While often the subject of movies and hit television shows, stories about a post-apocalyptic world involve a lot of imagination on what that the planet will look like once food and water are scarce or gone. But what does science have to say? While humans can survive longer without food than they can without water, both are still necessary and conservation efforts need to continue.
The earth has a great deal of water, but much of it, about 75%, is saltwater. The oceans and other bodies of water might be evaporating, but that water returns to earth in the form of rain, so we do get some back. The problem is that the water is being used up faster than the cycle of evaporation and rain can replenish it, leading to violent conflicts over access to water, such as the civil unrest in the Darfur area of the Sudan. The same is true for the food supply. Crops cannot be grown and livestock cannot be raised without a water supply. When food becomes scarce, the same kinds of violent scuffles over food will happen as have happened over access to clean water. Seed vaults are in place in many countries so that if the worst did happen and crops were wiped out, the earth could start over again, but it's a better idea to settle disputes now and continue to conserve resources rather than have to begin again after an apocalypse.