The amount of water in the world is limited. The human race, and the other species which share the planet, cannot expect an infinite supply.
Water covers about two-thirds of the Earth's surface, admittedly. But most is too salty for use.
Only 2.5% of the world's water is not salty, and two-thirds of that is locked up in the icecaps and glaciers.
Population is rising, but water supplies are not
Of what is left, about 20% is in remote areas, and much of the rest arrives at the wrong time and place, as monsoons and floods.
Humans have available less than 0.08% of all the Earth's water. Yet over the next two decades our use is estimated to increase by about 40%.
Water shortages set to grow
In 1999 the United Nations Environment Programme (UNEP) reported that 200 scientists in 50 countries had identified water shortage as one of the two most worrying problems for the new millennium (the other was global warming).
We use about 70% of the water we have in agriculture. But the World Water Council believes that by 2020 we shall need 17% more water than is available if we are to feed the world.
So if we go on as we are, millions more will go to bed hungry and thirsty each night than do so already.
Today, one person in five across the world has no access to safe drinking water, and one in two lacks safe sanitation.
Today, and every day, more than 30,000 children die before reaching their fifth birthdays, killed either by hunger or by easily-preventable diseases.
And adequate safe water is key to good health and a proper diet. In China, for example, it takes 1,000 tonnes of water to produce one tonne of wheat.
Inefficiency behind water crisis
There are several reasons for the water crisis. One is the simple rise in population, and the desire for better living standards.
Another is the inefficiency of the way we use much of our water. Irrigation allows wastage on a prodigal scale, with the water trickling away or simply evaporating before it can do any good.
In China it takes 1,000 tonnes of water to produce one tonne of wheat
And pollution is making more of the water that is available to us unfit for use. The Aral Sea in central Asia is one of the starkest examples of what pollution can do, to the land as well as the water.
Increasingly, governments are seeking to solve their water problems by turning away from reliance on rainfall and surface water, and using subterranean supplies of groundwater instead.
But that is like making constant withdrawals from a bank account without ever paying anything into it.
Looking for solutions
And using up irreplaceable groundwater does not simply mean the depletion of a once-and-for-all resource.
Rivers, wetlands and lakes that depend on it can dry out. Saline seawater can flow in to replace the fresh water that has been pumped out.
And the emptied underground aquifers can be compressed, causing surface subsidence - a problem familiar in Bangkok, Mexico City and Venice.
Pumping groundwater is like making constant withdrawals from a bank account without ever paying anything into it
There are some ways to begin to tackle the problem. Irrigation systems which drip water directly onto plants are one, precision sprinklers another.
There will be scope to plant less water-intensive crops, and perhaps desalination may play a part - though it is energy-hungry and leaves quantities of brine for disposal.
Climate change will probably bring more rain to some regions and less to others, and its overall impact remains uncertain.
But if we are to get through the water crisis, we should heed the UNEP report's reminder that we have only one interdependent planet to share.
It said: "The environment remains largely outside the mainstream of everyday human consciousness, and is still considered an add-on to the fabric of life."