Living in the Bay Area, we know that a large earthquake could happen at any time. After a major disaster, your home could be without running water for many days. It will take time for DSRSD and relief agencies to set up temporary water stations. Other events can interrupt water service, too, including pipe breaks, pump failure, contamination, extreme drought, or a major fire.
It's essential to be prepared. While you can survive three weeks without food, you can die after three days without water. Every family should store water for emergencies.