PEOPLE kill each other over diamonds; countries go to war over oil. But the world's most expensive commodities are worth nothing in the absence of water. Fresh water is essential for life, with no substitute. Although mostly unpriced, it is the most valuable stuff in the world.

Nature has decreed that the supply of water is fixed. Meanwhile demand rises inexorably as the world's population increases and enriches itself. Homes, factories and offices are sucking up ever more. But it is the planet's growing need for food (and the water involved in producing crops and meat) that matters most. Farming accounts for 70% of withdrawals.

Our special report this week looks at the increasingly visible consequences of the rising demand for water. Few of the world's great rivers that run through grain-growing areas now reach the sea all the year round or, if they do, they do so as a trickle. Less obvious, though even more serious, are the withdrawals from underground aquifers, which are hidden from sight but big enough to produce changes in the Earth's gravitational field that can be monitored by NASA's satellites in space. Water tables are now falling in many parts of the world, including America, India and China.

So far the world has been spared a true water war, though the belligerency in Darfur comes close to being one, and competition for water can sometimes bring rivals together as well as drive them apart. But since over 60% of the world's population lives in a river basin shared by two or more countries, the scope for squabbles is plain. Even if acute water shortages were to become widespread in just one country—India, say, or China—they could lead to mass migration and fighting.

Although the supply of water cannot be increased, mankind can use what there is better—in four ways. One is through the improvement of storage and delivery, by creating underground reservoirs, replacing leaking pipes, lining earth-bottomed canals, irrigating plants at their roots with just the right amount of water, and so on. A second route focuses on making farming less thirsty—for instance by growing newly bred, perhaps genetically modified, crops that are drought-resistant or higher-yielding. A third way is to invest in technologies to take the salt out of sea water and thus increase supply of the fresh stuff. The fourth is of a different kind: unleash the market on water-users and let the price mechanism bring supply and demand into balance. And once water is properly priced, trade will encourage well-watered countries to make water-intensive goods, and arid ones to make those that are water-light.

Solutions on tap

All four approaches have a part to play in the solution, but none is likely to end the over-drawing of water at all quickly. For instance, although crop yields may be improved by new technologies and even new breeds (especially following this week's genome advance), no one has yet managed to produce plants that offer dramatically more crop per drop. Without such a breakthrough, growing more crops inevitably means losing more water, since each extra plant transpires water vapour into the atmosphere during photosynthesis.

Desalination looks more hopeful, since new technologies are being developed and prices are falling. But it is still expensive, especially in terms of energy use, which is why at present it provides only 0.4% of the world's fresh water. It will undoubtedly add to the supply of drinking water, of the ultra-pure water required for some industrial processes, even of irrigation for high-value crops, especially if grown for export. But it won't solve most farmers' water problems.

Liquid assets

As for the market, when it approaches water it meets all sorts of obstacles: water is difficult to move, difficult to measure, difficult to price and often difficult to charge for, since many people think it should be free. Even in arid market economies where every drop is precious, the price of water seldom reflects scarcity. Trading in water rights may one day bring order to the 20m well-users in India, but not in time to feed the 1.4 billion Indian mouths expected by 2025.

If that is to be done, it will be done largely by managing demand. This is already beginning to happen in parts of both India and China, where farmers are learning how to measure the water they pump, how to use it to greatest effect and thus how to sustain aquifer levels. The urgent need now is to spread their principles and practices at home and to other countries.

Even if the world manages to limit depletion, many water-related problems will persist. About 1 billion people are still without access to a decent water supply, while others suffer from flooding, pollution and poor sanitation. Yet if man wants to solve these problems, he can. He has applied far more money and know-how to issues far less important than the shortage of water. And if he does tackle them successfully, the big causes of human suffering—disease and poverty—will be automatically alleviated. Investing more thought and cash in the better use of the world's most valuable commodity is surely worthwhile.