PDA

View Full Version : water requirements



Turk
2006-07-24, 00:52
I have a question about long sections of trail without a water source.
I am trying to plan my route to never be more than 2 days, 1 night hiking
from available water.

Does anyone have a good formula, or at least a generous rule of thumb
for how much water you need to carry for 2 days active hiking, cooking
and cleaning/hygiene ?

Take-a-knee
2006-07-24, 01:31
I don't think you can get by on less than 4 quarts/day. More of course if you are sweating. The British SAS found out that a man couldn't carry enough water to operate in the desert during WWII. If you go more than three days without a water resupply, you'll be in trouble.

Mutinousdoug
2006-07-24, 12:09
I think Take-a-knee is pretty close. When I was in the army I carried 3 1/2 gal for a dry season 3 day re-supply. Everyone was pretty much out of water by morning of the third day if there was no flowing water around and if we didn't see re-supply by noon or so, we were all hurting. Actually, except for re-supply day when we had beer and ice and sodas, we were always dehydrated. This was 100-110f tropical environment and we were humping 80+ pound loads, so YMMV.

Seeker
2006-07-24, 14:47
while this isn't a proper answer, it is an attempt to steer you toward one...

ray jardine (no, i'm not a huge fan, but sometimes he has good info) has a portion in his 'beyond backpacking' book that deals with how to get over long waterless stretches. i don't remember the exact quantities he carried for what length, but they were in the 'water' chapter, i think. you may own it, or your local library may have it.

jimtanker
2006-07-24, 15:49
Depending on climate of course, I would say that 1 gal per day would suffice. This depends on your fitness level, how you cook, how hydrated you normally keep your body and other things. I would, however, give up on the idea of "personal hygene" if you are having to pack alot of water.

I was drinking 3-5 gal per day when I was stationed at Fort Irwin in the Mojave desert. 125F adverage temperature and there I was working on a big steet tank. I think it was something like -10% humidity too.

Take-a-knee
2006-07-24, 15:50
I did a mission in the desert many years ago. We got off the chopper with an ALICE pack and web gear with 2 1qt canteens, each man carried a five gallon can on the top of the rucksack. We only had to move a few miles so it was no big deal and that was the longest movement we made for five days. It was 100+ but we were just looking through binos under a poncho. Each man's 5gal can was empty at the end of the mission. No bathing or washing, just drink and eat MRE's.

dropkick
2006-07-25, 01:32
This isn't going to help, but I thought I'd clutter your thread with a barely relevant story.

When I was working in Atlanta for a fiberoptic installation company, every once in a while I'd fill in on a truck that was short a man.
In the middle of summer our 3 man crew would start out with a 15 gal. cooler full of ice water. At noon we would each get a Gatorade type drink, plus what we drank with lunch. Shortly after lunch our cooler would normally run dry and we would go to a house and ask if we could fill it with their hose. We would drink the cooler dry before the day was through. occasionally we refilled it a 3rd time. This made over 10 gallons of water drank per man per day .


--When thinking about it later, the thing that amazed me most was that we never had to urinate, we lost it all in sweat.

Hiking I carry at least a gal. a day.

JAK
2006-07-25, 03:15
Good question.

It would depend on whether you were out of the sun, temperature, humidity, exertion level. If you get sick, like any sort of fever, or diarhea, you may need a lot more. Very risky because there are so many variables. I would eat less food during those sections. Sticking to stuff like gatoraid and water. If I could resupply at the beginning of such a section I would lots of oranges also. At night I would do that trick with a plastic sheet and a pan and rock to try and get water from condensation from the ground.

I think the answer is all you can carry. On the Fundy Footpath, which has plenty of water, I drink about a litre per hour. The first time I did it I was overweight and overpacked and had a bit of a fever from paddling a couple of days before in a cold rain. I went through a litre every km and only covered about 12 km in 12 hours of hiking. In early May on year in rain with some snow still in the woods I did the 60km including the extra trails on each end in about 24 hours of hiking and I think I consumed about 24 litres of water over 72 hours, noon to noon.

