Fresh water supplies are going to run out, so what can we do to make the taps keep running?

This may seem like a surprising statement, but the world's supply of fresh water is finite. As global population rises, the demand for food – and the water that produces it – grows inexorably. Globally, farming accounts for 70 per cent of our withdrawals from this fixed "bank account", this in the face of ever-greater domestic and industrial usage.

Water tables are falling in many parts of the world. Himalayan glaciers will shrink massively in the next century, reducing natural water storage in the mountains. The shortfalls will have to come from groundwater and surface storage. Many great rivers have drastically diminished flows.

Bangladesh is suffering from the diversion of Ganges River water and increased salinisation. Underground aquifers in many places are shrinking so rapidly that NASA satellites are detecting changes in the Earth's gravity. The Water Resources Group has estimated that India may face a 50 per cent lag in water availability relative to demand by 2030 and that global availability may lag demand by as much as 40 per cent; the statistics have been questioned. Sixty years ago, the world's population was about 1.25 billion people; few people, even in arid lands, worried about water supplies. Then came the Green Revolution, with its new, high-yielding crops, which depend on fertilisers and a great deal more irrigated farming. Global populations skyrocketed to nearly seven billion by 2009, with a projected nine billion by 2050. By the same year, the five hundred million people living in areas chronically short of water in the year 2000 will have grown by 45 per cent to four billion. A billion of us currently go hungry because there is not enough water to grow food. Much of the world's water is still unpriced, but it is now the most valuable commodity in the world. To compound the problem, 60 per cent of the world's people live in crowded river basins shared by several countries, often with daggers drawn.

The problems are acute, especially in arid areas with growing populations, where boreholes and aquifers are thought to be the answer. Seemingly a miraculous solution, but not if the drawdown exceeds the replenishment rate, as is the case with the ground water beneath a now-sinking Mexico City's 20 million inhabitants and with Bangkok, Buenos Aires and Jakarta, where pollution and rising salt levels combine with overdrafting.

In China, deep groundwater levels have dropped as much as 295ft (90m) in places. We have perforated the Earth's surface with boreholes to deplete a resource that we all, ultimately, hold in common. Now we stand at the threshold of what I call a third stage in our relationship with water; one where, apparently, cataclysm looms on every side. Vivid Doomsday scenarios espoused by numerous writers have Phoenix imploding as its water supplies fail, the Nile drying up, tens of thousands of people crossing national boundaries to find water.

Futurist after futurist warns that water wars are a certainty in coming centuries. Alas, at least some of these cataclysms could descend upon us if we persist in denying the seriousness of the water crisis and deluding ourselves into thinking that uncontrolled growth and more dams are the solution. They are not.

Yes, there will be shortfalls, people will go thirsty and die, but in the end, as has happened so many times in the past, human ingenuity, quite apart from technology, will find solutions. And in the process, we will develop new, much more respectful relationships with water, even if they do not necessarily have the profound spiritual intertwinings of earlier times.

In the short term, there are four potential ways of improving the situation, but none of them will solve the problem of chronic overdrawing. One lies in spending large sums on systematic improvements to storage and delivery, to the infrastructure behind water supplies. Underground reservoirs have potential. So do simple things like replacing leaking pipes, lining earth-bottomed canals and irrigating plants at their roots with just the right amount of water, among many others. A second solution also makes sense: make farming less thirsty, by using drought-resistant, higher-yielding, even genetically-modified crops. This is much easier said than done, for significant technological breakthroughs lie a long way in the future. Also, we should not forget that planting more crops means more use of water, since each plant transpires vapour into the atmosphere through photosynthesis. One possible solution may lie in developing plants that can grow using saline water but, again, this development is in the future. Then there's another seemingly attractive option: desalinisation. Surprisingly, this has been around a long time. Aristotle remarked that "salt water, when it turns to steam, becomes sweet and the steam does not form salt water when it condenses". Julius Caesar's legions drank fresh water condensed from sea water during his siege of Alexandria in 48-47 BCE. As self-appointed visionaries keep reminding us, desalinisation seems like the answer to all our problems but, in spite of improvements in efficiency, there remain significant environmental and technical problems. Desalinisation, which involves creating and recondensing steam, consumes prodigious amounts of energy, even in its most efficient iterations, so it is currently confined to nations where oil is cheap and abundant.

