About 80% of the world's population lives in areas where the fresh water supply is not secure, according to a new global analysis.
Researchers compiled a composite index of "water threats" that includes issues such as scarcity and pollution.
The most severe threat category encompasses 3.4 billion people. Writing in the journal Nature, they say that in western countries, conserving water for people through reservoirs and dams works for people, but not nature.
They urge developing countries not to follow the same path. Instead, they say governments should to invest in water management strategies that combine infrastructure with "natural" options such as safeguarding watersheds, wetlands and flood plains.
The analysis is a global snapshot, and the research team suggests more people are likely to encounter more severe stress on their water supply in the coming decades, as the climate changes and the human population continues to grow.
They have taken data on a variety of different threats, used models of threats where data is scarce, and used expert assessment to combine the various individual threats into a composite index.