The body cannot store water, so it is vital to constantly supply it with water to ensure proper function of body systems.
Contrary to popular belief, simply drinking water when you are thirsty is not always good advice when it comes to hydration. By the time the a person feels thirsty, it is usually a sign that thy body is already under hydrated.
How much water should the athlete drink? That depends on many factors including temperature, humidity, length of practice, and the unique needs of the individual. The American College of Sports Medicine (ACSM) recommends that athletes drink 16-20 ounces of water within a two hour window before practice begins and 4-8 ounces of water every 20 minutes during practice.
Although the ACSM does recognize that athletes that practice in high humidity for more than 90 minute sessions do regain lost nutrients by consuming sports drinks, they emphasize the fact that pure water is essential for an athletes to be at their best.