I would guess over 2 days, 1 night, and 16 hours of rough hiking, I would want about 8 litres per day. Might get by on half that. Might get into trouble and need twice that. Depends on what your emergency strategy is. Can you stop and rest and make water or is it total desert? 16 litres would be 35 pounds of water, which would be very doable if I was 35 pounds lighter. You are young and in good shape so it should be very doable for you, but you need a plan B for sure.

It would be fun to carry that much and then try to conserve water to see how little you can get by on, still drinking when thirsty, but wearing the right clothes and walking at an effcient pace and so forth. Then perhaps you might carry 4 gallons but get by on only 2 gallons. That would be a good learning experience, but you might want a buddy to make sure you don't get stupid.

dropkick
2006-07-26, 01:52
Turk,
Just remembered one of the things I was taught long ago during survival training (Army) and thought I should pass it along in case you go light on the water:
You can only recycle your own urine twice, after that it becomes poisonous.



--I can see people going EWWW! in my mind... but an interesting fact none the less...
....What I wonder is how they found this out.....

Seeker
2006-07-26, 13:29
prolly the same way they did the navy dive tables... dunk a guy at a certain depth for so long, haul him up, and see if he gets the bends... do it again until he does. change divers. rinse, repeat.



'here soldier. drink this. yeah, it tastes like piss, but don't worry... it's a new electrolyte drink.'

JAK
2006-07-26, 14:30
How do you know when it's been around more than once?
Maybe bar codes.

"Only drink beer when it's fresh." - that's a bar code. :beerglass
It's OK to share your beer, just don't share your beer. :beer:

Turk
2006-07-26, 18:14
thankyou all for the comments. I will definately consider my water requirements
more seriously in the highland sections of the VT. Even if it means 20lbs extra
in water. I am young and strong.. and it really wasn't that long ago that I
was hauling close to a 40lb pack anyway.

This weekend I think I will try JAK's suggestion of 8 litres per day under very
controlled circimstances and see how I feel.

SowthEfrikan
2006-07-26, 19:50
The idea of a water still is absolutely inspired.

Also remember to camel up at the water source.

deadeye
2006-07-26, 20:15
[QUOTE=dropkick]Turk,
Just remembered one of the things I was taught long ago during survival training (Army) and thought I should pass it along in case you go light on the water:
You can only recycle your own urine twice, after that it becomes poisonous.
QUOTE]

I wonder if the Army has changed their teaching - the word now is never drink it. Never.

Take-a-knee
2006-07-26, 22:07
You should never drink urine, just like you should never drink sea water. You can pour it in a solar still and reclaim the pure water from it, however. In SERE at Bragg they teach that urine can be used as a wound irrigant though. It is sterile, or nearly so, much cleaner than most water in the wild. The salt and nitrates in it are a problem for consumption.

ripvanarkie
2006-07-27, 01:21
You can pour it in a solar still and reclaim the pure water from it, however."

I'd hate to lug my still around, it is almost 100lbs of copper and SS, would be nice to have along for a zero day!

Later,
Rip :vollkomme

JAK
2006-07-27, 09:50
thankyou all for the comments. I will definately consider my water requirements
more seriously in the highland sections of the VT. Even if it means 20lbs extra
in water. I am young and strong.. and it really wasn't that long ago that I
was hauling close to a 40lb pack anyway.

This weekend I think I will try JAK's suggestion of 8 litres per day under very
controlled circimstances and see how I feel.Go for it. You look like you have lot's of voyageur blood in you. An extra 20 pounds of water should be a joke. I'm carrying an extra 40-60 pounds on me right now even as we speak. I believe the Top Rock uses the term "All Cameled Up". :biggrin:

Raises an interesting question. If you are lean, like you should be, do you dehydrate quicker? I would think mile for mile it would be better to be lean and carry the extra water externally, but can the body store some water in the fat tissue?

Iceman
2006-07-27, 11:30
I know what you are thinking...

