Heavy rain does not guarantee clean tap water. Floods, landslides, and power cuts can shut a treatment plant or contaminate a well even while water is falling from the sky.

A practical starting point used by emergency agencies is about one gallon (roughly 4 liters) of drinking water per person per day. That covers drinking and basic hygiene, not long showers. Plan for at least three days, and two weeks if you have the space. Add extra for children, pregnant people, anyone who is ill, and pets.

Where to keep it

Use food-grade jugs or containers made for drinking water. Washed milk jugs and containers that held chemicals are a poor choice. Store them in a cool, dark place, off the concrete if you can, and away from gasoline, paint, and household cleaners.

Label each container with the fill date. Sealed commercially bottled water can last longer than water you fill yourself. Home-filled containers should be emptied and refilled about every six months.

If the tap water becomes unsafe

• Bring clear water to a rolling boil for 1 minute. At elevations above 6,500 feet (2,000 meters), boil for 3 minutes.
• If you cannot boil, unscented household bleach can disinfect clear water. Public-health doses change with the bleach strength, so follow the current instructions from the CDC or your local health department, then let the water stand for 30 minutes.
• Cloudy water should be filtered through a clean cloth first, then boiled or disinfected.
• A backup filter or purification tablets is useful if you have to leave home or the stove will not light.

Do this before the season

Fill the containers while the tap water is still safe. Learn how to shut off the main water valve in case a pipe breaks. If you use a well, ask a local health office how to test and disinfect it after floodwater recedes. Floodwater itself is not drinking water, even after it looks clear.