How Much Water is Enough?

“It is better to rise from life as from a banquet -- neither thirsty nor drunken.” - Aristotle

It has become accepted wisdom: "Drink at least eight glasses of water a day!" This is equivalent to 2 to 2.5 quarts or 64-80 onces per day. But a study in 2002 suggests that this is an urban legend: Dartmouth Professor Finds No Scientific Evidence for '8 x 8'. Heinz Valtin, MD, believes that the water obsession can be linked to a recommendation in 1945 by the Food and Nutrition Board of the National Research Council for "1 milliliter of water for each calorie of food." What most people don't realize is that the next sentence in the same recommendation states, "most of this quantity [of water] is contained in prepared foods."

So how much water is enough?
There is an excellent chart on the site http://www.survivaltopics.com/survival/how-long-can-you-survive-without-water/ which shows how long you can survive at different max daily temperatures. With rationing and shade, the chart indicates that 2 quarts will allow you to survive 9 days in temperatures up to 90 F / 32.2 C. Water needs vastly increase as the temperature increases; the same 2 quarts will only keep you alive two days at 120 F / 48.9 C.

Many preparedness sites recommend a great deal of water, the Red Cross and FEMA say one gallon of water per day per person in their “Preparing for Disaster” pdf (on page 8). The site http://www.72hours.org/ says "three gallons per person per day will give you enough to cook and for limited personal hygiene.” The average person in the United States uses about 65 gallons per normal day with toilets, showers, dishwasher, lawn sprinkler, etc. Storing gallons of water is great for home storage but not practical for go kits or backpacks. A gallon of water weighs just over 8 pounds so a three-day supply is about 25 pounds per person.

In emergency situations you should separate water in to three categories:

1. Water safe enough to drink and put into your mouth (i.e. teeth brushing)
2. Water safe to cook with after boiling. This includes washing dishes.
3. Grey water used for body/clothes washing but not safe to consume

Given the weight of water, I personally plan on using found water for items 2 and 3 and applying filters, iodine, boiling, etc. I carry bottled water only for item 1 - drinking. How much? Answers vary on conditions. A minimum is 1 quart per day. Some sites recommend at least 2 quarts (perhaps biased by the 8x8 glasses of water meme). Many, like the National Park Service (NPS), recommend 4 quarts or 1 gallon per day. The NPS offers this warning for hiking in the Grand Canyon, "During the summer months, your fluid/electrolyte loss can exceed two quarts per hour if you hike uphill in direct sunlight during the hottest part of the day. Because the inner canyon air is so dry and hot, sweat evaporates instantly making its loss almost imperceptible."

Recommended Reading...
http://www.scribd.com/doc/2218172/Survival-Water-Storage