Hope you're thirsty. "Backpackers typically require about 4 liters per day," says Mary Drumm, program director for High Sierra Outward Bound. But that's just an average. Cold, heat, body size, and altitude all raise the count. "At high altitude, you can lose 4 to 5 liters of water per day just through respiration, and that's on top of the water you're excreting through sweat and urine," says Drumm.
Don't forget that dehydration means more than just thirst. "A loss of 2 to 3 percent of your water weight can reduce performance by 10 percent," says Neal Henderson of BCSM. That means that even small drops in hydration can hold you back.