I live in the Arizona desert, and during the summer, despite my best intentions, I always seem to end up doing some hiking in the summer heat.
What is a good formula to figure out how much water I need to bring, so that I'm not running out before I'm done, but don't end up carrying 2 liters all the way through and back to my car?
I led extended backpacking trips in Grand Canyon country for several years, and we required everyone carry at MINIMUM a gallon (almost 4 litre) per person per day -- which adds up. (Though rarely did we plan trips that did not have dependable water sources within a day's walk, meaning we would start and end the day with bellies full of water.)
Different people consume (and lose) at different rates, so the above would usually balance out in a group.
Addendum: As some have pointed out, there are varying recommendations, and even Grand Canyon National Park seems to have some difference of opinion: exhibit A, and exhibit B
Considerations such as difficulty of the terrain, whether there will be a water-source where you are going, whether you start the hike well-hydrated, what time of day you are hiking, etc all play into it. As does the consideration of more water = more weight = more energy expended = more water needed. For some people, adding 2 gallons (7.6 litre) to their pack would make it difficult for them to go anywhere...
In short, if you think you need more, take more. Cache water (if possible). Be smart. Know your limits.
Hiking in the desert? As much as you can carry! I tend to prefer packing more water than less, and especially in a desert area where if something goes wrong, your access to resupply is very limited.
You can lose up to two quarts (almost 2 litre) of water per hour hiking in the desert in middle of the day, but you can only absorb around one quart (almost one litre) per hour, so that's the minimum you should shoot for if you're planning a hike.
I visit AZ every summer and include some hikes in the desert when I'm there.
First, don't just take water. That can lead to electrolyte loss, as actually happened to me the first year I did this. Now I dissolve some gatorade powder in the water, and I haven't had that problem since. I usually mix it 1/2 to 2/3 strength relative to what the directions say. The first thing I do when I get to AZ is to buy one of those large tubs of gatorade powder. I go thru most of it in the 10 days or so I'm out there.
I don't like carrying more than 1 gallon of water, so I plan my hikes accordingly. Depending on where exactly and the weather that day, that can be good enough for a all-day hike of 8 hours or more, or only 4 hours in some cases. In cases where I know the conditions are severe, I either plan to be out no more than 4 hours or I'm going out and back along the same route and turn around when the container gets half full.
A good example of the severe case is when I went hiking in the southern White Tank mountains west of Phoenix. (I recommend avoiding the White Tanks, too many yahoos with guns, dirtbikes, and trash everywhere, but I didn't know that at the time.) It was a hot day, even for that location in August. I drank all I could stand before leaving, and took a gallon of diluted gatorade. When that got down to 1/2 gallon, I turned around and went back to the car. I only got a few miles in, and I think the whole hike lasted only about 3 hours. I had a little liquid left when I got back to the car, which I then drank and felt about right.
Other times at somewhat higher elevations and a bit cooler temperatures, I've gone the whole day on one gallon. One example was a whole-day hike around Juniper Mesa in the Prescott NF.
So in the end it matters a lot where you're going and what the conditions are in that location on that day. For the real serious desert in the heat of summer, I'd say a gallon per 3-4 hours, then derate according to weather from there.
I carry 6L in the Mojave dunes but find that I only drink 3 to 4L. I may carry as much as 8L. Better safe than sorry.
Water is very heavy, but you can always ditch gear. I pack my backpack like a lightweight hiker, so I can carry all that water without my backpack getting too heavy to be any fun. My backpack weights around 40-45 pounds (18-20 kg) with water, but it's sub 16 (7 kg) without it.
I recently hiked in the desert in April and drank 8 oz (~¼ litre) per half hour, which was a pint (~570 mL) an hour. That was not enough. I felt like a dried out sponge for a couple days after, and this was only a six hour day hike. I will plan a quart an hour in the future if at all possible, so that means a gallon (3.8 litre) for every four hours out.
It depends a bit but I would recommend twice as much water as you would need in a cooler time of the year for a similar hike. Perhaps 3 times as much if it is particularly hot.
As best as possible, don't travel in the heat of the day.
Apart from what has been said, your need of water also depends on what you eat. If you eat salty or sweet food, or meat, you will need more water than when eating cereals for example.
I live in Arizona and worked at Philmont scout Ranch in new mexico. I also am an eagle scout. When hiking it is recommended that you carry a litre of water for every two hours of hiking you plan on doing. If there is a certain water source where you are hiking I recommend taking a water filtration or purification system to be certain you can refill on clean potable water. Never sacrifice water for pack weight, that is foolish and deadly thinking, especially if you are hiking somewhere dry or hot. If you have trouble conserving water or rationing, chew gum or have something like sunflower seeds, these keep your mouth busy and moist and will keep you from constantly using water to moisturize your mouth. Just remember to still drink enough water to stay healthy, and happy hiking!
It depends on how well hydrated you are to start with, whether or not there is a water source at the end of the day, how steep the terrain is, how much food you are eating, how you breathe (mouth or nose), how good of shape you're in, how much weight you're carrying, how hot it is, and what type of pack you are using. If you are totally hydrated before heading out, eat small meals, are in excellent physical shape, keep your pack weight under 35 pounds, use a pack with a well-ventilated back (like an external frame) so you don't overheat, breath through your nose, and lay low during the afternoon... you could easily get by on 2 liters a day, I do it all the time. On the other hand, someone in poor fitness with a 50 pound pack who is chronically mildly dehydrated, using a back-hugging pack that insulates and overheats them, breathes through their mouth, overeats big salty meals, and hikes in the afternoon heat through steep terrain... could easily drink 3 gallons or more in a day, if they have that much available.
