Jiang and co-author Jennifer Burney of Stanford University quantified how much of global rainfall begins as land-based evaporation. They found that when more than roughly one-third of precipitation originates from land, croplands become significantly more susceptible to drought, soil moisture declines and drops in yield. Ocean-driven systems generally produce heavier and more consistent rainfall, while land-driven systems tend to produce lighter and less predictable showers, making crops more vulnerable during stages when water is essential.
“For farmers in areas that rely heavily on land-originating moisture — such as parts of the (United States) Midwest — 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.”
The research identifies the U.S. Midwest as one of two prominent global hotspots, the other being tropical East Africa.
In the Midwest, Jiang notes that droughts have grown more frequent and intense in recent years, despite the region’s status as one of the world’s most productive agricultural zones.
“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 the region is a major contributor to global grain markets, disruptions there can influence food supply far beyond U.S. borders. Jiang says farmers in the Midwest may need to focus on soil moisture conservation, irrigation efficiency and strategic timing of planting to limit the risk of compounding drought effects.
Jiang’s work introduces a new framework that links land use decisions, rainfall patterns and agricultural planning. This approach may become increasingly important as regions look for strategies to strengthen drought resilience.
The study also presents a satellite-based mapping method that could guide investments in irrigation, soil water retention and forest conservation to help stabilize rainfall.
