Morimorimori

Hope this is the right sub-forum for such question.

They say a proper hydration is especially important for cyclists - as drop of liquids in their body will substantially increase probability of (permanent) knee injuries. But how much water is actually enough?

First of all, what I'm interested in is a solid, educated, well-grounded opinion. Ideally, approved by majority of athletes and sports doctors. As it seems like there are a lot of controversies around this subject. Some say it's enough to have 1 liter of water/isotonic per 30-50km, other sources claim you must drink 0.5-1 liter per 10km, or 1-1.5 liters per hour (in hot weather).

Those numbers vary greatly, and many of those who provide them only have their personal experience to back it up. So far, the most scientific approach to this I've seen is related to amount of water a human loses when exercising. Different sources claim it ranges from 0.5 liter to 1 liters (depends on the temperature) per hour.

Could somebody either confirm those numbers, or provide an alternative viewpoint? Let's structure it like this:

1. Recreational ride in a warm day (50 kms at 15 km/h, with a few breaks to have some rest, at temperature 18-25C)
2. High-performance ride in a warm day (50 kms at 20-30 km/h, no breaks, at temperature 18-25C)
3. Recreational ride in a very hot day (50 kms at 15 km/h, with a few breaks to have some rest, at temperature 35+C)
4. High-performance ride in a very hot day (50 kms at 20-30 km/h, no breaks, at temperature 35+C)

How much water per hour should one consume in each of those 4 cases?

Also, how much isotonic is more efficient than plain water? Is there some approximate coefficient, like "you will need 0.75 liters of isotonic instead of 1 liter of water"?