Rainfall tipping point predicts drought risk for crops

It matters where the rain that irrigates your food comes from.

In one of the first global analyses tracing the origins of rainfall for major crops, researchers at Stanford University and the University of California San Diego used satellite data and physical models to map where rainwater is recycled from land versus drawn from oceans. They found that regions relying more on land-sourced moisture—such as the U.S. Midwest, southern Africa, and parts of Asia—face greater drought risk and crop yield losses when rainfall falters.

The study, published recently in Nature Sustainability, offers a new way to pinpoint vulnerable farming regions and guide adaptation strategies. It also identifies a key threshold—when roughly a third of rainwater comes from land sources—beyond which crops become far more likely to suffer water stress.

Below, study coauthors Jen Burney, the deputy director of the Stanford Center for Food Security and the Environment, and Yan Jiang, a postdoctoral scholar in at the University of California San Diego, explain what this means for global agriculture, why it matters for food prices and policy, and how science is helping farmers anticipate water scarcity before it strikes.

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Why does it matter for our food supply where rainwater comes from?

Burney: More than 80% of the world’s cropland is rainfed, and all of that water originates either from evaporation of ocean water, or evapotranspiration from land. It was not clear to us prior to this study whether these two sources are equally reliable from a cropping and food security perspective, and it turns out they are not. So in short, tracking where rain comes from helps us understand crop vulnerability in a new way and even guides adaptation and resilience efforts.

You identified a “tipping point” where croplands become water-stressed when roughly 36% of rainfall comes from land moisture. Why is that threshold important — and what happens beyond it?

Burney: This 36% value is a critical empirical dividing line that emerged in our analysis. It effectively separates global croplands into two groups: one that is generally water-secure and the other that tends to be highly water-stressed. Regions where cropland depends more on land-originating water – beyond the 36% threshold – often have insufficient water supply during most sensitive parts of the growing season. On top of the higher annual risk of soil moisture deficits, these croplands are much more prone to experiencing frequent and intense droughts.

Were there any surprises your findings revealed about where and when crops are most vulnerable to water stress?

Jiang: Two regions really stood out to me: the U.S. Midwest and tropical East Africa. The Midwest is home to one of the world’s most productive and technologically advanced corn belts, but it’s experiencing worsening droughts. Our findings suggest that its high reliance on land-sourced moisture could amplify these self-reinforcing dry spells, which can exert prominent impacts on global grain markets. In East Africa, where cropland expansion and forest loss are accelerating, rainfall often depends on moisture recycled from nearby forests. This creates a dangerous conflict: the act of clearing forests to create farmland could eliminate the source of rain needed to sustain those same farms, posing a direct threat to local food security. This makes it a frontline for implementing smarter land-use policies now, while there is still time.

How could this research help farmers — in the U.S. Midwest or elsewhere — prepare for worsening droughts or shifting rainfall patterns?

Jiang: Our findings show that farmers in regions that rely heavily on land-sourced moisture need to pay close attention to local water availability and soil moisture, since changes there have the biggest impact on yields. Investments in irrigation, water storage, and soil-moisture management will be especially important. It also sends a wider message that protecting upwind forests and ecosystems matters. They help generate the evaporation that feeds downwind rainfall. In contrast, croplands that depend more on ocean-sourced rain should focus on adjusting planting schedules to better align with or avoid the worst impacts of large-scale climate disruptions, such as El Niño and monsoon storms.

Your method uses satellite measurements of water isotopes — essentially tracing the “fingerprints” of rain. What does this new technology let us see that we couldn’t before?

Jiang: This type of satellite data is a game-changer. It has been around for a while, but we are able to leverage the longer-run observational record of almost two decades in this new way. Water isotopes act like unique “fingerprints” of moisture in the atmosphere. Even though they make up only a tiny fraction of water vapor, they are closely tied to moisture evolution, including where that moisture came from and how it moves, mixes, and turns into rain. By using satellite observations of these isotopes, we can track the journey of water through the air — something that wasn’t possible with traditional measurements that only showed how much moisture or rainfall was present.

Burney is also a professor of Earth system science and of environmental social sciences in the Stanford Doerr School of Sustainability; and a senior fellow at Stanford’s Freeman Spogli Institute for International Studies.