Lt.Schueller
2006-07-27, 13:48
very very funny

Amigi
2006-07-27, 21:40
You should never drink urine, just like you should never drink sea water. You can pour it in a solar still and reclaim the pure water from it, however. In SERE at Bragg they teach that urine can be used as a wound irrigant though. It is sterile, or nearly so, much cleaner than most water in the wild. The salt and nitrates in it are a problem for consumption.
Exactly. The Army does not now teach you drink urine ever.

The ideas about how much water have already been exhausted here, but really the amount needed depends on ambient temp and exertion.

In 30F weather, I've gotten by with half a gallon of water ( drinking water only ). In Colombia in summer in a rain forest ( thank God ) I drank 2-3 gallons per day.

Always camel up when you can, and remember to try to hike with your mouth closed and dont spit. You would not believe the amount of hydration you lose hiking or marching with your mouth open.

JAK
2006-07-27, 22:05
That's very interesting about breathing through the nose. That would also be a good way to pace yourself so that you are maintaining a good steading pace to burn body fat rather than carbs. I understand nose breathing conserves heat and moisture in winter also, and a scarf also helps.

Iceman - How's she goin? :biggrin:
I'm way behind on your put up or shut up post. Hope to get out this weekend.

What do they say about eating while rationing water? I understand you need water to burn carbs, but that the way the carbs are stored also releases some water, so you only need to replenish half the water that gets used. Of course the other side of that is that when you eat to replenish the carbs you also need to drink water with it to store the carbs. So I would think that walking slow, like 1/2 to 2/3 of your max heart rate, you will burn more fat and less carbs so you could probably save water by eating less. But I also understand you need water to burn fat, so I am not sure. I think the answer is to maintain a steady pace and drink when thirsty and eat when hungry, but everything steady and in moderation. I think you would get to know your body quite well with more practice. I find a heart rate monitor is helpful just to give you a better read on what is going on. Keeps things interesting also. Of course hiking at night would be a good trick also, or at least starting early and finishing late and taking a siesta in between.

Iceman
2006-07-28, 01:33
Iceman - How's she goin? :biggrin:
I'm way behind on your put up or shut up post. Hope to get out this weekend.


Thanks for asking...I need to shut up in this case. Been my worse summer for fun of all time. My mother in law passed away, and we have been spending the last 12 weekends 60 miles away, going thru and cleaning her home in preparation for it's sale. No hiking, tons more stress. I hope to get away for two days of hiking/hunting next weekend for our bear season opener, 100 degrees and steep, should burn off a few pounds.... Enough of my crying...

JAK
2006-07-28, 02:08
Sorry to hear about your Mom. We only have one.
I lost my Mum bunch of years ago and still think about her lots.

Don't be afraid to just take a 30 minute walk now and then.
They don't all have to be thru-hikes, or even in the woods.
Now if I could just take that advice.

Cheers.

Take-a-knee
2006-07-28, 08:51
There was an article many years ago in Outside mag about the results of a university study on water intake and exercise. They put three different groups of atheletes on treadmills. The first group had no access to water, they didn't last long, not a suprise. The second group had access to all they wanted to drink, they lasted longer, again, no suprise. The third group was forced to drink a specified amount of water every hour, they lasted the longest, this was the only group that could go to glycogen depletion.

On public TV many years ago a university tried to recreate Icarus' mythical flight. They bulit a pedal-powered airplane and got these cyclists to fly it. They had to keep redesigning the cockpit to get enough air flow to keep the pilot from overheating. Also, they had to provide water for the pilot (they were trying to save weight) or he couldn't last long enough to make the flight.

oops56
2006-07-29, 11:33
Water how long will it keep in a canteen befor it goes or if kept in a trunk of car for two or three weeks if heated up can it be used for coffee or noddles :confused: :confused:

JAK
2006-07-29, 11:54
Water how long will it keep in a canteen befor it goes or if kept in a trunk of car for two or three weeks if heated up can it be used for coffee or noddles :confused: :confused:I have wondered that also. I think it must depend on the state of the water when it was stored. I have had cached water in a 4 litre jug after a year. Smelled OK. Made tea. I think it depends on what you expect might be in it. I doubt it changes much after one month, better or worse, as long as it is well sealed. If in doubt, boil it or use aquamira, or whatever water treatment you have, like you would water from a stream or the best mud puddle available.

