Even then there might seem to be plenty. Enough rain falls on the land each year to cover all the world's countries two and a half feet deep. True, two-thirds of this evaporates, and two-thirds of what is left runs away in floods - but there is still enough, in theory, to meet the needs of twice the world's present population.
But there are two big snags. First the rain does not always fall where the people are: Iceland gets enough to provide each of its 250,000 people with 674,600 cubic metres of water a year, while Kuwait, with seven times as many people, scarcely gets a single drop.
Secondly, our use of water has multiplied sixfold this century, at twice the rate of population growth, largely because of a soaring increase in its use in agriculture.
Some two billion people in 80 countries are already afflicted by serious water shortages. Over the last half-century by United Nations' estimates, the amount of water available to each Asian has slumped by two-thirds, and to each African and Latin American by three-quarters. All over the world, people are trying to meet shortages by mining water, sinking wells into underground stores which have taken thousands of years to fill. The Ogallala aquifer, a vast subterranean sea that lies under eight Great Plains states in the US, is being drawn down by a metre a year to help fill the world's main breadbasket, which provides food for 100 countries.
This cannot go on forever - every year only a half of an inch works its way down to replace what is taken.
Mexico City, says the United Nations, has sunk more than 30 feet over the last 70 years because so much water is being taken from the ground beneath it. Cities as diverse as Bangkok and Houston are sinking too. Two-thirds of China's cities are short of water, while a quarter of the wells in the Indian state of Maharashtra have run dry. The water of Colorado River is now so fully used that it no longer reaches the sea.
Everyone expects things to get worse as populations and environmental degradation increase: cutting down forests, for example, cuts supplies.
Trees trap water and allow it to filter down into the earth to replenish springs and underground aquifers. And the enormous disruptions to the world's weather predicted as a result of global warming are expected greatly to aggravate the crisis.
As supplies tighten and demand grows, water will increasingly cause conflict. The first flash point is likely to be the Middle East where, the World Bank estimates, supplies per person will fall fivefold between 1960 and 2025. Already there have been tensions between Ethiopia and Egypt over sharing the waters of the Nile, and between Turkey, Iraq and Syria over the Euphrates.
Boutros Boutros Ghali, when Secretary-General of the United Nations, said: "The next war in the Middle East, will be fought over water, not politics."