Exercise, heat and hydration

Running during the current heatwave is hard work.

In normal conditions, I carry a backpack of isotonic drink to keep myself hydrated for longer runs, but for a short few kilometers, I tend to wait till I get home.
However, in this heat, I need to carry water on even the short runs.

Without good hydration, lactic acid builds up, sapping strength and stamina. Extreme cases will do serious damage.

Drowning in rules
Most role-playing games don't deal with this particularly - they may have some rules on general fatigue, or on heat damage from high temperatures, but there tends to be less about water consumption.
3rd Edition D&D, which is based on the d20 rules, which I'm using for my own game, does have a rule for how much water one needs each day - a gallon, apparently - double or treble that for "very hot climates". (It's in the Environments section of the DMG - under starvation and thirst.)
This sort of quantity seems to me to be so excessive as to be useless. Also, there's no accounting in that rule for exertion or lack of it.

So, I thought I'd see if I could find a simple mechanism to reflect dehydration in RPGs, including under hot conditions, and when exerting oneself.
(I'm not looking to make real world recommendations regarding survival rations here - just to get some verisimilitude for rulings regarding water intake in RPGs.)

When I run for 3 hours, I'll drink about two litres of isotonic. I will tend to need to top up my water intake after I've run. This seems to be in line with recommendations - although I've just planned this based on how I feel about my own thirst while I run.
In hot weather, I'll drink more - but only by about a third or so. Of course, this is British hot that I'm talking about: just 30 Celsius or so in the shade.

Dehydration damage
What I don't know in detail - and what I'm not keen to find out first hand - is the point at which I start to get injured or ill from lack of water.
Thankfully, the internet provides answers. We start to get ill when we exceed 2% body water loss. At 5% - 10%, headaches and other symptoms set in, including seizures, and even death.
In game terms, let's say that at 2% dehydration, there's a risk of symptoms - penalties to act, and so on. You're not at your best, but you can still manage to get things done.
At 5% or more, the penalties get worse, and real damage starts to set in.

It appears that body water is estimated at about 60% of the mass of a person. There's bound to be some leeway in that, depending on physique, but we can stick with that figure for now. 2% of that 60% gives us about 1% of mass. I'm rounding off, of course.

Litres per kilo
To translate that into litres per kilo, a 100kg person starts to experience dehydration symptoms once they've lost about a litre of water. This seems to be in the right order of magnitude: it's said that an average person needs to top up a loss through sweat, urine and breath of about 1.2 litres per day.
We get a bit of water from normal food intake, so if we use 1 litre to replace 1.2 litres, it seems close enough for our purpose. Let's stick with 1% of mass as our benchmark for dehydration symptoms - with severe symptoms appearing at 5%

Remember my drinking 2 litres for 3 hours of exertion? That's double my daily need, in an eighth of a day. It seems like exertion increases your dehydration sixteen times. But let's also assume that you spend only 10 hours per day active, with the other 14 hours split into relaxing and resting / sleeping - and then compress all our water loss into that 10 hour slot.
(D&D assumes an 8 hour day in terms of travel, so we seem to be in the right region. I know that's fairly artificial, but the time you spend working is going to be the major contribution to your daily dehydration, while the resting time is less important.)

That means that my 3 hour run is about a third of the day - and I've got a thirst for double my whole day water intake. That means I'm drinking six times more for that three hour exercise than I would normally.
Maybe I'm drinking more than I strictly need because I have water on tap - so let's drop that down to a multiplier of five. Very conservative - but we're looking for minimum intakes to avoid serious ill health.

So for any portion of the day spent exerting yourself, you need five times the amount of water you would normally.

That means that for every hour spent in strenuous exertion (forced march, mass combat, frantic digging, hard rowing, and so on - generally if you're expending about double the usual effort to achieve your task), you need to drink an extra half pint per 100lbs, or 500ml per 100kg.