Does anyone have ideas for a portable solar still, like a sheet of plastic on the ground over a bowl of water with a small stone on top. Does this work? Should the plastic be clear or dark? Will it work at night also? Here is one link:

http://www.desertusa.com/mag98/dec/stories/water.html

Take-a-knee
2006-07-29, 14:54
Ideally water should be stored in the dark (to prevent algae growth), somewhere cool, like a basement. Sanitized GI water cans full of water stored in a basement will keep indefinitely. Pathogenic (disease-causing) bacteria reproduce somewhere around body temp. Each time that water stored in a vehicle in the sun heats and cools it becomes a potential medium for growth, if the bacteria were present to begin with. I would use it for cooking if I knew the containers were clean (no milk jugs).

Solar stills work as advertised, the plastic must be clear to achieve the greenhouse effect. Once it is set up you should only open it to pour urine inside or add vegetation. It may work better in a streambed if you can find one, the more moisture in the ground the more condensation. As you can see you'll need at least four stills per person, and you have to remain in that area so it isn't any good except in a "sit-and-wait" survival situation. If I lived in the desert, these would be in my truck. 2 mil poly with a six-foot piece of vinyl tubing in a ziploc.

SowthEfrikan
2006-07-29, 15:44
What about space (emergency) blankets - would those work for a still?

Take-a-knee
2006-07-29, 16:10
A space blanket would be a very poor choice, the plastic needs to be clear to allow the light into the still (like your parked car) to allow the still to heat up, this is what raises the humidity and allows the condensation to drip into the cup. The plastic needs to be durable enough not to get a hole in it, that would reduce the effectiveness of the still and it must be flexible enough to acheive the slope with a stone to drip into the cup.

Amigi
2006-07-29, 18:49
Solar stills work as advertised, the plastic must be clear to achieve the greenhouse effect. Once it is set up you should only open it to pour urine inside or add vegetation. It may work better in a streambed if you can find one, the more moisture in the ground the more condensation. As you can see you'll need at least four stills per person, and you have to remain in that area so it isn't any good except in a "sit-and-wait" survival situation. If I lived in the desert, these would be in my truck. 2 mil poly with a six-foot piece of vinyl tubing in a ziploc.

Quick correct sir. I would leave the urine part out, but that's just me.

A good solar still will have a hole dug to below the frost line ( if you know it ) or at least down until the ground becomes moist. Use vegetation that looks moist ( duh ) but regular grasses work too in a pinch. Focus on ground or near gound flora. Water lilies, cactus, ferns, and ground cover type plants are best. Shiny leaves should be broken up and mashed a little. Stills set up near river banks ( best place ) should be put on the north or west side to maximize sun exposure, assuming the ground is angled correctly.
The water can be filtered further using black charcoal from a fire strained through a towel or sock. My grandpa taught me that trick.

And remember, if you cant eat it, dont put it in a still.

SowthEfrikan
2006-07-30, 17:12
Turk, check out The Complete Walker IV, as I've also been thinking about water. Colin Fletcher and Chip Rawlins cover water pretty thoroughly, including building stills. I looked at Beyond Backpacking again to see what was there - what a waste of time.

dropkick
2006-07-31, 01:33
I just thought I'd throw this in to stir things up:
Years ago I read about a man who in the 1900's disagreed with the commonly believed idea that you couldn't drink sea water. His belief was that if you drank small amounts from the beginning of your time at sea you would survive and your body would handle the salt.
He said that the problem was that normally people waited until their bodies were damaged from dehydration, and then drank to much of it.
In support of this idea he spent a month (not sure about the time period any more, could have been weeks) in a boat with no water except sea water and survived.

