New study reveals source of rain is major factor behind drought risks for farmers
UC San Diego–led research shows that understanding where rain comes from could reshape drought planning and land management across the globe
University of California - San Diego
A new University of California San Diego study uncovers a hidden driver of global crop vulnerability: the origin of rainfall itself.
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 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.
In contrast, East Africa faces a more precarious but still reversible situation. Rapid cropland expansion and loss of surrounding rainforests threaten to undermine the very moisture sources that sustain rainfall in the region.
“This creates a dangerous conflict,” Jiang said. “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.”
However, Jiang sees opportunity as well as risk:
“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.”
Forests as Rainmakers
The research underscores that forests and natural ecosystems are crucial allies in farming. Forests release vast amounts of water vapor into the atmosphere through evaporation and transpiration (when plants produce moisture), effectively seeding the clouds that bring rain to nearby croplands.
“Upland forests are like natural rainmakers,” Jiang said. “Protecting these ecosystems isn’t just about biodiversity—it’s about sustaining agriculture.”
A Tool for Smarter Land and Water Management
Jiang’s research provides a new scientific framework connecting land management, rainfall patterns and crop planning — a relationship that could become central to future drought resilience strategies.
The study’s novel satellite-based mapping technique could help governments and farmers identify where to invest in irrigation infrastructure, soil water storage and forest conservation to maintain reliable rainfall.
Read the full paper, “Crop water origins and hydroclimate vulnerability of global croplands.”
Journal
Nature Sustainability
Method of Research
Data/statistical analysis
Subject of Research
Not applicable
Article Title
Crop water origins and hydroclimate vulnerability of global croplands
Rainfall tipping point predicts drought risk for crops
Stanford University
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.
Watch related video
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.
Journal
Nature Sustainability
Article Title
Crop water origins and hydroclimate vulnerability of global croplands
The crystal that makes clouds rain
How silver iodide seeds ice: TU Wien researchers uncover how a tiny crystal triggers ice formation at the atomic level
image:
Experiments with silver iodide:The experiments have to be performed in the dark
view moreCredit: TU Wien
No one can control the weather, but certain clouds can be deliberately triggered to release rain or snow. The process, known as cloud seeding, typically involves dispersing small silver iodide particles from aircraft into clouds. These particles act as seeds on which water molecules accumulate, forming ice crystals that grow and eventually become heavy enough to fall to the ground as rain or snow.
Until now, the microscopic details of this process have remained unclear. Using high-resolution microscopy and computer simulations, researchers at TU Wien have investigated how silver iodide interacts with water at the atomic scale. Their findings reveal that silver iodide exposes two fundamentally different surfaces, but only one of them promotes ice nucleation. The discovery deepens our understanding of how clouds form rain and snow and may guide the design of improved materials for inducing precipitation.
Surface structure holds the key to ice formation
“Silver iodide forms hexagonal structures with the same sixfold symmetry familiar from snowflakes,” says Jan Balajka from the Institute of Applied Physics at TU Wien, who led the research. “The distances between atoms also closely match those in ice crystals. For a long time, the structural similarity was believed to explain why silver iodide is such an effective nucleus for ice formation. A closer examination, however, reveals a more complex mechanism.”
The atomic structure of the surface where ice nucleation occurs differs from that inside the crystal. When a silver iodide crystal is cleaved, silver atoms terminate one side and iodine atoms the other. “We found that the silver-terminated and iodine-terminated surfaces both reconstruct, but in completely different ways,” says Johanna Hütner, who performed the experiments. The silver-terminated surface retains a hexagonal arrangement that provides an ideal template for the growth of an ice layer, whereas the iodine-terminated surface reconstructs into a rectangular pattern that no longer matches the sixfold symmetry of ice crystals.
“Only the silver-terminated surface contributes to the nucleation effect,” explains Balajka. “The ability of silver iodide to trigger ice formation in clouds cannot be explained solely by its bulk crystal structure. The decisive factor is the atomic-scale rearrangement at the surface, an effect that had been completely overlooked until now.”
