Nothing is zero sum, IMHO especially water. You can’t kill water. Everything that uses water for life urinates the remainder, except perhaps birds and they don’t drink much anyway, and that use is minuscule IMHO. To quote the President and Algore, the oceans are rising, meaning even more water. If anything, water is cyclical. A few years ago our area lakes were at fifty percent of full, and this year after heavy spring rains, all are full to capacity and outflows still heavy.
There are also vast lakes, pools and rivers of water underground, deeper than the ground water we tap with wells.
http://eamazings.com/index.php/eamazings/10-incredible-underground-lakes-and-rivers.html
There has been a lot of debate concerning new water brought to Earth via small comets.
http://smallcomets.physics.uiowa.edu/faq.htmlx
Also google “water cycle”.