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Taking freshwater as being:

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, icebergs, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams.

How much has the total available freshwater change over time? How much has it gone from one reservoir to another? How can we estimate future changes?

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    $\begingroup$ If the estimates of total seawater volume and total all water volume on Earth are accurate, perhaps one could use climatology of sea level to determine change in seawater volume, and thus, freshwater volume? $\endgroup$ – milancurcic Dec 25 '14 at 22:22
  • $\begingroup$ @milancurcic this would only work if the ratio fresh:sea water is (about) constant, wouldn't it? But water is also found in the atmosphere and "stored" in the biomass. $\endgroup$ – BmyGuest Dec 26 '14 at 7:54
  • $\begingroup$ @BmyGuest actually milancurcic's comment doesn't use a ratio but a mass balance, V_tot = V_salt+V_fresh = ~constant, so any increase in salt water volume would mean an equal decrease in fresh water volume. $\endgroup$ – hugovdberg Dec 27 '14 at 23:09
  • $\begingroup$ @hugovdberg that's what I meant. It will work if the other aspects could be considered 'constant', but are they? is there any time-development of the earth's total biomass? But you're right in that I used ratio rather than sum - still the same problem though. $\endgroup$ – BmyGuest Dec 27 '14 at 23:40
  • $\begingroup$ @BmyGuest well off course the mass balance as stated here is oversimplified, there have been changes in total biomass for sure (once there was none..). So, depending on the timescales you want your estimates for, several other factors need to be taken account of indeed (there is a big mistake in my previous comment, using volumes in a mass balance). But looking at this link biomass isn't your biggest error in the balance ;-) $\endgroup$ – hugovdberg Dec 27 '14 at 23:48

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