What is the minimum amount of water per day to survive?

Question

I am disabled in the wildness, there are no natural water sources available. I have 2 gallons (7.5 liters) of water. The only practical solution is to sit and wait for rescue. I will be resting in the shade. How much water should I drink each day to survive as long as possible?

I have a memory from school that the kidney is obligated to produce 500ML of urine each day. That would seem to be lowest possible consumption.

I found a variety of amounts during an internet search.

With the possibility of urine output dropping to 500 ml, the minimum could be just over 1 liter a day or for the sake of making it easy to understand, this would be just over 32 ounces a day for a 154 lb man

Source

A normally active person needs about three quarters of a gallon of fluid daily, from water and other beverages.

Source(ready.gov)

Absolute Minimum Requirements...for replacement purposes, for an "average" person, has been estimated to be approximately 3 liters (3.2 quarts) per day, given average temperate climate conditions.

Source

one gallon of water per person, per day

Source (FEMA)

a minimum of 7.5 litres per capita per day will meet the requirements of most people under most conditions. This water needs to be of a quality that represents a tolerable level of risk. However, in an emergency situation, a minimum of 15 litres is required

Source (WHO)

With daily consumption of 500ML (~ 1/2 quart) the water would last 2 weeks, but would I survive? A 7.5 liters (~2 gallons) the water would last a single day and I would die in about 3 days.

How much water should I ration myself each day?

Answer 1

According to NAP.edu, the minimal (obligatory) daily water loss for an adult is at least 1 liter (~1 quart), which includes losses by the urine, feces, insensible perspiration and breathing (but not sweating). So, if you are sweating minimally, you would need to drink about 1 liter of water per day to keep yourself well hydrated. You can know you are well hydrated, when you do not have any symptoms of dehydration (dry mouth, thirst, strong yellow urine, poor skin turgor).

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About rationing water (only some theory at this point)

Let's say you are a 70 kg with 60%, that is 42 kg (liters), of water in your body. Death can occur when you lose 20% (~8 liters) of the body water.

If you lose 1 liter of water per day and drink 1 liter each day, 2 gallons (7.5 liters) of water would keep you well hydrated for 7.5 days. After that you will continue to lose 1 liter of water per day, you'll become progressively dehydrated, but you could survive for the additional 8 days, so 15.5 days in total.

If you drink 0.5 liters per day from the beginning, it seems you will develop a negative balance of 8 liters of water in 16 days, so basically the same as in the above scenario.

But, when you become dehydrated, you lose progressively smaller amounts of water, and this could make you survive a bit longer when you start to ration water from the beginning.

However, as a result of rationing water, the toxic waste products (urea) will start to build in your blood. It's then the question about the balance of harms of the buildup of urea in your blood and benefits of the sparing of water by your kidneys.

The question "How much water should I ration myself each day?" can be answered only by showing several convincing reports of how someone did it successfully (news reports...). I'm not sure if I know one. And, they don't usually do any "human trials" on this topic...

Practically, you would probably lose more than 1 liter of water per day because sweating occurs even if you sit in a shade, especially in summer.

Answer 2

Rationing water is not a good idea,

If activity is reduced, healthy people can survive on half their usual food intake for an extended period and without any food for many days. Food, unlike water, may be rationed safely, except for children and pregnant women.

...

If supplies run low, never ration water. Drink the amount you need today, and try to find more for tomorrow. You can minimize the amount of water your body needs by reducing activity and staying cool.

Source

As far as how long you could live, Over the edge: Death in the Grand Canyon has data from as study called Physiology of Men in the Desert by Edward Adolph and the estimates for time until death starting from a properly hydrated state are

• Not moving and staying in the shade - 72 hours
• Moving only at night - 48 hours
• Moving during the day - 24 hours

Its also going to depend on whether it is likely that someone will come to your aid in time to save your life. If nobody knows where you are or when you should check in and the odds of someone coming along are slim, well then you are going to have to decide whether or move or stay.

People have died because they choose to stay put and wait for rescue instead of trying to fix the problem themselves.

Her cell phone couldn’t get a signal. Instead of continuing to hike she stayed put. For 26 days she wrote in a journal until she died quietly of exposure and starvation.

Source

The camp was less than two miles from the Appalachian trail. Adam wrote that walking south from the campsite, the dense forest became open woods with good visibility after 60-70 yards, and after another 25 minutes he found “a clear logging road” that led to lodging. In total the walk took about 30 minutes.

Source

In that specific case the search and rescue teams did have a decent idea of where to look, but still failed to find the person who ended up dying of starvation after the search was called off.