When I used to do chart reviews, I was always struck by how many patients were quite dehydrated when they finally arrived at the hospital.

But, for day to day living, research has determined that it's probably wisest to drink when your thirsty, and not ingest copious amounts "just to be sure".

However, while vigorously exercising (especially in the heat) dehydration can occur very quickly, so it is prudent to ingest water at regular intervals (i.e. sip the camelbak).

This forum has hashed and rehashed electrolyte replacement to death, but it does bear repeating how important it is to hydrate.

I get chills when I read the stories of desert travelers dying from dehydration/heat stroke with water still in their canteens.

As Doug says, store water in your stomach!