1470. Where Do the World's "Water Towers" Get Their Water? —A New Perspective on Atmospheric-Water Cycles

The high-altitude regions that support our water resources are called "water towers." These regions function as headwaters for rivers and play a crucial role in stabilizing seasonal water supply by storing water as snow and ice. However, in recent years, global warming has led to glacial retreat and reduced snow cover, significantly altering the hydrological balance. While precipitation is becoming increasingly important, its "moisture source" has not been fully understood.

Approximately half of precipitation originates from "terrestrial evaporation."
A study published in Geophysical Research Letters analyzed water vapor transport in the atmospheric water cycle and quantified moisture sources in 73 water tower units (WTUs) worldwide. The results revealed that approximately 50% of precipitation originates from terrestrial evaporation. This is a process called "moisture recycling," where water that has evaporated from the Earth's surface is transported through the atmosphere and returns as precipitation. In particular, in inland WTUs, the contribution of terrestrial water vapor was found to be greater than that of oceanic water vapor.

Transpiration Plays a Dominant Role
Looking at the breakdown of terrestrial evaporation, plant transpiration is the most important. The study revealed the following characteristics:

• Short vegetation such as grasslands → Major global water vapor source
• Forest transpiration → Prominent in areas like the Amazon basin
• Bare soil evaporation → Important in the inland areas of the Tibetan Plateau

In particular, transpiration, unlike simple evaporation which depends on soil moisture and solar radiation, is influenced by plant root systems and canopy structures, enabling a more stable water vapor supply.

Different Water Vapor Origins in Precipitation Phases (Rain and Snow)
This study analyzed precipitation by dividing it into rainfall and snowfall. The results showed:

• Snowfall → Large contribution from oceanic evaporation
• Rainfall → Large contribution from terrestrial transpiration

This is thought to be because during cold periods, vegetation activity decreases and transpiration decreases, while water vapor transport from the ocean becomes relatively important.

Water resources are connected by "teleconnection."
Interestingly, water supply is not always confined to a single region. Due to atmospheric water vapor transport,

• water is supplied from distant regions rather than upstream.
• Land-use changes in downstream areas affect upstream precipitation.

This highlights the importance of the concept of an "atmospheric precipitationshed."
For example, deforestation in the Amazon can reduce precipitation in the downwind Andes region.

Resilience of Water Towers under Climate Change
This research demonstrates that water resources in high-altitude areas are closely related not only to topography and precipitation, but also to atmospheric-water cycles and land cover. In the future, integrating factors such as:

• Vegetation changes (greening/deforestation)
• Evapotranspiration dynamics
• Fluctuations in atmospheric circulation

will be essential for assessing the sustainability of water resources.

(References)
Bomei Zhang, Hongkai Gao, Lan Wang‐Erlandsson, Yan Li. 2026. Origins of Precipitation in the World's Water Towers. Geophysical Research Letters. https://doi.org/10.1029/2025GL118244

Contributor: Miyuki IIYAMA, Strategic Coordination Office