Normal Requirements: The standard advice is "Drink eight 8-ounce (8x8) glasses of water a day." That's about 1.9 liters.
The Mayo Clinic says,
"Adequate intake (AI) for the average, healthy adult living in a temperate climate according to The Institute of Medicine is roughly 3 liters (about 13 cups) of total beverages a day for men. The AI for women is 2.2 liters (about 9 cups) of total beverages a day."

Hydration Myths

1. "You need to drink eight 8-ounce (8x8) glasses of water a day." You get significant amount of water from food. You also get it from drinking milk, coffee, tea, juice or soft drinks, so it does not have to be plain water.
   Drinking too much water can actually be dangerous by causing low blood sodium levels (a condition called hyponatraemia) and exposing people to pollutants in the water.

   The origin of the 8x8 rule was a Food and Nutritional Board of the National Research Council recommendation in 1945. But, It also noted most of that intake level was already satisfied through consumption of food without the need for the additional water.

   According to Jurgen Schnermann, a kidney physiologist at the NIH, to replace daily losses of water, an average-sized adult with healthy kidneys sitting in a temperate climate needs no more than one liter of fluid.

   Exception: See Exercise below.

2. "Caffeinated or alcohol containing drinks don't count because they are diuretics."
   Caffeine does cause a loss of water, but only a fraction of what you're adding by drinking the beverage. In people who don't regularly consume caffeine, for example, researchers say that a cup of java actually adds about two-thirds the amount of hydrating fluid that's in a cup of water.

   Caffeine weakens the detrusor muscles in the bladder, which provokes the need to urinate, but does not affect diuretic hormones the way alcohol does.

   Exception: A few alcoholic drinks the night before could have a large negative effect on hydration levels when exercising the next day.
   Alcohol and thin air at high altitude has more of an effect than at sea level.

The best rule is to:

1. Under normal conditions let your thirst be your guide.

   Exception: During strenuous exercise or high temperatures, waiting until you are thirsty is not good. By the time thirst sets in, the body has already lost at least two percent of its fluid, and dehydration occurs.
   Most recommend you start hydrating the night before strenuous exercise.

2. Pay attention to your urine. It should be clear or slightly yellow.
   Too yellow means you are dehydrated.

Most experts recommend drinking at least a cup (four to 10 ounces) of water every 15 minutes of exercise.
Drink about 8 oz. every 20 minutes while you run.

Research shows that your heart rate increases eight beats per minute for every liter of sweat lost during exercise.

You can become dehydrated in as little as 30 minutes of exercise depending upon the environment and your intensity.

Hydration is more important when hiking at high altitude. The marked dryness of the high mountain air, combined with the increased breathing rate from less oxygen. This is a major cause of altitude sickness.

Immediately following exercise the muscles are able to replenish glycogen stores at a much higher level than a few hours later, Try a protein-carb mix to more rapidly replenish your glycogen stores after a long run. But don't forget your overall fluid replacement needs should be met with a sports drink or water. so a sports drink or a fluid source such as water with another source of carbohydrates for glycogen replenishment would be fine. Don't forget that if you are using Sport Gels or Bars for carbohydrates drink 4 oz to 8 oz of water with them to aid your digestion and assist in their absorption. There is some evidence that a protein-carbohydrate drink may assist in glycogen replacement better than carbohydrates alone.

The thinking that beer replenishes liquids as well as supplying carbs after exercise is controversial.
See note on the beer page.

In 2004, the Food and Nutrition Board of the nonprofit Institute for Medicine (part of the National Academy of Sciences) released a report on the recommended intake of water (and some minerals). While exact water requirements were not specified, general daily recommendations for women were set at 2.7L (91 fl. oz.) and 3.7L (125 fl. oz.) for men. This figure includes "total water"--water derived from all beverages and foods. About 20% of a human's daily water intake comes from foods. Exercise intensity and high temperatures clearly will increase a person's daily water needs.

Excessive water intake can actually be life-threatening. But by far the greater risk facing active outdoor athletes is under-hydration and dehydration.

Play it smart. Step up your awareness of body signals when active outdoors, particularly if the intensity, temperature or elevation is higher than normal for you. Drink fluids regularly and generously, even before your thirst-alert mechanism kicks in. Using a hydration system gives you an advantage. Because fluids are so easy to access, you'll drink more often and as a result perform better.