Why Farmers Should Care Where Rain Comes From: Insights from UC San Diego’s Latest Climate Research

Insights from a new UC San Diego study on the hidden drivers of drought risk 

I’ve always appreciated having the University of California, San Diego practically “up the street” from where I live. Beyond the ocean views and the innovation culture, UCSD continues to deliver research that reshapes how we think about climate, agriculture, and water. A recent study published in Nature Sustainability is a perfect example. Led by Yan Jiang, a postdoctoral scholar jointly appointed at the School of Global Policy and Strategy and the Scripps Institution of Oceanography, the research offers a completely new way to understand drought, and the findings couldn’t be more relevant for farmers and policymakers around the world. 

A New Lens on Rainfall 

For decades, drought forecasting has focused primarily on how much rain falls. But Jiang and co-author Jennifer Burney of Stanford University decided to dig deeper, literally into the atmosphere, to trace where that rainfall originates. Their work distinguishes between two very different sources of atmospheric moisture: 

• Ocean-sourced moisture is transported long distances by large-scale weather systems such as atmospheric rivers, monsoons, and tropical storms. 

• Land-sourced moisture comes from nearby soils, lakes, forests, and vegetation. This process—often referred to as “recycled rainfall”—is highly dependent on local land conditions. 

Using nearly 20 years of satellite data, the researchers created global maps showing how much of a region’s rainfall comes from land versus the ocean. What they discovered reframes the entire discussion around drought vulnerability. 

According to Jiang, “It’s not just about how much it rains, but where that rain comes from.” And that insight changes everything. 

Why the Source of Rain Matters 

The study found that when more than roughly one-third of a region’s rainfall originates from land evaporation, the risk of drought intensifies. Ocean-sourced systems tend to deliver heavier, more reliable rain. Land-sourced systems, on the other hand, tend to be weaker and more variable. If local soils dry out or forests are cleared, the moisture they would normally contribute to the atmosphere disappears, along with the rainfall that depends on it. 

This means that drought isn’t simply a product of low precipitation; it’s heavily influenced by land health, water management, and ecosystem stability. Regions that rely primarily on land-originating moisture can experience sudden, cascading difficulties when soil moisture drops or vegetation is removed. 

Two Global Hotspots: The U.S. Midwest and East Africa 

Jiang’s team highlighted two regions that are particularly vulnerable under this framework. 

1. The U.S. Midwest 

Often considered one of the world’s most productive agricultural regions, the Midwest is surprisingly dependent on moisture recycled locally from soils and vegetation. This land-driven cycle makes it more susceptible to “rainfall feedback loops.” When the ground dries, less evaporation occurs. Less evaporation means less rainfall in the future, and the cycle continues downward. 

Given the Midwest’s importance in global grain markets, these drought dynamics can ripple far beyond the United States. Jiang recommends closer attention to soil moisture management, irrigation efficiency, and planting strategies to avoid reinforcing these feedback loops. 

2. East Africa 

In East Africa, the situation is more fragile. As cropland expands and surrounding forests are cleared, the very systems that sustain rainfall are being disrupted. Forests are major contributors to land-sourced moisture, and losing them directly decreases rainfall reliability. 

“This creates a dangerous conflict,” Jiang explains. “Farmers are clearing forests to grow more crops, but those forests help generate the rainfall those crops depend on.” 

Still, the study stresses that East Africa’s trajectory is not fixed. Conserving forests, restoring vegetation, and improving land management practices could help protect rainfall systems and regional food security before the balance tips too far. 

Forests as Natural Rainmakers 

One of the most important takeaways from the study is the critical role forests play in rainfall generation. Through transpiration and evaporation, forests release vast amounts of water vapor into the atmosphere. This vapor helps seed clouds and contributes to the rainfall that supports nearby croplands. 

“Upland forests are like natural rainmakers,” Jiang notes. Protecting them isn’t only about biodiversity or carbon storage; it’s a direct investment in agricultural sustainability. 

A Framework for Smarter Decisions 

Jiang and Burney’s research offer a practical tool. By mapping the origins of rainfall, governments and farmers can identify regions most at risk of land-driven drought cycles and adjust their strategies accordingly. 

This information could guide decisions such as: 

• Where to prioritize irrigation infrastructure 

• Where soil moisture conservation efforts will have the greatest impact 

• Which regions might benefit most from forest conservation or reforestation 

• When to adjust planting schedules to avoid peak drought vulnerability 

It’s a new way of thinking that aligns land management, water policy, and crop planning with the physics of the atmosphere. 

Why This Matters Now 

Climate change continues to make weather patterns more unpredictable. Understanding not just how much rain falls, but why it falls, gives us an entirely new set of tools to build resilience. As someone deeply engaged in water management and agriculture, I find this especially relevant. The idea that rainfall itself has a “supply chain” is both intuitive and profound. 

And it’s happening right in our backyard, literally. UC San Diego is producing research that can reshape how the world grows food and prepares for the future. 

A Final Word 

Kudos to lead author Yan Jiang, her co-author Jennifer Burney, and the UC San Diego and Stanford teams for this groundbreaking work. Their contribution provides farmers, policymakers, and water professionals with a new lens for planning for drought, protecting ecosystems, and sustaining global food systems. 

Sometimes the answers we’re looking for (like the origins of rainfall) have been above us all along.