Humans can survive 3 days on average without water—less when it is very hot or one is very active (as opposed to weeks without food). A single day without water significantly reduces bodily and mental performance.
Water is an essential item on any camping or hiking trip. Some campgrounds have tap water, drawn from wells and purified. This water is of reliable quality under normal circumstances and should be used whenever practical. When tap water is not available, it is usually best to bring all necessary water from home. However, it is impossible to bring more than a couple of days' worth of water on a backpacking trip, and survival situations may create an unforeseen demand for water. If this demand is not met, dehydration will result, leading to heat exhaustion, heatstroke and death within days.
Please avoid the thought of rationing water. The best storage container in a survival situation is your own body. Even experienced soldiers have been found dead of dehydration with a full canteen.
Finding Water 
- You should have a map that shows water sources in the area where you plan to operate.
- Water is most likely found where it is carried by natural terrain features such as ravines, gullies and washes.
- Game trails leading down-slope may lead to water, but it may be miles away.
- Water is more likely found in green areas with a good amount of vegetation.
- Collect falling rainwater with a tarp, tent, or even clothing. It is safe. Funnel or wring it into containers (bottles, canteen, pots, you).
- Quickly running water is more likely to be safe than large slow moving rivers or water in ponds and lakes.
- Dew can be a water source when it condenses on any surface. Sop it up with a sponge or cloth.
- You can get water by distillation from common leaves, grasses, and other green plants. Place an armload in a plastic bag/container, then leave it out in an area where it will get a lot of warmth and sunlight. Make sure that you don't use any poisonous plants, and that the container is sealed, so no water vapor escapes. Several such containers will be needed for a single person.
- DO NOT drink seawater unless it is distilled! It takes twice as much water for your body to process seawater as you get from it.
- NEVER drink urine, even if it is filtered.
- Avoid eating snow in cold weather; doing so can significantly lower body temperature (as can becoming dehydrated) and even lead to shock.
- Try to avoid stagnant water. It is more difficult to make safe.
- Try to avoid brackish water. Purification or disinfection will not remove salt from brackish water.
- Of the methods discussed above, only distillation and charcoal reduce risks from chemicals dissolved in water. Try to be aware of the risk of chemically-contaminated water in your area of operation.
Making Water Safer 
- You should always presume that water found in the outdoors contains disease-causing organisms. They are present far from any signs of human habitation. These organisms may have no effect on you if you drink the water without treatment OR they make cause debilitating illness or even death (See, for example, giardia, cryptosporidium, cholera, typhoid.). Therefore, you should do what you can to make the water safer.
- Boiling is the most reliable method to kill all organisms in water. Bring the water to a rolling boil. Boil several minutes if at high altitude since water boils at a lower temperature at altitude. Even bringing water to temperature uncomfortably hot to the touch kills most organisms, but boil if you can.
- Commercial filters in working condition and used correctly remove all organisms except viruses.
- Chlorine dioxide (NOT "bleach" which is chlorine hydroxide) used according to directions will kill all organisms in water. Wait times may be four hours.
- Water in a closed, clear container (Think 2 ltr. softdrink bottle.) if exposed to a day of strong sunlight is usually safe (See SODIS).
- Water obtained by distillation (See above.) is usually safe.
- A well dug ten feet/3 meters or more from a body of water and dug to a depth below the water level in that body of water, will gradually fill with water that is safer than the source body of water.
- A filter of layers of grass, sand, and charcoal (from a fire site) inside a cone or cylinder of bark, plastic, aluminum foil or the like, will make water safer.
- A filter of 6-8 layers of tightly-woven cloth will make water safer.
- Allowing particles in water to settle to the bottom of a container and then treating the clearer water towards the top improves the resutls of treatment. Such "settling" of water, alone, makes water safer.
- Any method that makes water safer, such as use of "bleach" (Chlorine hydroxide) or iodine, is better than not treating the water at all. Putting it another way, killing or otherwise eliminating some of the wee nasties is better than leaving them all for your body to fight.
- Bring "enough" safe water with you where ever you go OR have a method to render safe water from known sources. Be prepared.
- If you have no water, avoid eating fats and proteins because your body uses water to process them. Juicy vegetable foods may sources of water.
- Always try to keep the sun off of you, regardless of how much clothing or sunblock you are wearing. It heats your body up over time, so you lose a LOT more water from your body from the extra sweating.
