New Study Shows Rain and Ag Have Closer Relationship Than Previously Assumed
A recent paper published by Yan Jiang, a hydroclimatologist completing a postdoc at the University of California, San Diego and Jennifer Burney, a professor of Global Environmental Policy and Earth System Science in the Doerr School of Sustainability at Stanford, sought answers to where, exactly, rain comes from in agricultural areas around the globe.
The water cycle is a well-known scientific process. Rain falls from the sky, goes into the ground or bodies of water, where it evaporates via the sun’s energy, and then falls from the sky again. Jiang’s research shows that on a global level, rain comes both from the ocean, where it is moved around the globe by the jet stream and other air currents, and from the land, area where the majority of rainfall is the result of evaporation over land. Together, these form precipitation sheds.
Jiang and Burney utilized two decades of satellite data to measure how much of the world’s rainfall comes from land-based evaporation. They found that forests and native ecosystems — wetlands, uplands, downlands, grasslands, etc. — are remarkably important for how much rain falls from the sky. These ecosystems release large quantities of water vapor through evaporation and transpiration, and that vapor seeds the clouds, then rain falls.
Jiang noted that the amount of rainfall directly traceable to water already on the landscape was generally closer to 40% in the croplands that they tracked via satellite to quantify the origin of rainfall.
“Keeping water on land makes more rain. When moisture evaporates, the majority of moisture will fall back to the land as rainfall. It is a recycling system, a positive feedback loop,” she added.
“According to our map … mainly in the growing season, the summer or rainy season, a large portion of the moisture comes from land-based moisture,” Jiang said. “And it is a relatively high number … Up to 60% of rainfall comes from the land.”
Does this hold true for the Colorado River Basin?
The research driving this paper focused on areas that tend to be much wetter than much of the Colorado River Basin. But the study does raise global-scale questions that have major implications for the Colorado River Basin and other areas seeing changes in the regularity and intensity of their precipitation.
While the study focused on areas with more precipitation, Professor Burney is familiar with how important irrigation is for dry, inland areas like those in the Colorado River Basin.
“This study is not resolving fine scale processes in the Colorado Basin, but one of the things we showed is that if you are growing food in a place that relies on water from the land, you care about land management a lot more,” she said.
The Colorado River Basin has seen enormous changes in the amount of water on the landscape. Large scale drying of wetlands and diversion of water into reservoirs and for other uses has radically shifted how the land looks and is managed. Irrigation in the lower basin has reduced the water supply feeding millions of acres of wetlands. But the tricky thing with the vast scale of precipitation sheds and cause and effect on the water cycle is that to find out if human action is directly reducing precipitation, you would need to run the biggest experiment of all time.
To be clear, we don’t know if irrigation in the Colorado River Basin has any impact on the precipitation patterns in the region. But to find out, imagine a sixth-grade science project that crafts a waterproof and airtight terrarium, where the water cycle takes place before your eyes as it passes through plants, rises to the top of the terrarium, condenses, and precipitates back to the soil where the cycle begins again. Now imagine that on a landscape scale across the Colorado River Basin, with a few hundred parallel experiments running at the same time.
It’s not the most feasible project. That’s why modeling like the kind designed by Jiang and Burney is useful to answer these questions, and brings up questions about how farmers, ranchers, and land managers are partially responsible for processes like rain falling from the sky.
Read the research
Jiang, Y., Burney, J.A. Crop water origins and hydroclimate vulnerability of global croplands. Nat Sustain 8, 1491–1504 (2025). https://doi.org/10.1038/s41893-025-01662-1
