A human being can survive an average of three days without the intake of water, assuming sea-level altitude, room temperature and favorable relative humidity. In colder or warmer temperatures, the need for water is greater. Need for water also increases with exercise.
A typical person will lose 2-3 litres of water per day under ordinary conditions, and more in hot, dry, or cold weather. Four to six litres of water or other liquids are generally required each day in the wilderness to avoid dehydration and to keep the body functioning properly. The U.S. Army survival manual recommends that you drink water whenever thirsty. Other groups recommend rationing water through "water discipline".
A lack of water causes dehydration, which may result in lethargy, headaches, dizziness, confusion, and eventually death. Even mild dehydration reduces endurance and impairs concentration, which is dangerous in a survival situation where clear thinking is essential. Dark yellow or brown urine is a diagnostic indicator of dehydration. To avoid dehydration, a high priority is typically assigned to locating a supply of drinking water and making provision to render that water as safe as possible.
Many sources in survival literature, as well as forums and online references, list the ways in which water may be gathered and rendered safer for consumption in a survival situation, such as boiling, filtering, chemicals, solar radiation + heat/SODIS, and distillation. Such sources also often list the dangers, such as pollutants, microorganisms, or pathogens which affect the safety of backcountry water.
Recent thinking is that boiling or commercial filters are significantly safer than use of chemicals, with the exception of chlorine dioxide.
The issues presented by the need for water dictate that unnecessary water loss by perspiration be avoided in survival situations.