Nearly half the existing desalinisation plants are in the Arabian Peninsula and along the Persian Gulf, especially in Saudi Arabia and the Gulf states. In most other places, the cost of desalinisation is three or four times that of conventional water sources.

The cost of oil is rising, so the alternatives are either coal or nuclear power, both of which have their own environmental consequences and political baggage. Desalinisation plants operate along sea coasts; many of the most water-hungry areas are far inland, thereby adding huge transport costs to the already high price of a gallon of desalinised water. What, also, are we to do with the brine resulting from desalinisation, which has to be disposed of? Once again, breakthroughs lie in the long-term future. At present, desalinisation is no panacea, for it contributes only about 0.4 per cent of global water supplies.

Finally, there's conservation, which involves both profound changes in our mindsets and completely new attitudes toward water as a marketable commodity. Water is scarce, but it is also a complicated thing to market. It is difficult to move, hard to measure accurately in large quantities and complex to price and charge for. Most people resent paying for water, for they think it should be free or very cheap. Even in dry parts of the world where every drop is precious, the price of water seldom reflects its true scarcity. However, we are entering an era of potentially ferocious trading in water rights and a time when water could cost more than oil, as managing demand becomes an international priority. It's no coincidence that privately owned companies are quietly and aggressively purchasing water rights in many countries. Increasingly, municipal and other authorities are pricing water according to usage. Judging from experience in Australia, Los Angeles and other water markets, the strategy leads to reduced water use, especially when combined with measures to save water, such as reduced-flow toilets and strict timetables for watering. Like oil, water is a commodity that will be the subject of market forces, with price mechanisms that will bring supply and demand into balance. Once water is priced properly, the economics of international trade may encourage water-rich countries to produce water-intensive goods and arid ones to make those that are water-light. Mindsets are notoriously difficult to change, especially in societies accustomed to abundance and seemingly unlimited water supplies.

Using the forces of the marketplace and stricter allocations will not be strategies of first choice, especially in urban settings with high levels of poverty. Nor will conservation in the form of another commonly proposed measure, yet more dam construction, prove effective. History from the near and remote past tells us that dams are no panacea, for they silt up and silt has to be removed or the dam becomes shallower and ever less useful. And, even more important, where is the water to fill them going to come from? No dam ever creates water; it merely captures what is a finite supply. How can new dams provide more water in the era of prolonged global drought that lies ahead? Besides, there's adequate dam capacity in the American West to store any water that will come from the smaller snowpacks of future decades. Short of creating more water, more efficient allocation, extensive water recycling for landscaping and other purposes, drastic reductions in agribusiness water subsidies and miserly use of current supplies are some viable strategies for the future. And this kind of conservation, on scales small and large, is the responsibility of us all. Our survival depends upon it. We have much to learn about water conservation from the experience of our ancestors. Humans have managed water successfully for thousands of years in ways that are often far from the historical radar screen. We learn from their experiences that it is the simple and ingenious that often works best – local water schemes, decisions about sharing and management made by kin, family and small communities. These experiences also teach us that self-sustainability is attainable.

Such ingenuity comes in many forms. It may be a simple idea in the field or, in this day and age, more likely the inspiration for a social and political initiative that changes the way people think. We are moving into an entirely new water future, where equity of use, sustainability to protect future generations and affordability for everyone are major components.

A new paradigm for water management, based on well-defined priorities in which all stakeholders have a voice, will have to govern our future water use. Our salvation lies in long-term thinking, indecisive political leadership and in a reordering of financial priorities for, after all, investing heavily in water management will alleviate much disease and poverty automatically. Above all, the future will need a shift in our relationship with water to one that equates, at least approximately, with that of those who went before us – characterised by a studied caring and reverence.

Elixir: A Human History of Water by Brian Fagan is published by Bloomsbury (£20). To order a copy for the special price of £17 (free P&P) call Independent Books Direct on 08430 600 030. www.independentbooksdirect.co.uk