SGT Rock
2006-07-31, 08:34
Army still talks about drinking urine, but only if you have to. It was mentioned in SEER-B training in January so it is still in recent doctrine. It is not my first level of defense though. Speaking of desert survival, has anyone seen Lion King's clip from his new walk on the PCT? http://walkingwithfreedom.com/episode1.mov

It is in the desert section and he is not doing well in it when it comes to water. In one scene he gets to a place where there is a stream bed listed in his guide but cannot find water - but he is complaining about all the insects in the stream bed. I was always told that the presence of a bunch of insects like that in the desert, especially in a stream bed is a good sign of water near the surface, start by looking at the low spots in curves along the stream bed.

Now on to how much water. It depends of course on weather and exertion level. On the AT I need about 2 gallons a day in normal weather. In Iraq I can use 4 liters in 4 hours by simply standing in the sun in my body armor in 106F (40C) weather. If I were to do a desert hike again I would probably plan to hike between 0400-1000, take a siesta, and then start up again at about 1600-2100 and create some shade with air flow for that 6 hour break. Also remember that in the desert, especially on the desert floor, the temperature right next to the ground is hotter than it is a few feet up, so if you can find a place to break that is on high ground you will be better off.

Pappyhighlife
2006-08-01, 14:12
Couple of years back in Uwharrie Forest (NC) around July I came up on these rough looking survival types, once I realized I was safe we started to chat, and quite naturally the conversation turned to water, as it is hard to find in this part of the Forest.
A guy named Reuben said they had a water factory back in the brush and asked if I wanted to see it. Feeling comfortable I wasn't going to get Deliverence raped I followed him a few yards to a grove of trees and bushes.
Looked like Christmas or something, they had like 50 or 60 sandwich baggies
hanging at various heights in the vegetation. The baggies were secured with large rubber bands to the branches.

Reuben said by morning each bag would produce about a half ounce of water sometimes a little more depending how cool it got during the dawn period.
He said with the bags he had hanging he would be able to nearly fill a Nalgene bottle, the system was used to augment their current water supply they had carried in.

I asked about bugs and taste, Reuben said they shake off the bugs before they bag the branch and depending on the foiliage the taste of the water can be bitter, but you can't tell it in Tea or Coffee.

Nice enough folks I suppose, but I put some serious distance before I made camp.One day I will try the baggies a quart of water is better than none.

Take-a-knee
2006-08-01, 14:41
Pappy, I don't think you had anything to worry about those Uwharrie Cats. They had most likely helped train many Green Berets, as had their fathers. Army SF has been running around that neck of the woods since the fifties, that is probably where the guy learned that trick. People from that area have volunteered to work as SF auxilary for two generations. Just don't cross 'em, and leave their stills or cannabis patches alone and you'll be fine.

Pappyhighlife
2006-08-01, 15:05
Yeah, lol
I hear they got a training village in those parts called Pine Town or Piney town.
Now that you mention it ole Reuben may have been sitting on a sack of seeds when I inadvertantly walked up on them.
Thought that water factory was cheap and easy to make, going to Pisgah later this month I will try it.

Turk
2006-08-14, 00:25
Okay I have now completed 6 days, 5 nights testing of water
requirements based on JAK's suggested 8 litres per day.
Based on very light duty weekend trips. Car camping, with
day trail hiking. I realize this is no great simulation for my thru-hike,
but it was the best I could devise this time of year having used up
most of my vacation days.

Results are somewhat varied. Worst day = Flat ground hiking, in a small
local conservation park, I set out exactly 8 litres of water. It was 102
deg F by 11:30 AM. Aproximately 12km of trail completed and water
was nearly exhausted. I continued the experiment and finished the last
4km of trail and returned to my camp. I tried to see how much I could
push in terms of exertion and how I responded with no water left.

