How much water do you need to drink on the trail?
On our trip to Death Valley, we took what we hoped would be enough water for the two of us for an overnight backpacking trip: Slightly more than a gallon per person for 24 hours. In terms of water bottles, it was 14 quarts, and that turned out to be about right. We drank three quarts during our hike to the campsite (this was, after all, Death Valley) and then used another three quarts for dinner. And then used another two quarts for breakfast the next day…and drank two more on the way out. So we drank ten quarts (five quarts per person) over the 24 hours of the hike. We were a little under-hydrated on the first day, as we were hiking in the afternoon sun. And we had some water left over (which is not a bad thing in the desert). If we were to do it again, we’d probably take about the same.
That's a photo of Death Valley at right...looking down towards Badwater and the lowest spot in the Western Hemisphere...
In the Sierra, of course, you can fill your water bottles along the way. We generally only take four quart bottles for the two of us on those hikes. We always camp near water, so we don’t need to worry about carrying the water for dinner or breakfast. And we start the day with four full bottles---enough to get us through lunch and into dinner. Some people prefer to carry less weight, and may only carry one bottle per person---or even hike from stream to lake and drink what’s available. But we don’t like to take the time to pump and filter while we are on the trail—we prefer to hike. So we carry a little extra weight, and stop less often to pull out our water filter.
Either way works, as long as you keep drinking enough water.