Our existence requires water and our activities can affect that need. How much water are we talking about? All human beings are not same. Their size decides the amount of food and water that must be consumed during a period.
Let's assume that we are talking about one day. According to some sources from Internet we can see that men and women have different needs. Other sources though do not publish that difference but they mention both groups as adults.
However the need is around 2-3 liters per day for adults but it can differ depending on the body size, activity and environment. During a hot day we normally need more water than a colder day. And if we are very active we need even more than people who lay down and rest.
How is it when we run? Is it enough if we drink before and after the run? Well, we have done some tests and it depends on the route. If we run up to 10 kilometers we do not need to drink some extra water but if we run longer it is smart to have some close to hand.
Running is an activity that makes our body working harder than normally and therefore it consumes more water that it normally does. Our tests shows that running a half marathon without extra water is very exhausting and difficult. But if we can drink while running we can make the whole run easier. We can say that drinking water during a longer run can charge our batteries and lead us right to the goal.