The Earth may be the "Blue Planet", but the amount of Freshwater available on it is actually quite small, and upon which the demand from human population both on a rising numerical basis and through the aspirations of its individuals is exacting an unsustainable toll. The Earth is the "Blue Planet" because almost 70% of its surface is covered by water, but this is mainly salt-water which constitutes the seas and oceans, and which without desalination is unfit for drinking, washing-in or for most industrial processes. Only 2.5% of the Earth's water is fresh, and about two thirds of that is locked-up in glaciers, mainly in the Antarctic and Greenland ice-sheets, and in permanent snow cover. Nonetheless on inspecting the detailed figures ("Freshwater Resources - The Atlas of Canada") the volume of liquid freshwater available appears vast, at close to 11 million cubic kilometers (km*3). However, near to 10.5 million km*3 of that is located in deep underground aquifers.
The principal sources of water available for human access are lakes, rivers, soil moisture and relatively shallow groundwater basins. There are various estimates of that total volume contained therein, ranging from as little as 60,000 km*3 to >300,000 km*3, from which I estimate as a reasonable average that around 200,000 km*3 of water is available, which is less than 1% of all freshwater and only about 0.01% of all water on earth. To be sure, we are not going to hit "Peak Water" in the same way as "Peak Oil", gas or uranium, since water as an entire resource is completely renewable, being "distilled" around by the planet's hydrological cycles - it is never transmuted permanently into any other form by Nature - rather the problem is one of providing "clean" water free from contamination, and to meet a seemingly inexorable and increasing demand for it.
Strain on water resources is particularly acute, unsurprisingly, in more arid regions where large-scale agricultural production and or large populations are. Hence, the Middle East, Central Asia, North Africa, South Asia, China, Australia, Mexico and the western United States are notably prone to water shortages. Globally, use of freshwater has tripled during the past 50 years, while population has more than doubled, and improved technologies permit farmers to extract groundwater from increasing depths and harness rivers with more inclusive dams. Almost 70% of all such water is used for agriculture (irrigation mostly) while industry and domestic use takes 20% and 10% of it respectively.
Between 1950 and 2003, the area of irrigated land increased globally from 94 million to 277 million hectares (three-fold) but this expansion is levelling-off as the water needed to drive it is becoming increasingly scarce. As demand for water continues to grow mainly to meet the rising demands from agriculture, industrialisation and residential needs, aquatic ecosystems struggle to respond. Since the dawn of civilisation, countless communities have depended on rivers (many having been deliberately established thus, including my own village of Caversham, located on the north bank of the River Thames). However, in many locations, demands being made up-stream have become so great that there is almost no down-stream, and so communities there have been dessicated to extinction.
Aquifers are being drained in major areas of food-production, including the North China plain, which provides half of China's wheat and one third of its corn; Punjab, Hayana and other "bread basket" regions of northern India; along with the southern Great Plains of the United States. Altogether, these countries produce around half the world's grain, and along with Pakistan together account for over 60% of the world's total agricultural water extraction. I am making an educated guess, here, but the combined populations of India, China and the U.S., must be close to 3 billion, or around half the burgeoning total of 6.5 billion humans on the face of the Earth, so such an immediately large proportion (60%) in fact merely reflects the number of mouths in must feed.
To all intents and purposes, water "is" running out, and there must be a "Peak" where supply can no longer sustain demand. The rider lies in the provision of "clean" water (not the total volume of H2O, as already noted); accordingly, much effort is being made to improve methods for "desalination" of sea-water. Thus, water thus purified by its means might be drawn inland from the coasts around the western U.S., the Middle east, Mexico, Central Asia, and elsewhere, although even then, a highly intensive infrastructure would be necessary to pipe it to those regions of necessity. However, desalination in a nutshell involves pumping salt-water through a semi-permeable membrane which excludes salt at a pressure greater than the osmotic pressure (reverse osmosis), and so lets the pure water pass through it to the other side. In principle this is feasible, and there are thriving cities in the Middle East (e.g. Kuwait and Qatar) that have been "taken back from the desert" using desalinated water. Indeed, desalination is used in over a hundred countries, with Saudi Arabia accounting for about 24% of total world capacity. Kuwait built the world's first large-scale desalination plant in the 1960s and, on account of its enormous energy reserves, is unique in using desalinated water to support agriculture. Interestingly, he world's largest desalination plant is in Ashkelon, Israel, which began operating on August 4th, 2005 is capable of producing 100 million cubic meters of water per year.
There are a number of drawbacks to desalination, which suggest that it will never provide a long-term solution to the world's water shortages, which are now inevitable. (1) The process is energy intensive (...here we go again... yet more pressure on energy reserves which are already under pressure). (2) The salt that is removed from the water which ends up "pure", has not "disappeared", it is merely concentrated in the sea-water on the opposite side of the membrane, and needs to be disposed of. This is not a trivial matter, and the U.S. Environmental Protection Agency has classified it as "industrial waste". Simply pouring it onto land is likely to render that unfit for agriculture, and sea disposal in sufficient concentration could disrupt sensitive eco-systems. (3) The projected rise in sea-levels could inundate ("swamp-out") coastal desalination plants as it threatens to do to similarly located nuclear plants.
We have to save water... let's start thinking in terms apart from those of unsustainable production; the prevailing "more and more" attitude, is getting us nowhere fast.