* Over population * Building too quickly and taking too many trees away * Weather changes * Not caring for our forests * Heat can evaporate water * People waste water with washing machines, flushing toilets, having long showers. Of course we don't often realize that we are using so much water, but take it away for a week and people would soon get the drift. * Governments that are not environmentally friendly. Another answer: The problem, like food, isn't total quantity, it's availability and distribution. Barring the few molecules that come to Earth from comets etc., there is a finite amount of water on earth. Scarcity comes from not only lack of water to meet a local demand, but a need for CLEAN drinkable water. As the above answer suggests, scarcity is also caused by libing beyond our means, more people in an area that need more water than the system can provide.