- When melting snow in the winter, first melt a small amount in the bottom of a pot and then add more snow slowly. If you fill a pot with snow and put it over a fire, the snow at the bottom may sublimate directly into a gas, leaving the bottom of the pot dry and vulnerable to melting.
- Pouring boiled water back and forth between pots after boiling will remove the flat taste. If you are very thirsty, you will not notice a flat taste.
- If in the desert, do not remove your clothing as your sweat will evaporate more quickly and you will dehydrate faster. Ideally you want light colored, lightweight, loose clothing that covers as much skin as possible.
- Do your work in the early mornings and at dusk to avoid the hottest part of the day. Get extra sleep during that time.
- Your stomach is the best water container. People have been found dead of dehydration with water still in their canteens. Don't drink more than you need to, but don't be stingy with it either.
- Always try to protect yourself as much as possible from the wind, it can dehydrate you in mere hours of exposure in areas with little cover, particularly mountains and plains.
If a natural water source is available 
It is not difficult to obtain water from a natural body of fresh water such as a river or lake, but this water should not be used untreated. Natural water often contains organisms that cause infectious disease, most notably Giardia lamblia. There are four ways to remove this threat and make natural water potable.
- Water may be boiled over a campfire or portable stove. At high altitudes, water boils at a lower temperature, so the boil must be maintained for several minutes to kill the microorganisms.
- Water may be filtered with a portable water purification device. Water purifiers differ widely, so if you own one, familiarize yourself with the instruction manual. If the water is visibly dirty, pour it through a clean cloth to remove large particles and avoid prematurely clogging the purifier.
- Certain chemicals, such as dilute chlorine solution, are commercially sold as antimicrobial additives. Some leave an unpleasant flavor that may be masked with powdered drink mix.
- Water may be added to an evaporation still or solar still to purify the water through the natural evaporative process. Questionable water is added then when it evaporates it rises and condenses on a cooler surface. It then drips into your collection container where it can be collected using tubing to maintain the efficiency of your still. The warmer the still becomes the faster you will be able to collect pure water. Only a few chemicals will evaporate with the water one of which is benzene. This is usually only a concern when trying to purify flood waters.
This method can also be used to purify salt water. As long as the salt water is not allowed to get on your collection surface or in your collection container the water will be potable.
If a natural water source is not available 
The evaporation still 
Water may also be obtained from the soil or from plant matter.
Probably the best way to get water from the ground is the evaporation still, shown in cross-section in the diagram at right. To build an evaporation still, you need only four items: a shovel, a sturdy sheet of transparent plastic, a cup, and a piece of flexible plastic tubing long enough to reach from the middle of the plastic sheet to the side with some excess length.
Begin by digging a hole with sloping sides in the shape of the sheet of plastic, but slightly smaller. Avoid digging in hot weather, as you will lose considerable amounts of water through sweat. Sink the cup in the middle of the hole so that the rim is almost flush with the sides of the hole. Place one end of the tube in the cup, run the other end to the outside of the hole, and place the sheet of plastic over the whole assembly. Weigh down the sides of the plastic sheet, or anchor them with stakes, and place a small weight directly over the cup.
The evaporation still will produce water continuously. The plastic sheet will create a greenhouse effect in the still, accelerating the natural evaporation of water from the soil. When the water vapor hits the plastic sheet, it will condense and drip down into the cup. The tubing may be used to drink from the cup without disturbing the still. For added effectiveness, use a second cup to pour any available water-based fluids, such as urine or water collected from roofing or tent surfaces into the pit. A second tube would also work for this same purpose.
The vegetation still 
An easier method uses just a plastic bag. Gather enough succulent vegetation (big leaves, cacti stripped of their thorns, etc.) to mostly fill the bag. Mash it to break through the leaves' outer water-resistant cuticle. As in the evaporation still, a greenhouse effect will cause water to evaporate from the leaves. It will then condense on the plastic and run down into the bottom of the bag.
The water in the bag will pick up chemicals from the leaves. These will give it a strong leafy flavor, and may include toxins, so make sure not to gather any poisonous plants.
Vegetation may also be added to an evaporative still to avoid the unpleasant taste but the used vegetation will have to be cleared out after the water has been collected from it. Preferably at night when the still is less effective.