My 4 year old qh colt has had sunken in flanks for about 6 weeks now. I was always told that that indicated dehydration so I started monitoring his water intake. Seeing as he is a colt he is by himself so it is easy to tell just how much water he is drinking.
He is only going through maybe 10L (2.5 gallons) of water a day and we are in summer here. Well autumn now but it is still hot. His water is cleaned and refilled fresh daily and kept in the shade so it doesnt heat up in the sun so there is no reason there that should stop him from drinking. By my calculations he is drinking only about a 1/4 of what he should be. Even after exercise I can't get him to drink much and he would surely need it. I've tried adding molasses to his water to encourage him to drink but that hasnt helped.
I know every horse is different but all my other guys average around 40L a day which is considered normal.