5 Ways to Purify Water -Stocking your water reserves is one of the most important things you can do to get your family prepared for disaster. You’re going to need about a gallon of water per person per day. You can minimize the amount of water your body needs by reducing activity and staying cool, but you should never ration water. If you’re thirsty, drink! If the disaster lasts long enough, eventually, you’re going to run out of water. What will you do then?