Megacity precipitationsheds reveal tele-connected water security challenges

Area of Science:

• Hydrology
• Urban Planning
• Environmental Science

Background:

• Urbanization concentrates populations, increasing demand on finite freshwater resources.
• Many megacities depend on precipitation-fed renewable water sources, not groundwater or desalination.
• Understanding precipitation origins is crucial for urban water resource management.

Purpose of the Study:

• To identify the sources of precipitation supplying renewable water to the world's largest cities.
• To quantify terrestrial moisture recycling for 29 megacities and analyze variations between dry and wet years.
• To assess megacity vulnerability to disruptions in precipitation generation due to land-use changes.

Main Methods:

• Utilized the precipitationshed concept to trace precipitation origins.
• Quantified precipitation sources for 29 megacities.
• Integrated indicators of water stress, moisture recycling, economic capacity, and land-use change to assess vulnerability.

Main Results:

• 19 out of 29 megacities depend on terrestrial evaporation for over one-third of their water supply.
• Terrestrial moisture recycling dependence is often higher during dry years for many megacities.
• Four megacities (Karachi, Shanghai, Wuhan, Chongqing) exhibit high vulnerability; six others show medium vulnerability.

Conclusions:

• Megacity water security is significantly influenced by upwind terrestrial processes affecting precipitation.
• High dependence on terrestrial evaporation creates vulnerability to land-use changes impacting water supply.
• Integrating precipitationshed analysis is vital for understanding and ensuring urban water security.