Where Rain Comes From Matters: Study Finds Midwest and Eastern Africa Face Higher Drought Risk

New research shows that not all rainfall is created equal: regions that rely heavily on moisture recycled from land — rather than moisture advected from the oceans — are more vulnerable to prolonged droughts. Using satellite observations and climate models, researchers mapped how much precipitation in different regions originates from land-based evaporation and transpiration versus ocean sources.

The analysis, published in Nature Sustainability and led by scientists from Stanford University and the University of California San Diego, finds a clear pattern: areas where more than 36% of seasonal rainfall comes from land-originating moisture are likelier to experience dry spells during the main growing season. Examples highlighted in the study include parts of the U.S. Midwest and portions of eastern Africa.

How the feedback works

The mechanism is a reinforcing negative feedback: drier soils reduce evaporation and plant transpiration, which in turn lowers the amount of local moisture available to form rainfall. That reduction in rainfall can further dry soils and vegetation, increasing drought risk and making crop yields more sensitive to weather variability.

"Our work reframes drought risk. It's not just about how much it rains, but where that rain comes from," said Yan Jiang, a co-author of the study and a postdoctoral scholar at UC San Diego.

The researchers emphasize agricultural consequences: in regions that depend on land-sourced moisture, local water availability becomes a deciding factor for crop success. Changes in soil moisture, land cover or deforestation can therefore have immediate, cascading impacts on yields and local food security.

"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 added. "Changes in soil moisture or deforestation can have immediate, cascading impacts on yields."

Implications and responses

Rising temperatures are likely to intensify these risks by increasing evaporation rates and altering atmospheric circulation, potentially prolonging droughts in some regions. To reduce vulnerability, the authors recommend both broad climate mitigation and targeted land-management actions: cutting greenhouse gas emissions to limit warming, conserving forests, restoring vegetation, and improving soil water retention can help maintain the land-sourced moisture that sustains regional rainfall.

Protecting and restoring vegetation is particularly important where local evapotranspiration supplies a large share of rainfall: lost forests or degraded soils can directly reduce the rainfall that agriculture depends on, increasing food-security risks. The study offers policymakers and farmers a new way to assess drought vulnerability and prioritize interventions before dry spells damage crops.