Water

A human can survive a maximum of three days without the intake of water, assuming you are at sea level, at room temperature, and a relative humidity.

In colder temperatures and/or with rain or snow the length or likelihood of survival would be greatly reduced. In addition to the aforementioned priorities, length of survival also depends on amount of physical exertion. A typical person will lose 2-3 liters of water per day in ordinary conditions, but more in hot, dry, or cold weather.

A lack of water causes dehydration, resulting 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. Your body requires 4L to 6L of water or other liquids each day in the wilderness to avoid dehydration and to keep your body functioning properly.

Dark yellow or brown urine indicates dehydration. Because of these risks, a safe supply of drinking water must be located as soon as a shelter is built (or even before, depending on conditions).

Water can be gathered in numerous ways: scooped out of a creek or pond; rainwater can be caught in makeshift containers; collect dew from vegetation in clothing articles. Many tree roots and vines contain water. The cactus is also a source of water, confirming the myth- pulp can be removed from the broken stem.

In a survival situation, any water supply may be contaminated with pollutants or pathogens (see Potability of backcountry water). Although little can be done to remove molecular contaminants, particles and microorganisms can be removed and/or killed (see Portable water purification). In a beach situation, digging in the sand below sea level, the sand well will fill with drinkable water; it may taste salty or brackish, but the sand acts as a filter reducing the salt content the further you dig inland. Stagnant water can be made drinkable by filtration through a sieve of charcoal.

Animal blood is not suitable for rehydration as it may be diseased. In addition, because of the nutrients it contains, it requires energy to digest. Mammals all have blood-borne pathogens so the animal must also be cooked. Urine contains salt and other toxins, which also makes it unsuitable to drink, although it can be refined in a solar still.

Many birds, mammals, and some insects, such as bees, ants, and mason flies, are reliable indications of water, either through a stream or a soaked patch of earth.

The U.S. Army survival manual recommends that you use your water whenever thirsty to avoid "voluntary" dehydration. Other groups recommend rationing water through "water discipline".

While finding water is most important, preventing water loss is also an issue. Resting, avoiding smoking, and breathing through the nose are recommended.