Amount of Water we Need
It is important to measure or estimate the amount of water that are needed. A human must absorb at least 2 to 3 liters of water per day [Ref. needed] (more in times of drought, heat), it usually obtains drinking 1.5 liters daily, and eating the equivalent of 1.5 liter of water in its solid food.
In times of dehydration, urine is darker [ref. necessary], which is a simple criterion to estimate (not measure reliably and precisely) the dehydration of a possible subject. Water from a human body renews itself in 6 weeks. Quantity to drink 40 ml / kg body weight [Ref. necessary]. Half is paid by food and half liquid. So, 20ml per kg. We must add to this 1 liter per hour of sustained physical activity. Example: I am a tennis player and have a mass of 140 lbs. My weight in kg: 63.6. So I have to drink 1.2 liter per day minimum and if I play for 2 hours I have to drink 3.2 liters.