<p>Water availability critically influences crop phenology and agricultural productivity. Here we use satellite-derived water isotope observations and physical models to trace atmospheric moisture origins for major global rain-fed crops from 2003 to 2019, distinguishing between oceanic and terrestrial sources. Our analysis shows that the fraction of rainwater originating from land (<i>f</i>) varies both geographically and seasonally, with an important threshold at ~36%. Regions with higher <i>f</i>, that is, more dependent on land-originating water, are more prone to insufficient rainwater supply and soil moisture deficits during the main growing season. Crops in these regions show higher sensitivity to hydroclimate—with reduced productivity in lower-rainfall years—and a higher likelihood of drought. Notably, more than 40% of global maize and 60% of winter wheat is grown in regions where rainfall depends heavily on land-originating moisture (<i>f</i> ≥ 36%), underscoring the vulnerability of key staple crops to hydroclimate stress. Our results highlight the importance of managing local land moisture sources and reveal where targeted water management strategies would be most expected to enhance agricultural resilience.</p>