Its a simple numbers game.
There are on average 140,000,000 births and 57,000,000 deaths each year; if you figure each person needs about a gallon of water each day just to survive this means they need to come up an additional 30 billion gallons of drinkable water each year.
If you figure each person around the globe on average "uses" 80-100 gallons of water a day this is an increase of 2.4-3.0 trillion gallons of potable water each year.
Its not that water disappears-its just harder to get clean fast enough, to the people who want it, cheap enough.
Now when asked when I think things will change I answer "The next time Thomas (aka the fed) robs Peter (aka the 53%) to pay Paul (aka the 47%) and Peter pulls a gun...things will change"