Unraveling ice nucleation through experiments and simulations
The TU Wien team investigated these effects using two complementary approaches. First, experiments were conducted under ultrahigh vacuum and at very low temperatures. Water vapor was deposited onto small silver iodide crystals, and the resulting structures were examined using high-resolution atomic force microscopy.
“One of the challenges was that all experiments had to be performed in complete darkness,” explains Johanna Hütner. “Silver iodide is highly light-sensitive, a property that once made it useful in photographic plates and films. We only used red light occasionally when handling the samples inside the vacuum chamber.”
In parallel, the team simulated the surfaces and the water structures covering them using density functional theory, an advanced computational method for quantum mechanical modeling of atomic-scale interactions. “These simulations allowed us to determine which atomic arrangements are energetically most stable,” explains Andrea Conti, who performed the calculations. “By accurately modeling the silver iodide–water interface, we could observe how the very first water molecules organize on the surface to form an ice layer.”
“It is remarkable that for so long, we relied on a rather vague, phenomenological explanation of silver iodide’s nucleation behavior,” says Ulrike Diebold, head of the Surface Physics Group at TU Wien, where the study was conducted. “Ice nucleation is a phenomenon of central importance for atmospheric physics, and the atomic-scale understanding provides a foundation for evaluating whether other materials could serve as effective nucleation agents.”
Journal
Science Advances
Method of Research
Experimental study
Subject of Research
Not applicable
Article Title
Surface reconstructions govern ice nucleation on silver iodide
New study shows high-resolution cmip6 models better capture long-term precipitation trends in high mountain Asia
Institute of Atmospheric Physics, Chinese Academy of Sciences
image:
Linear trends of summer precipitation during 1951–2014 (units: mm·month⁻¹·decade⁻¹). (a) Observed trends based on GPCC data. (b) Trend differences between low-resolution models and GPCC. (c) Same as (b), but for differences between high- and low-resolution models.
view moreCredit: Lan Li
High Mountain Asia (HMA), the source region of major Asian rivers, plays a vital role in sustaining downstream water and ecosystem security. Over the past 50 years, summer precipitation in HMA has exhibited a dipole pattern—drying in the south and moistening in the north. While global climate models are widely used to explore the mechanisms and projections of these changes, their performance remains limited by the region's complex terrain and unique climate conditions. A key question thus arises: Can enhanced model resolution yield greater fidelity in simulating HMA precipitation?
A new study published on October 15 in Journal of Climate addresses this issue, revealing the added value and physical mechanisms of increased horizontal resolution in simulating HMA long-term precipitation trends. The work was led by Ph.D. student Lan Li from the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences, and the University of Chinese Academy of Sciences (UCAS).
Using six pairs of CMIP6 models with different horizontal resolutions, the team analyzed how higher resolution improves the simulation of long-term summer precipitation trends (1951–2014) and explored the physical mechanisms driving this improvement.
“The high-resolution models capture observed precipitation trends much more accurately than their low-resolution counterparts—especially over the southern margin of the HMA and nearby regions—reducing the wet bias by roughly 65%,” said Lan Li, the study's lead author.
What drives this improvement? “The enhanced performance of high-resolution models primarily stems from remote forcing associated with Indian Ocean SST warming, rather than local orographic effects,” explained Professor Tianjun Zhou, the study's corresponding author.
In-depth analyses of moisture budget and moist static energy budget reveal that the high-resolution models can better capture a warm sea surface temperature (SST) pattern over the central tropical Indian Ocean. This SST anomaly suppresses precipitation over the South China Sea and the Maritime Continent, which in turn triggers a Rossby wave response that generates an anomalous anticyclonic circulation over the northern Bay of Bengal. The resulting anticyclonic flow transports dry air into the southern HMA, suppressing local convection and alleviating excessive precipitation in the region.
This study demonstrates that, under the same physical configuration, climate models with higher horizontal resolution more accurately reproduce precipitation trends over High Mountain Asia. The researchers therefore recommend using high-resolution models when studying water cycle changes in regions with complex terrain. They hope these findings will offer valuable insights for improving the next generation of climate models.
Journal
Journal of Climate
Article Title
High-Resolution CMIP6 Models Better Capture Southern High Mountain Asia Precipitation Trends
