In an extreme climate, especially an arid one, the average person can lose 2.5 to 3.5 liters of water per hour. In this type of climate, you should drink 14 to 30 liters of water per day. The average diet can usually keep up with these losses but in an extreme situation or illness, additional sources need to be provided. A mixture of 0.25 teaspoon of salt to 1 liter of water will provide a concentration that the body tissues can readily absorb. Of all the physical problems encountered in a survival situation, the loss of water is the most preventable.
Water is used and consumed as a part of the digestion process and can lead to dehydration.
The body performs more efficiently in extreme conditions when acclimatized. Conserve sweat not water. Limit sweat-producing activities but drink water. Until you find a suitable source, ration your water sensibly. You can estimate fluid loss by several means.