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Much has been made lately of the growing prospect "water shortages". The impression is given that we must all conserve, lest we run out of usable water.

To my understanding, the planet earth is a "closed loop". Isn't it true that, barring a tiny amount of sublimation off into space and natural/man-made conversion to/from its component hydrogen and oxygen, the planet has always, and will always contain pretty much the same amount of water? Obviously, the form it takes - liquid, solid, vapor - will vary but in the end, the actual quantity available to humanity shouldn't change much?

If so, is the issue really the possibility of actually "running out", or is the problem the expense or the difficulty of finding viable technologies to recover or clean or convert all of that existing water?

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    Nice question - welcome to sustainability.SE! – Flyto Jul 14 '14 at 17:01
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    It's potable water that we're running out of, not water per se. Seawater is not actually very useful to us. – Móż Jul 14 '14 at 21:15
  • Most of the <3% of freshwater found globally is locked in ice caps and aquifers. So a global water shortage is eminent if water resources management is poor. – Chimango Chisuwo May 3 '17 at 7:10
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As you surmise, the problem is in cleaning/purifying the water we want to use. Desalination is extremely expensive, to the point where it's cheaper to have a war rather than build a desalination plant. This is currently most obvious in the middle east but changes to the Indian Ocean monsoon could have dramatic effects all around that area. India is working to mitigate the likely effects.

Desalinating Seawater is expensive. To give you some idea of numbers, they built a desalination plant in Victoria (Australia) recently, and that cost about $5 billion to build and another $150 million a year to run. That produces about 150 gigalitres of drinking water per year. By comparison, the adjacent Powlett River is part of a group of 10 short rivers that drain the surrounding area with a flow of 900Gl/year and Melbourne city (pop 4 million) uses about 360 gigalitres/year. So they'd need three of those $5 billion plants to supply Melbourne with drinking water.

(edit based on Simon's comment below) The cost of desalination is mostly energy - desalination is very energy-intensive. Usually that's electricity to run pumps for reverse osmosis systems, but distillation could be used (has been in the past). The worldwide implications from energy use if lots of people used desalination on a large scale would be significant - those giant solar electricity schemes proposed to supply Europe could instead provide electricity to desalinate... not enough water for North Africa. Energy and water tend to be quite closely linked (most methods of obtaining one require the other), and both are becoming increasingly scarce. If you add a renewable energy plant to feed the desalination plant you're probably adding a billion dollars to the cost. Money that most water-short nations don't have.

That doesn't cover irrigating farmland, which is done on a relatively small scale in Victoria, but in the Murray-Darling basic (Australia's foodbowl) they use about 3500Gl/year to irrigate about 1.4 million hectares. That's 25 desalination plants, going on $3 billion a year in electricity to run them, and the build cost would be astronomical. Obviously using desalination plants to water crops, especially the broad-acre irrigation that Australia uses, is insane. With water that expensive it would be economical to completely change farming techniques to reduce water use, as is done in the middle east and other places.

Mining Water

Many countries use aquifers for irrigation and there are two problems. Often that water is heavily mineralised and sometimes the minerals are poisonous (arsenic for example), but mostly they use them at non-sustainable levels. Either they draw more water from an aquifer than is going in (exceed the replenishment rate) or there's no detectable replenishment and the aquifer is fossil water. Once you've mined out that water, it's gone. Many countries have ongoing problems with wells needing to be drilled ever deeper as the water level drops, or seawater flowing into the aquifer (seawater intrusion).

Water Wars

With numbers like that, you can see why going to war is a attractive strategy (to an economist, anyway). And for people who don't have the money to build desalination plants, that's often the only option. It's likely that one aspect of the current conflict(s) in Syria and Iraq is a contest for control of water sources. There a conflict shaping up between Ethiopia and Egypt over dams on the Nile (Ethiopia will shortly be able to turn the Nile off, which is obviously a concern for Egypt).

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    Just a suggestion to add to this - money is not the only limitation on desalinaton, but energy resources too. You've mentioned the energy required in terms of the cost of electricity, but for some nations (e.g. those who can't easily buy oil on the world market) such energy simply isn't available. Not to mention the worldwide implications from energy use if lots of people used desalination on a large scale. Energy and water tend to be quite closely linked (most methods of obtaining one require the other), and both are becoming increasingly scarce... – Flyto Jul 15 '14 at 9:07
  • Great answer. In the case of aquifers, can you explain why our waste water doesn't filter back into the aquifer and replenish the water we mined out? – michelle Jul 16 '14 at 17:12
  • @michelle that's not really relevant, and would be a better questions for geology.SE or geography.SE – Móż Jul 16 '14 at 22:03
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I think it is also worth noting that as human population grows, significant ammounts of water are 'locked up' in the body itself. If the body is roughly 94% water by weight (a figure I've heard thrown around.. but you get the point), and the average person 150lbs, that is 141lbs of water per person on the planet.

  • There's about 1.5 billion cubic kilometres of water on the planet. At 60 litres each, that's enough for 2.5 x10^16 people, or 25 million billion, roughly 5 million times as many people as exist today. – Móż Aug 27 '14 at 23:36
  • Wow... So I guess I can stop feeling bad about drinking my 8 cups a day then. – Scott Goodgame Aug 28 '14 at 0:32

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