Heat and thirst
I also found I was drinking about a third more water when the temperature was 10 degrees more than normal. (That's Celsius, of course, I'm a scientist at heart.)
Compare that with the D&D rule that you need double for "very hot climates". The weather section of the rules gives temperature categories of "cold" (0 - 40 degrees), "moderate" (40 to 60 degrees), "warm" (60 to 85 degrees), and "hot" (85 to 110 degrees), with scope for cold snaps and heat waves (subtracting or adding 10 degrees each). Of course, this being a darn tootin' Yankee creation, these temperatures are in archaic Fahrenheit - so my 10 degree jump seems to take us pretty much from one temperature category to the next.
What this seems to tell us is that you need to add one third to your water intake for any given time period for each 10 C or 20F above normal temperate weather conditions - for each weather category, in D&D terms.
But the maths for adding thirds of 100ths is going to make hard work - let's simplify, and make it one half.

Attempting a rule
Let me try to put all this into a rule:
Humanoids need to drink 1/100 of their body weight in water or water-based liquids each day, or risk dehydration. For this calculation, 1lb = 1 pt.
Every hour of strenuous exertion adds one half that amount to the total needed that day, as a top up drink. Strenuous exertion includes mass combat, frantic digging, hard rowing, and so on - generally if you're expending about double the usual effort to achieve your task, then the effort is said to be strenuous.
For each weather category hotter than "moderate", the amount of water required is increased by 1/2. That is: In warm weather, you need 1.5 times the amount of water; in hot weather, you need 2 times the amount; and in weather exceeding 110 F, you need 2.5 times the amount.
Make this increase after adding any extra amount for strenuous exertion.
Round all these numbers up to the next half pint.
 Each hour of strenuous exertion prompts you to either drink the required top up amount, or make a save, as below.
Each day, or as prompted above, if you've drunk a whole 1/100 less than your required water, you must make a Fortitude save, DC 15 (+1 for each time you've had to make this save since you were last fully hydrated). Failure indicates that your are fatigued.
If you've drunk 3/100s less than your required water, you must make a new Fortitude save, DC 20 (+1 for each time you've had to make a dehydration save since you were last fully hydrated). Failure indicates that you are exhausted.
Once exhausted through lack of water, every hour you must either drink the required amount, or make a new Fortitude save, DC 20 (+1 for each time you've had to make a dehydration save since you were last fully hydrated). Failure means that you take 1d4 lethal damage.

Not really very simple, though is it? Maybe we should just use D&D's rules after all...

But let's compare what I've got with the 1 gallon rules from D&D:
Consider a 200lb fighter. He needs to drink 2 pints per day to keep hydrated. In a typical adventuring day, though, he has a few encounters (lasting a few minutes in total), runs about a bit, and so on - amounting to maybe one hour of strenuous effort. So he needs an extra 1 pint for that hour.
We're still nowhere near to the D&D gallon - less than half. It'd take another hour of activity, plus some hot weather to get there...

But maybe we need to imagine that a normal state for a hero is that she's putting in some strenuous activity each day - that seems right, doesn't it? She's not very heroic if she's just lounging about all the time, is she?
Resting, or light duties would be the exception, not the norm for a heroic character.

So, let's try another approach, aiming for the lower intake values, but keeping the relative simplicity of D&D rules:
Medium characters need at least half a gallon of water-based liquid per day, assuming they are actively adventuring. When resting, undertaking light professional tasks, sedately travelling (such as aboard ship as passengers, or at a regular pace on mounts), and so on, this requirement is halved. (Small creatures need half as much in all cases.)
For each weather category hotter than "moderate", the amount of water required is increased by 1/2. That is: In warm conditions, you need 1.5 times the amount of water; in hot conditions, you need 2 times the amount; and in conditions exceeding 110F, you need 2.5 times the amount. For each 20 degrees above 130F, the requirement increases by another half.
A character can go without water for 1 day plus a number of hours equal to his Constitution score. For each pint that a character drinks during that time, add another 6 hours. In hot weather, halve these times.
After this time, the character must make a Constitution check each hour (DC 10, +1 for each previous check) or take 1d6 points of nonlethal damage. Characters that take an amount of nonlethal damage equal to their total hit points begin to take lethal damage instead.
Characters who have taken nonlethal damage from lack of water are fatigued. Nonlethal damage from thirst or starvation cannot be recovered until the character gets water, as needed - not even magic that restores hit points heals this damage.
That'll do - the crazily excessive gallons are gone, hot weather is accounted for, and strenuous activity is built into the base line requirement, with provision for lazy layabouts.

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