In its annual World Water Development Report released on Friday, the world body said if current trends of water usage continue, the demand for water will exceed its replenishment by 40 percent by 2030.
The report said the rise in the world's population by some 80 million people per year was one of the main factors behind this looming global water deficit, with the current population of some 7.3 billion likely to reach 9.1 billion by 2050.
The increase in the number of people on the planet means that agriculture, which already uses some 70 percent of water resources globally - a figure that rises to over 90 percent in most of the world's least-developed countries, will have to step up output by some 60 percent, the report said.
It said climate change, which will affect rainfall patterns, and growing urban populations across the world will also exacerbate water shortages, with global demand for water likely to rise by 55 percent by 2050.
Groundwater in jeopardy
The report highlighted the fact that some 20 percent of groundwater supplies - which provide drinking water to some 50 percent of the world's population - are now suffering from over-extraction, which leads to freshwater in coastal areas often being contaminated by saline intrusion.
India is among the countries most affected by overuse of groundwater, with regions such as Maharashtra and Rajasthan being subject to significant water stress, according to the report.
"The fact is there is enough water to meet the world's needs, but not without dramatically changing the way water is used, managed and shared," the report said, pointing to a number of current abuses, including agricultural and industrial pollution and contamination by untreated sewage.
Higher water tariffs?
The report called for the introduction of measures to curb waste and punish pollution, increased education about the problem and possibly rises in the price of water.
"Present water tariffs are commonly far too low to actually limit excessive water use by wealthy households or industry," the report said, adding, however, that "responsible use may at times be more effectively fostered through awareness-raising and appealing to the common good."
The UN report was released two days ahead of the UN's World Water Day on 22 March, which aims to draw attention to water as a finite resource.