Competetive athletes (triathletes) say that a decline in performance will occur when you loose 2 percent of your body weight in water. For a 160 pound person (like myself) this is 3.2 pounds, or about 2 liters of water. As you will learn as you continue reading, you can loose this much in less than 2 hours of hard exertion.
I recently read an account of a couple hiking in 100 degree plus weather in the Grand Canyon with only 3 liters each per day. This was a 2 day hike, and by the second day the pair was in big trouble with judgement seriously impaired due to dehydration. The account of the tragedy suggested that 9 liters per day would have been more realistic. This is a heavy load of water (almost 36 pounds of water for two days), but in those conditions there is no alternative. If you are not willing to carry enough water to survive, the only alternative is not to go. An experienced friend who just returned from a Grand Canyon hike in temperatures around 100 degrees (not extreme summer Grand Canyon heat, but hot spring weather) says that he was carrying 10 liters each day, but only drinking about 6. The extra was insurance because they were unsure of being able to reach (or find) water sources each day.
Another consideration is staying hydrated. If you are carrying water, it is essential to drink it. This probably sounds silly, but apparently people need to be told this. People have died with water in their packs -- the best place for water is inside of you, not in your canteen - there is no benefit to holding water in reserve.
The need for continual water intake becomes a serious argument for hydration bladders. The human body will loose water by sweating at a rate from 0.8 to 1.4 liters per hour, depending on the person and conditions. The human body can absorb water at a rate from 0.8 to 1.2 liters per hour. The conclusion to draw from this is that you want to continually supply yourself with approximately 1 liter per hour of water. If you don't do so, you will become dehydrated, and "batch drinking" at rest stops will not allow you to catch up. Some sources indicate an absorption rate more like 0.5 liters per hour. You can also loose considerably more water, a person training for a marathon in southern California heat was found to be loosing 3.7 liters per hour. If you really want to get serious about this, you need to calibrate yourself by weighing yourself before and after training runs or hikes.
Note also that consuming 1 liter per hour over an 8 hour period yields 8 liters in a day, remarkably close to the 9 liter per day recommendation quoted above.
And don't neglect the need to replenish electrolytes. Some people like sports drinks, but an excellent method is to carry a proper electrolytle replacement "salt pill" like the Endurolyte product by Hammer Nutrition. Drinking water without replacing salts can lead to hyponatremia, which can be fatal in extreme situations. This is not rare or uncommon, it is estimated that 1/3 of the heat related incidents in the Grand Canyon involve hyponatremia. In the past it was not recognized as a specific issue and lumped together with heat and dehydration. Salt is not evil.
Tom's hiking pages / tom@mmto.org