There are hidden sources of water in your home. You can use the water in your hot-water tank, pipes, and ice cubes. As a last resort, you can use water in the reservoir tank of your toilet (not the bowl).
How to use water in your pipes: Shut water off to your house. Let air into the plumbing by turning on the faucet in your house at the highest level. A small amount of water will trickle out. Then obtain water from a faucet located at the lowest point in the house.
How to use hot-water tank water: Be sure the electricity or gas is off before draining the hot-water tank. Open the drain at the bottom of the tank. Start the water flowing by turning off the water intake valve and turning on a hot-water faucet. Do not turn on the gas or electricity when the tank is empty.
Be sure to purify water from outside natural sources before drinking it. Avoid water with floating material, an odor or dark color. You should not drink floodwater.