Origins of Precipitation in the World's Water Towers

High-mountain systems act as the planet's vital water towers, sustaining freshwater supplies for billions of people. Climate change is exacerbating hydrological imbalances in these regions, yet the moisture sources maintaining their precipitation—the primary water input—remain poorly quantified. Here, we combine two atmospheric moisture tracking methods to analyze 73 water tower units (WTUs), revealing that terrestrial evaporation contributes approximately half of total precipitation (52% UTrack, 50% WAM-2layers) over these regions, with inland WTUs relying more on land-sourced moisture than coastal systems. Transpiration from short vegetation dominates terrestrial contributions globally, while forest transpiration (e.g., Amazon) and bare-soil evaporation (e.g., interior Tibetan Plateau) are regionally significant. Importantly, oceanic moisture drives snowfall, whereas terrestrial transpiration sustains rainfall—except in the southern Tibetan Plateau. These findings refine the atmospheric water cycle's role in high-mountain hydrology, offering a mechanistic basis to project water tower resilience under global change.