Dehydration

Dehydration or Thirst, is a derived statistic in Fallout: New Vegas and Fallout 4.

Characteristics
Different foods and beverages have different effects on hydration. Any type of water will reduce dehydration/thirst. A drink from a water fountain, or any body of water will increase Hit points, but may also incur radiation damage. Most food has no effect on dehydration/thirst, however, some farmed or wild fruits and vegetables and crafted recipes may reduce dehydration too. Alcoholic beverages, soda and some food may also increase dehydration/thirst.

Fallout: New Vegas
In Fallout: New Vegas, dehydration is measured by the H2O bar only in Hardcore mode.

Dehydration increases over time, and can be affected by certain consumables. Much like radiation, it has increasing effects with each increment of 200. It primarily affects Endurance, though Perception, Intelligence, and Agility will also suffer with more severe dehydration. A level of 1000 means instant death. Without outside interference, it will take three days to reach fatal levels of thirst. It is the fastest-acting stat in Hardcore mode, accumulating twice as fast as food and about five times as fast as sleep.

Maize, Nevada agave fruit and prickly pear fruits are some of the grown food that reduce dehydration, and Caravan lunch, desert salad and gecko kebabs are crafted recipes that reduce dehydration. Beer, Nuka-Cola, Sunset Sarsaparilla and Pork n' Beans are examples of food and beverages which increase it.

Fallout 4
In Fallout 4, Thirst is indicated under the Action points (AP) bar only in Survival mode. Thirst increases over time. It can be reduced by hydrating type consumables, but could also increase from use of healing or aid consumables.