Thirst

Thirst is a derived statistic in Fallout 4 Survival mode and Fallout: The Roleplaying Game.

Gameplay
Thirst occurs if the Sole Survivor does not drink water after a long enough period of time, whether dirty or purified. Thirst accumulates slowly at all times. Increased levels modify SPECIAL stats. Higher thirst levels induces Fatigue, at which Action Points will become affected. Ultimately, damage will be taken, reducing Hit Points. Most types of hydrating drinks will reduce thirst by some amount, although certain drinks and soups are more effective than others. Chems and healing aids are known to increase thirst.

Stages of thirst
There are five stages of thirst that span in-game hours. It doesn't appear that the thresholds are linear. For example, it takes about 4 to 5 hours to go from hydrated to parched to thirsty, then 9 hours to go from thirsty to mildly dehydrated. The following table shows the approximate times that it takes for a character to go from completely satisfied to the listed condition in each area. In-game time is expressed in "hours" and real-time is expressed in "minutes".

''Note: The In-game / real-time data in this table are approximations only. The time expressed is not truly accurate. This table may be incomplete.''

Fallout: The Roleplaying Game
Thirst is a survival statistic that must be tended to by drinking sources of hydration. All character origins (with the exception of Mister Handies) must hydrate themselves or risk increasing their thirst states and incur fatigue points. All sources of - even rainfall - draws from contaminated sources and must be purified or risk radiation.

Any kind of strenuous activity or hot conditions can speed up this process. This includes combat or similar intense activity immediately moves a character down one step at the end of a scene. Hot weather from the exposure rules halves the amount of time listed on each step.

Drinking anything moves a character up one step from their current thirst stage. Drinking purified water moves two steps.

Soups
Soups, stews, and similar foods contain a lot of water, and can be a good way of keeping both thirst and hunger at bay. Consuming a food item with soup or stew in the name counts as both eating and drinking something, improving both Hunger and Thirst states by one step each.