I read through some old posts here and think we need to set a 'best practices' description for those with less experience in dealing with such a situation and what to do.
Not to be harsh but a number responses to this issue were providing advise which is just not the right thing to do.
An 'example' Problem (there are an infinite number of different situations):
Hiker is starting to suffer dehydration symptoms (moderate to severe).
Hiker is hiking in the desert sun in 'hot' temps - this would be defined at 85-105 F in the shade, but there is no shade so 100-120 F in the sun.
Hiker has 5 miles or more of rugged terrain to the next CERTAIN water.
Time of day is earlier than 6pm - the earlier the more critical the situation
Hiker has no more than 1 liter of water left
Caveats:
What to do is 'somewhat' dependent on experience, fitness, age, etc. If you have never been in this situation take the most conservative approach. It might save your life.
Solution:
LAY UP - do not, I repeat, do not continue to hike. That is how you die. This is ESPECIALLY true if you are suffering severe symptoms like nausea, throwing up, bad cramping. If you have stopped sweating all together death is sitting on your shoulder already.
Find shade, any kind of shade, make shade - you have gear you know.
Keep as cool as possible
Do not move around
If you need to pee - SAVE IT as you might need to drink it. Yes I am dead serious.
That 1 liter you have will last a long time if you do the above. It will disappear in a heartbeat if you keep walking.
Wait till o' dark thirty before you go for the water source. Move SLOWLY. Do not get out of breath. Try to move slowly enough that you do not sweat.
Cache your gear!!! Extra weight in this situation is trying to kill you. And come back for it after you get to the water and rehydrate. If you die then you won't care what happened to your gear anyway. If you live you will be happy. win win
If you are on a trail like the PCT then DO NOT leave the trail to take a shortcut or anything like that. There are lots of other hikers out there who do have water and they are heading in your direction. Desert trails almost always route to the next nearest water source so leaving a trail, unless you can see the water, normally adds time and effort which is not good. Bushwacking in the desert can easily consume 2-3 times the effort required by a trail so this is often the worst thing you can do. The vast majority of hikers out of water are saved by other hikers coming down the trail.
Do not panic. Panic kills. Leaving the trail is a form of panic.
Discussion
The hiker in the above situation cannot hike at any speed at all due to his/her physical symptoms for one. Another issue is that attempting to hike in this kind of heat on rugged terrain will result in heavy sweating at a rate of at least 1 liter per hour. Since the hiker is already declining physically and is at least 3 hours from the water due to physical infirmity and under a prime requirement not to lose any more water going for the water source is not an option. A high percentage of people who make the decision to go on in this situation die. In AZ each year we have an average of about 15 hikers die this way. Most of them die within 2 miles of water and quite a number right inside Phoenix itself on the hiking trails. If you are already suffering moderate to severe dehydration one finds that the range of human capabilities comes into play. Those with exceptional physiques can often grit their way through but those who are average or below tend to die. So there is no reason to use this opportunity to find out which kind of body you have is there? Just take the safest options and, if you make it out alive, use the experience positively. The above example situation is just a snapshot of what is possible and there are endless other possibilities. What one does in a real situation requires serious thought and a mature decision tree. There are lots of answers which can lead to a good result as well as a host of responses which can lead to disaster. Make your decisions when your head is still clear.
I am just one guy here but I do have a LOT of experience in desert hiking in extreme conditons - 10,000 + miles and I have been out of water in them a couple of times. Plus living where I do I see many times a year people dying due to not having planned well and not knowing what to do to extend their chances.
Being out of or almost out of water in the desert in hot conditions is not like anything one can experience along the AT or in the East. One can even do a hike of the PCT and never see real bad conditions due to timing - being up high when it is hot and low when it is cool instead of vice versa.
I have saved hikers in the desert who were out of water due to bad decision making on how much to carry or due to being lost. l I have been off trail and running low on water and had to think hard to stay safe. Be smart and be cautious.
Here are a few suggestions:
Throw away the (family blog)ing water bladders. Those pieces of gear are for day hikers, trail runners and mountain bikers. No long distance hiker should be carrying such an item. Why? Weight - they are heavy. They promote overly quick water consumption as the sipping thing they promote is not the way to drink nor can you monitor how full they are. Plus you cannot keep them clean of course.
Water bottles. By far the best water bottles to have are one's like the Smartwater bottles and their competitors. The are very light - much better than anything sold for backpacking use, they are rugged, they can be replaced at almost every resupply point, redundancy to failure by having a bunch of them with you. My standard load out for the AZT for instance is 6 Smartwater bottles and 2 Sawyer bladders for a total of 8.5 liters - and I would not hesitate to add to that if the situation called for it.
Water bottles allow one to monitor their water consumption accurately. Ration your water consumption at all times. IF consumption and progress are indicating you might run short then take proactive action to improve the situation. Don't be the typical optimistic fool - remember optimists die young because they think things just work out while the pessimist knows that Murphy was a prophet and the world is just waiting to turn to crap and is thus prepared for it.
Do NOT sip water in the conditions described in this post. Take a mouthful (or two) hold it for a bit and swallow. Sipping all the time results in a lot more water loss through the mouth. Walking with your mouth open consumes lots of water as well so keep your mouth shut and slow down so you don't breath heavy. Time your water consumption based upon how long it needs to last.
Your hat should be light colored and mesh to allow better cooling. An umbrella is a huge benefit as it cuts the effective temperature a lot.
How much water to take in really harsh conditions is a different answer than one gets under other circumstances. The decision is based upon multiple factors and not just how many miles it is to the next source. Serious heat and walking in direct sun require much larger amounts of water. On the AT in average conditions you might get by on 1 liter per 8 miles and some don't even carry water between water sources. In extreme desert hiking conditions you can consume 1 1/2 liters per hour. In the most extreme conditions possible you simply cannot hike at all. In the desert water holes are most commonly much further apart than they are in other kinds of terrain and this makes the situation worse. Ruggedness of the terrain is also a factor. The reliability of the water source is critical information also. Many depend on trail angel water caches but this is also very risky - I have seen empty caches on the PCT and lots of empty caches on the AZT for instance. People who are short will take the water you cached for your self frequently also.
Calculate your water load on the following: miles, terrain, expected heat, reliability of supply, distance to the secondary and tertiary water sources and what reserve is needed to be safe. NEVER plan on arriving at a water supply out of water - only fools do this. Assume the 1's and 2's on the water report are dry. My rule is to have at least 1 liter left in case the water supply is dry (I one time had 3 water sources in a row be dry - yes I ran out). This also gives you some cushion on time which you might need if you twist your ankle and are dramatically slowed or some similar issue.
How much your pack weighs is not a reason to not carry the water you need. I remember a picture of Wired leaving a water source on the Hayduke with 10 liters (4 of which were in a 1 gallon jug she carried in her hand because her pack was full). So if she is tough enough to do it at about half the size of most of us you can be tough enough also.
I am sure I have left some stuff out so those who have been there as well please chime in and fill in the blanks.