[AndrewC]

The amount of water on the earth is constant

Interesting. What scientific law is that?

[redgolum]

Strictly speaking, "Water" or H20 is made and destroyed millions of times a day in your body. The water molecule is broken up and formed during the many chemical reactions in your body at any given time.

Kind of neat actually (in a chemical engineering mass balance kind of way).

[H. Paul Pressler IV]

The total water supply of the world is 326 million cubic miles (a cubic mile is an imaginary cube (a square box) measuring one mile on each side). A cubic mile of water equals more than one trillion gallons.

About 3,100 cubic miles of water, mostly in the form of water vapor, is in the atmosphere at any one time. If it all fell as precipitation at once, the Earth would be covered with only about 1 inch of water.

http://ga.water.usgs.gov/edu/earthhowmuch.html