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