What happens when there is shortage of water supply ?

When there is a water shortage, people will flock to the stores to clear the shelves of bottled water. Then what do we do if we run out of bottled water? Why not bypass that problem, and just use a high quality drinking water filter at home like www.mosteffectivefilter.com, and then you won't have to worry about rationing water and paying a lot of money if and when something happens? Hope that helps.