In a free market, the price of water rises so that there is no such thing as a "shortage." It would be uneconomical to waste water on lawns far before a significant amount of additional people would be unable to buy enough to drink. This is why you'll never find the gas station runs out of bottled water because a nearby farmer used it to water his strawberries, despite the selfish/greed factor of the bottled water industry being off the charts.