Thirst

Thirst is a derived statistic in Fallout 4's Survival mode and Fallout 2d20.

Characteristics
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.''