Each individual water requirements depend on various factors, like health, activity types, and environmental conditions. Your body's need for fluids will help you estimate how much water you need to drink each day.
According to the U.S. National Academies of Sciences, Engineering, and Medicine, an adequate daily fluid intake should be;
About 15.5 cups (3.7 liters) of fluids a day for men
About 11.5 cups (2.7 liters) of fluids a day for women
These recommendations include fluids from water, other beverages, and food.
Mild levels of dehydration can affect your mood and cognitive functioning. Especially in the very young, very old, those in hot climates, and those engaging in vigorous exercise it may cause serious health risks.
Mild to moderate levels of dehydration negatively affect short-term memory, perceptual discrimination, arithmetic ability, visuomotor tracking, and psychomotor skills.
Reintroduction of fluids during mild dehydration can reverse dehydration-induced cognitive deficits. Dehydration can be a great risk factor for delirium and for dementia in the elderly and in very ill patients. Also, older people have less thirst sensation and reduced fluid intake when compared to younger people.