More than 505,000 cubic kilometres of fresh water falls on the earth annually as rain or snow. But almost four fifths of this is on the oceans, and elsewhere much of it cannot be collected, leaving less than 0.5 per cent available as a resource.
Western Europe has access to only about four per cent of this and the phenomenon of climate change is expected to make the supply more difficult to guarantee over time.
A Friends of Europe report (“Saving Europe's Water: Its Place in the EU's Green Strategy”, 2006) says: "The effect of climate change on water could result in increased demand for irrigation in agriculture, reduced hydropower potential, less available cooling water, health problems stemming from water quality and economic downturn in water-related recreation, fishing and navigation."
The report points out that warming in the Alps is already twice the world average and that as a result, runoff patterns are changing. The depth of ice on the last mountain glacier in Germany has almost halved since 1910.
Although 75 per cent of the world's surface is covered with this precious liquid, 97 per cent of it is salty or otherwise undrinkable.
Of the remaining three per cent, two thirds is stored in glaciers and ice caps, leaving only one per cent of all the earth's water for humanity's needs. And even tapping into this presents many problems.
Not all water can be collected, and resources are distributed very unevenly around the globe. A fifth of all the freshwater reserves of the planet, for example, are concentrated in South America's Amazon basin.
As a result, already worldwide more than a billion people do not have access to safe drinking water and 4 billion people do not get enough water.
The situation in Europe is fortunately not as bad but water nevertheless remains a valuable resource. There are eight countries that can be considered water-stressed: Germany, England and Wales, Italy, Malta, Belgium, Spain, Bulgaria and Cyprus, representing 46% of Europe's population.
Droughts are already common in south European nations such as Spain, Italy or Greece, and are increasingly being seen in central or northern Europe countries such as in certain regions in the UK. Despite this, water consumption remains high in many western European households.
The water-stress is increasing in magnitude and impact. For example at the October 2007 meeting of the International Water Association (www.iwahq.org), the Flemish Minister-President stated that drinkable water is much scarcer in the region of Flanders than most people expect. Around 2000 litres of drinkable water is available per year for each inhabitant, which is one of the lowest numbers in Europe.
Meanwhile, European lifestyles have changed significantly over the last 50 years. The fact we have water on tap means we use it everywhere: in washing machines, dishwashers, baths, showers, sinks, pools and gardens.
Consequently there is a potential risk of a growing imbalance between the supply and demand for water and a need to reduce the amount we consume or see an increasing percentage of the population experiencing water-related problems.
For the developed world, this will involve overcoming a number of water-related challenges. But the good news is that, in many cases, saving water will be as easy as turning off a tap.