You rotate it every ~6 months:

* https://www.cdc.gov/water-emergency/about/how-to-create-and-...

The old stuff can be used to water house plants or your garden. If you have advanced warning of an event (e.g., hurricane), you can also fill up a large reserve 'just in time':

* https://thewirecutter.com/reviews/waterbob-bathtub-liner-rev...