The source of a region's rainfall — whether it comes from the ocean or the land — can be a key indicator of how vulnerable local crops are to climate change, according to a UC San Diego study.
Published in Nature Sustainability, the research traces atmospheric moisture back to its source. Moisture from the ocean can travel long distances through storms and other large weather events. Land-sourced moisture — also known as recycled rainfall — comes from water that evaporates from nearby soils and vegetation.
"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 UCSD. "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 and co-author Jennifer Burney of Stanford University measured how much of the world's rainfall comes from land-based evaporation using nearly two decades of satellite data. What they found was when more than a third of a region's total rainfall comes from land sources, crops are "significantly more vulnerable to drought, soil moisture loss and yield declines," owing to less reliable precipitation.
"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 said. "Changes in soil moisture or deforestation can have immediate, cascading impacts on yields."
In the American Midwest, the authors write, droughts have become more frequent and intense in recent years.
"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."
The authors of the paper suggest that due to the Midwest's role as a major supplier for export markets, farmers and agronomists may need to pay closer attention to soil management, irrigation and the timing of planting to avoid compounding the issue.
They contrast the American situation with East Africa, which faces a "more precarious but still reversible situation." Cropland expansion and loss of rainforests threaten to undermine moisture sources sustaining the region's rainfall.
"This creates a dangerous conflict. Farmers are clearing forests to grow more crops, but those forests help generate the rainfall that the crops depend on. If that moisture source disappears, local food security will be at greater risk," Jiang said. "Eastern Africa is on the front line of change, but there is still time to act. Smarter land management — like conserving forests and restoring vegetation — can protect rainfall and sustain agricultural growth."
The authors reiterate that forests and natural ecosystems are crucial allies in farming.
"Upland forests are like natural rainmakers," Jiang said. "Protecting these ecosystems isn't just about biodiversity — it's about sustaining agriculture."