Keep in mind I am car camping during this ... and many litres of water
were available just 10ft from my hammock. But for purpose of the
experiment I decided to see how my body would react. Around 7 or
8pm I was extremely parched, in poor spirits and with complete lack
of appetite due to thirst. I had also developed a pretty severe sun
exposure headache when I fell asleep reading in the hammock. I kept
a journal that I tried to log every hour (except when I fell asleep), how
I felt physically and mentally, amount I was perspiring, energy level,
as well as distance covered, or activity I was doing at time of log.

I feel pretty confident in the future that I will be able to recognize the
earliest signs of dehydration and with a few more cautious experiments
be able to come up with a margin of safety in terms of how much water
I need to ration, and how soon I need to get water, when the first
symptoms kick in.

On the flip side, here are some results of my easiest day in terms of
water consumption:
moderate terrain elevation, only 10km travelled for the entire day, lounging
in camp, cooking, relaxing, personal hygiene included a shave, brushing
teeth and a .5 litre sponge bath. temps all day aprox. 76- 80 deg F
I remained well hydrate and met all my cooking/cleaning needs with only
5.7 litres of water
consumed at end of day.

My next trick will be to come up with a meaningful and safe test to see how
far I can go when I am just starting to feel the onset of dehydration. I
think perhaps I should point out here that I am not looking to test "survival
endurance" here. Merely I am trying to get some useful data to determine
max distance, for minimum water carried without unpleasant side effects
and without imparing myself in any way for the following days hike. I am
hoping with this data I can get a good basis for my thru-hike.
Dont have any ideas right now exactly how I am going to test this.

JAK
2006-08-14, 01:21
Interesting stuff Turk.

One approach you should take to establish a minimum baseline would be how much water is used to burn calories. For example, I think if you are running in cool weather and butn 1000 calories and hour that will require 1000 ml or water, except that 500ml is gained from burning the calories so so only need to consume 500ml in that instance. Now that is based on running which is burning maybe 2/3 carbs 1/3 fat, except for the very fit folks that can run long distance burning perhaps 2/3 fat and 1/3 carbs. I think water is needed to store both fat and carbs, and some water is released when it is burned, but net net you need water, but I can't remember why exactly. I am not sure if water is used or released to convert protien into glycogen when you are low on carbs. All the tissue of the body can metabolize fat except the brain.

It makes sense though that you would need a lot of water when you eat. I would stick to mostly carbs when conserving water, and then only enough to keep the brain functioning safely. Do some book research to go hand in hand with you field research. Keep us posted. Thanks.

p.s. Look into whether there is any danger of conditioning yourself to not be thirsty when you should be, if you know what I mean. Also, various armed forces have probably done a lot of twisted research in this area, so that would be an area to search for material. I am sure its on the web someplace. I remember in basic training we saw some films on some Aussies playing around on an obstacle course with blistering agents. Nasty stuff. Tough bastards.

Take-a-knee
2006-08-14, 11:08
Always remember that thirst is a poor indicator of your level of hydration. There can be a significant time lag between having lost a lot of water and getting really thirsty, the faster you are losing water makes this phenomenon more likely. Your digestive system can only handle about a liter of water an hour, it is possible to sweat well over 2 liters every hour, especially if you are not acclimated to the heat. At about a liter low, you'll get cottonmouth, at about 2 liters low, you'll start to be incapacitated, light headed, mild nausea, anorexia. Once you get to this point, you are basically done for the day, your digestive system can't process enough fluid to get you back up to par, especially if you remain in a hot environment.

JAK
2006-08-14, 17:15
I would say a 1-2 litre loss would be primarily decreased performance and judgement, but you would be heading for trouble. Also, since it is often combined with heat stroke, sun stroke, or sun burn, and fatigue, that needs to be considered also. Depends on you body weight, but for a 165 pound person you are likely seriously incapacitated at 3 litres of water loss. Now body fat contains little water and cannot store much water, so you should base your calculations based on you healthy weight, not your current weight. Fatality due to dehydration alone occurs after you have lost about 15% of your normal water level, which might be 9-12% of your current body weight, but by then you have already gone through a lot of pain and convulsions and be long since incapacitated. I think as a rule of thumb that means, you never want to travel any distance without water since 6-8 hours of hiking could consume 3-4 litres even in ideal conditions. We are a species gifted with amazing endurance compared to most other animals, but we are not camels.

