The world will have only 60% of the water it needs in just 15 years unless countries dramatically change their use of the resource, a UN report warns.
Many underground water reserves are already running low, while rainfall patterns are predicted to become more erratic with climate change. As the world's population grows to an expected nine billion by 2050, more groundwater will be needed for farming, industry and personal consumption.
The report predicts global water demand will increase 55% by 2050, while reserves dwindle.
Having less water risks catastrophe on many fronts: crops could fail, ecosystems could break down, industries could collapse, disease and poverty could worsen, and violent conflicts over access to water could become more frequent.
"Unless the balance between demand and finite supplies is restored, the world will face an increasingly severe global water deficit," the annual World Water Development Report said, noting that more efficient use could guarantee enough supply in the future.
The report, released before World Water Day 2015, calls on policymakers and communities to rethink water policies, urging more conservation as well as recycling of wastewater. Countries may also want to consider raising prices for water, as well as searching for ways to make water-intensive sectors more efficient and less polluting, it said.
About 748 million people worldwide have poor access to clean drinking water, the report said, cautioning that economic growth alone is not the solution – and could make the situation worse unless reforms ensure more efficiency and less pollution.
World Water Day has been observed on 22 March every year since it was established by the UN General Assembly in 1993.
The idea is to make people think twice about how much water they waste. People who have plentiful access to water are encouraged to try not turning on their taps all day.