The planet earth is a closed system where materials cycle between the lithosphere, atmosphere, hydrosphere and biosphere. It does not lose materials to the outside and neither does it receive any from outer space.
The water that has been there since the earth was created is still the same. It is used and recycled over and over again through the hydrologic (water) cycle. It is never lost. but changes its form from liquid to vapor to ice or snow then back to liquid in a continuous cycle.
