Two-thirds of the world is covered in water, containing over a billion trillion liters of water. So how could we have water shortages?
The vast majority of water on earth is saltwater and therefore not fit for human consumption. Only 2.5 percent of all water is freshwater. But more than two-thirds of that is locked away in ice caps and glaciers.
That leaves us with a very tiny fraction of water to drink, cook with, irrigate crops and feed livestock.
But water is a renewable resource that moves in a cycle. The amount of H2O on our planet will always remain the same, and won't run out as such.
The question is whether we will have enough clean water available for all citizens at all times.
Local water scarcity
According to a 2016 study by the University of Twente in the Netherlands, 4 billion people could face severe water shortages for at least a month every year.
In some regions, people are already severely affected by droughts and water scarcity. Millions of people in the Horn of Africa face hunger and illness after years of recurring drought. And Pakistan could run dry by 2025, a UN report suggests.
"Locally, the problem is very acute," Johannes Schmiester, a water expert at WWF Germany, told DW. "And all available numbers and observations suggest that the situation will become more severe."
Climate change is expected to intensify the situation. It alters weather patterns and water cycles around the world, causing shortages and droughts in some areas, and floods in others.
Extreme temperatures are also to blame for physical water scarcity. But in many cases experts as well point out "economic" water scarcity, due to how we manage our water supplies.
Groundwater is over-extracted; rivers and lakes are drying up or becoming too polluted to use.
To combat economic water scarcity, governments have to invest more in infrastructure for water supply and water storage, says Vincent Casey, a water expert at WaterAid.
"A big challenge is that water isn't always where you need it and when you need it most. So investment has to go into water storage and distribution, to ensure people always have access to safe water," he told DW.
One of the biggest water consumers is agriculture. Around 70 percent of all freshwater on the planet goes into irrigation of fields and feeding of livestock.
Farmers in some regions are already seeking to water their fields more efficiently. But researchers say this is not enough.
In Spain's tomato-growing region, farmers using the latest technology have managed to decrease their water consumption over the years. But in the industry as a whole - which produces a quarter of Europe's tomatoes - still needs more water than local water resources can supply.
As a result, the region faces water scarcity.
Researchers say the solution is to consider the entire geographical area and think in units of river basins. Can the local ecosystem sustain agricultural production?
"It's important to change perspective and consider whether local fresh water supplies can support certain industries," says Schmiester from WWF.
An additional strain on water management is our ever-growing consumption.
According to the World Health Organization, human beings need at least 20 liters (5.3 gallons) of freshwater per day to prepare meals and for basic personal hygiene. Laundry and bathing are not included.
Water consumption is much higher in industrialized countries, though - for example, the average person in Germany uses 140 liters per day. Flushing the toilet alone uses 30 liters.
Our indirect water footprint is even higher: 840 liters of water are required to produce one pot of coffee. And more than 8,000 liters go into a single pair of jeans.
Growing consumption calls for better reuse of water and more efficient production. The theme of this year's World Water Week addresses "water and waste," and how to reduce use and recycle water.
More than 3,000 experts will exchange ideas and develop solutions for waste water treatment and poor water management.
But Casey warns that technical solutions won't be a silver bullet for the global water crisis.
"It ultimately comes down to political decisions over allocations and effective governance of the resource. Solutions have to be devised in the countries where the problems are - and only then can water scarcity be solved," he concluded.