During these hot days of ( much too early) summer all of us who work our horses should remember that access to water at all time is critical. Horses that have to wait till a human brings them water or are led to water might not drink enough or too much at one time. Dehydration is a very serious risk. Darker than usual urine can be an indicator of too little water intake.But also in winter , when they eat a lot of hay, it is mandatory that horses can have as much water as they need. Since nature built them as self-regulatory beings, their organisms know best how much. My horses, which have free access to water any time they aren't working, drink water only sparingly when they are unharnessed after work and wait a while till they quench their thirst. I guess because they are not so low on water in their bodies that they feel deprived. One of them, during winter , dunks ever so often while eating a mouthful in the nearby water bucket which I offer so that he doesn't have to travel over to the tank.
Obviously his body tells him that he needs that for digestion.