http://en.wikipedia.org/wiki/Dehydration
- 2%-5% water loss
Dehydration symptoms generally become noticeable after 2% of one's normal water volume has been lost. Initially, one experiences thirst and discomfort, possibly along with loss of appetite and dry skin. Athletes may suffer a loss of performance of up to 50%, and experience flushing, low endurance, rapid heart rates, elevated body temperatures, and rapid onset of fatigue. Symptoms of mild dehydration include thirst, decreased urine volume, urine that is darker than usual, unexplained tiredness, lack of tears when crying, headache, dry mouth, and dizziness when standing due to orthostatic hypotension.
- 5%-6% water loss
Around 5% to 6% water loss, one may become groggy or sleepy, experience headaches or nausea, and may feel tingling in one's limbs (paresthesia). In moderate to severe dehydration, there may be no urine output at all. Other symptoms in these states include lethargy or extreme sleepiness, seizures, sunken fontanel (soft spot) in infants, fainting, and sunken eyes. The symptoms become increasingly severe with greater water loss. One's heart and respiration rates begin to increase to compensate for decreased plasma volume and blood pressure, while body temperature may rise because of decreased sweating.
- 10%-15% water loss
With 10% to 15% fluid loss, muscles may become spastic, skin may shrivel and wrinkle, vision may dim, urination will be greatly reduced and may become painful, and delirium may begin. Judge Lynch of the Massachusetts Supreme Judicial Court argued that death by dehydration symptoms was "cruel and violent" in his opinion on the 1986 Brophy case: The mouth would dry out and become caked or coated with thick material. The lips would become parched and cracked. The tongue would swell, and might crack. The eyes would recede back into their orbits and the cheeks would become hollow. The lining of the nose might crack and cause the nose to bleed. The skin would hang loose on the body and become dry and scaly. The urine would become highly concentrated, leading to burning of the bladder. The lining of the stomach would dry out and the sufferer would experience dry heaves and vomiting. The body temperature would become very high. The brain cells would dry out, causing convulsions.
- Losses greater than 15% are usually fatal.
The respiratory tract would dry out, and the thick secretions that would result could plug the lungs and cause death. At some point within five days to three weeks, the major organs, including the lungs, heart, and brain, would give out and the patient would die. Be advised that death due to dehydration can occur in 3 days (or less in hot weather) and no one normally lives more than about 5-6 days without water.
----------------------------------------------------------------------

So I think the greatest risk when hiking is its effect on performance and judgement, especially when combined with heat stroke, sun stroke, sun burn, and fatigue. This reduction in performance and judgement will likely lead to further delays causing further dehydration, increased vulnerability to hypothermia, increased vulnerability to injury, incapacitation, painful suffering, and death. The body contains 60-80% water, depending on how lean you are. Serious trouble start at 5% water loss, which is 3-4% of your body weight loss, which for a 165 pound guy in good shape is 6.6 pounds or 3 litres, but for the same person at 220 pounds might also be only 3 litres. Painful convulsions and death are still off in the distance, but you are in serious trouble at this point.

Also beware of water intoxication, which can also be fatal. Becareful of this if you are exercising in a situation where you have nothing but water to drink. I don't think you can tank-up more than 1 litre, since any more will just pass through you.
http://en.wikipedia.org/wiki/Water_intoxication

p.s. Where I hike on the Fundy footpath thick woods that are humid but not that hot even in summer because of the Bay of Fundy. It is very hilly with lots of switchbacks. There is enough water than you never need to carry more than 2 litres, since there is a major or minor stream every 2-3km. I would guess that if I slowed down to half speed I could travel about 12 hours without water in cool conditions, but I would only get as far as I would normally travel in 6 hours, which for me is only 12km on the more rugged sections of that trail. That is when I am about 200# on a 6' frame with 20# of gear.