In a situation where you don't know when you will find water again, is it better to ration the water you have over time (and ever increasing dehydration) or to drink it to quench thirst as soon as needed?
It seems like there would be a tradeoff between longevity and performance in choosing one or the other course of action.
The literature is mostly pro-drinking and anti-rationing. Hopefully these sources will provide some details for you.
The U.S. Army survival manual does not recommend that you drink water only when thirsty, as this leads to under hydrating. Instead, water should be drunk at regular intervals.
Even in cold weather:
Don't take chances with hydration. Do not ration your water; it is better inside you than in your canteen or thermos. Push yourself to drink as much water as you can while on a wilderness trip or while stranded in the wilderness in a survival situation. http://boreal.net/articles/survival-training/water-balance.php
The usual advice to someone in an emergency situation in the wilderness is to stay put, so that it's easier for rescuers to find you. In this situation, performance isn't an issue.
There is a folk belief that "thirst is too late," and that people are commonly dehydrated without knowing it. If you believed that, then you might drink some of your water even if you didn't feel thirsty. The belief is false, however.
In reality, thirst is an extremely powerful physical sensation, and when you're actually dehydrated (not just thirsty), you will feel so thirsty that you will simply drink because the urge is so strong.
There is a referenced article on the subject at lightandmatter.com/article/hiking_water.html.
I have a very simple reasoning about it. It's only based on personal experience, so don't regard it as absolute truth.
Your body needs a certain amount of water to be comfortable, say, N. Comfort here includes urination (removal of poisons from body) and sweating (removal of heat). If you drink N or more, the body will function well.
If you drink less than N, the body will adjust to the deficit of water; it will decrease urination (so unwanted stuff will accumulate in the body), and then reduce sweating (body temperature will rise, leading to bad mood and less-than-optimal physical power).
You can often tolerate these effects (e.g. in emergency; also in general, depending on your goal and attitude), so you should drink less than your "normal" need.
In additon, imagine that you decided to conserve your water and drink 80% of N during the day, and you accept the ill effects of doing so. If you temporarily exceed that plan, drinking e.g. 30% of N in a quarter of a day, the body will "use" the excess water for natural but undesirable functions (urination/sweating). So you should be careful about how much you drink, and not succumb to thirst easily.
The tradeoff between longevity and performance is not easy to resolve; I feel that if you are carrying your water on your back, the tradeoff is always to drink less than needed for optimal performance. Some people will surely disagree though.
When I took a wildlands firefighting course in Arizona, the instructor said that if you find yourself lost in the desert, you should not ration your water. People have died of thirst with water in their canteens.
For what it's worth, I will refer to a single event: the outdoorsman Aron Lee Ralston survived 127 hours rationing his water.
Aron Lee Ralston had access to only 350 ml (12 imp fl oz) of water for a period of 127 hours. He was 28 years old and he was trapped in a slot canyon in eastern Wayne County, Utah, USA, between April 26, 2003 and May 1, 2003.
According to his book, Between a Rock and a Hard Place, he rationed his water.
