Our bodies can survive without food for a couple of weeks. But without water, in just a couple of days we'd be in real trouble.
Many experts suggest keeping a two week supply.
They are also light proof to keep algae growth down. These drums are great for storing water long term. See our Ion and Redichlor pages for instructions on how to do this. Osmonics also has a very informative area on this subject. And lastly, Alan Hagan's book has the best section on water that I've seen.