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PHB p. 185 (or the corresponding section of the basic rules) states:

Water

A character needs one gallon of water per day, or two gallons per day if the weather is hot. A character who drinks only half that much water must succeed on a DC 15 Constitution saving throw or suffer one level of exhaustion at the end of the day. A character with access to even less water automatically suffers one level of exhaustion at the end of the day. If the character already has one or more levels of exhaustion, the character takes two levels in either case.

DMG p. 110 states:

Extreme Heat

When the temperature is at or above 100 degrees Fahrenheit, a creature exposed to the heat and without access to drinkable water must succeed on a Constitution saving throw at the end of each hour or gain one level of exhaustion. The DC is 5 for the first hour and increases by 1 for each additional hour. Creatures wearing medium or heavy armor, or who are clad in heavy clothing, have disadvantage on the saving throw. Creatures with resistance or immunity to fire damage automatically succeed on the saving throw, as do creatures naturally adapted to hot climates.

What isn't clear is what 'access' entails. How much water does a character need each hour under Extreme Heat conditions?

  • 1 gallon, normally needed for an entire day?
  • Or 2 gallons, normally needed when it is 'hot' but also spread across a whole day?
  • Or, does 'access' mean that the amount stays the same under 'hot' conditions (2 gallons for the whole day), but the number of saving throws increases substantially?

Also, if a character has less than 1/2 of whatever that amount is, do they also suffer automatic exhaustion without saving throws?

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If you are the DM, you will really need a good reason to force everyone to pay that close attention to water during a play session and you should make sure players are aware that water rationing will play an important part of the session/campaign. Otherwise, the issue will become a source of frustration.

That said,

Characters need to be able to readily consume water to stay hydrated. "Access" means that the requisite amount of water exists and that the character is able to drink it in some way (eg, not chained up at a distance).

First, if a character has less than the necessary amount of water, they should be alerted to the fact that they need to find a source of water at some point during the day.

Failing this, if it's less than 100 degrees, at the conclusion of a day a character is either

  • Forced to roll for exhaustion if they had at least half a gallon at the start of the day
  • Suffers a level automatically if the initial amount was less than half a gallon

If the temperature is greater than 100 degrees (extreme heat), the characters should be directed to take stock of their water at the start of the day and given a timeframe in which they need to locate a source of water.

If a source is not found in that time, the result can be handled in one of two ways, depending on how important the timing of the exhaustion checks is.

  • If the DM intends to have the exhaustion factor into the day's events as a means of increasing the difficulty and adding to the dramatic tension, the rolls take place every hour after the established amount of time has elapsed.
  • If the exhaustion is not intended to be used in this way, the rolls might be done in series once the day ends or a water source is finally located, depending on how many hours elapsed after the water ran out. The exhaustion levels are then carried over the next day(s) until enough rest has accrued for it to wear off.
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    \$\begingroup\$ While this is all good advice it misses the key bit of information and doesn't say what volume of water the PC's need to drink each hour under extreme heat. \$\endgroup\$ – SeriousBri Sep 28 '18 at 9:08
  • \$\begingroup\$ I'm running my players through Tomb of Annihilation at the moment, so survival is an important aspect of the game (for us). I've seen one (un-official) publication state that extreme heat requires 1 gallon of drinking water per hour. I don't know where they got that exactly. Apparently they interpreted the 'access' required as ratcheting up the amount needed per hour as per the first option I listed above (same as a day, but now per hour). But that is not official. Do any of the other official adventures say anything? I only own ToA and it does not. Anyone asked this of the designers? \$\endgroup\$ – highbandwidth Sep 28 '18 at 20:42
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A little math...

The characters each need 2 gallons of water for the whole day due to hot conditions; and for each hour of extreme heat (i.e., temperature over 100 F), they each have to make the save if they don't have a portion of that water on them.

According to the container capacities given in the Player's Handbook on page 153, a waterskin holds 4 pints, which is a half-gallon (64 fluid ounces); thus each person needs four waterskins per day. (I'd use the waterskin as the basic unit of measure when they find a water supply, just to make it easier than dealing with ounces or something.)

If it's really important how much water they use on an hourly basis, you can estimate that there are twelve hours of daylight, and thus of extreme heat, so each hour they would need 1/12 of the daily total, which is 1/6th of a gallon, or a third of a waterskin (about 21 fl. oz., or 0.6 liters) during each daylight hour.

So, conclusion: a full waterskin should be good for three person-hours in extreme heat conditions (that is to say, it'll be enough for one person for three hours, or three people for one hour), and each person needs four full waterskins per day, minimum, to avoid fatigue.

Variations on the theme

This is all assuming the characters would use up the 2 gallons of water entirely during those hot hours of the day. If you spread it evenly across 16 hours instead (i.e., when they aren't asleep), they would be 16 oz per hour, or 4 person-hours per waterskin.

If you figure they'll use some amount of water after dark, but mostly need it during daylight, their per-hour water needs would fall somewhere between the two values, but for the sake of simplicity I'd just pick 3 or 4 person-hours per waterskin, and proceed from there.

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  • \$\begingroup\$ Thanks for the math - I guess you are basically saying it's the 3rd option I have above -- the volume is the same but you have to have it available at all times not just mostly/partially/terminally during the day. (terminally meaning they just glug all the requisite H2O at the end of the day when setting up camp) \$\endgroup\$ – highbandwidth Sep 28 '18 at 20:41
  • \$\begingroup\$ Right, the total volume needed is 2 gallons, based on the total weather condition. But you use some portion of it each hour, if it's important to know whether your guys are running out of water mid-day. "Access" is an intentionally loose term that doesn't specify, but it should be pretty clear they mean "have some water on you or in the party". \$\endgroup\$ – Darth Pseudonym Sep 30 '18 at 20:49

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