Abstract: Moisture pathways from land and ocean feed the rain that sustains global croplands. Since oceanic and terrestrial evaporation regulate rainfall patterns and seasonality in distinct ways, disturbances in these sources can expose croplands to different hydrological risks. Beyond numerical moisture tracing models, can we observe and separate terrestrial and oceanic vapor sources? And how do moisture origins link to crop production?

In this study, we combine satellite-derived water-isotope observations with physical models to trace the origin of atmospheric moisture – separating terrestrial and oceanic sources – over global croplands during 2003–2019. Our analysis reveals that the fraction of rainwater originating from land (f) varies both geographically and seasonally, with a significant threshold at ~36%. Regions with f ≥ 36%, i.e., more reliant on terrestrial evaporation, show greater odds of in-season rain shortfalls and soil moisture deficits, and exhibit stronger sensitivity to hydroclimate variability, including a higher likelihood of drought. Knowing where rain comes from helps anticipate where crops are more vulnerable. The results also point to upwind land management (soil/vegetation moisture retention) as a potential lever to strengthen agricultural resilience and facilitate food security.

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