Clean water is something that we all take for granted. We turn on the faucet and there it is. It is plentiful, it is clean and it is drinkable. Yes, it may have some undesirable chemical additions (fluoride come to mind) but that is a subject for another day. So, if a disaster occurred and the supply lines to fresh water were comprised, we would be in a pickle. There is a possibility that safe water would not be available for days and possibly not for weeks.
For this reason, the American Red Cross, FEMA, and just about every other authority out there recommends that the public store at least one gallon of water per person, per day for a minimum of three days. But if you think that a three day water supply is adequate, think again.