Question: How much water is enough during a disaster?
Most authorities suggest keeping at least a 3-day supply of water on hand in case of a disaster. The common definition of a 3-day supply is 3 gallons per person: half to drink and half to use for cooking and sanitation. The Centers for Disease Control and Prevention (CDC) even recommends drinking your half gallon every day during an emergency, no matter what. In hot weather, or for special cases like pregnancy or illness, the CDC recommends drinking closer to a gallon per day.
Most experts, including the CDC, warn against rationing. Drink it and more will come, they say. I agree that we shouldn't ration water supplies, but I'm not sure we should force ourselves to drink. There's a difference between depriving yourself of water in order to save it for later, and just not drinking all of it because you're not thirsty.
While not expressly stated, the suggestion to drink at least 64 ounces of water every day during a disaster comes from a longstanding myth that people need to drink 8 glasses of water. In reality, thirst is a good indicator for how much each person needs to drink and 64 ounces is an estimate of the total fluid intake the average person needs -- from all sources (tea, coffee, soda, fruit, water, clam chowder, baked beans...you get the picture).
Obey your thirst. Don't be afraid to drink water during an emergency. Indeed, you'll need to stay hydrated to stay healthy, but you don't need to force yourself to drink simply for the sake of drinking. Common sense suggests that you'll need the same amount of water during a disaster that you would need any other time.
If you follow the official recommendations for storing water -- a gallon per day per person -- and drink what you need, you should have more than enough to get by.
Valtin, H. "'Drink at least eight glasses of water a day.' Really? Is there scientific evidence for '8 x 8'?." American journal of physiology. Regulatory, integrative, and comparative physiology. Nov 2002.