Water is the most essential nutrient for the horse. Plenty of clean fresh water should be available to the horse at all time, but horses will also change their drinking pattern during the seasons. In more southern parts of the country, during the hot summer months, a horse's water intake increases from 4-8 gallons of water per day during 65-degree temperature days to 20-40 gallons of water during 100-degree temperature days. During extreme cool weather water intake may be compromised and the horse will drink significantly less than needed.
Feed intake is related to water intake and it is generally thought that a horse will drink about a 4 to 1 ratio of water to feed on average. Inadequate water intake has a direct effect of feed intake and maintenance of digestive health. Adequate water intake allows for a more uniform mixing of feed particles and aids in movement of digesta through the digestive tract and helps enzymes and bacterial to more effectively breakdown the digesta. Inadequate feedstuff breakdown increases risks of impaction and gaseous colics.
During very hot weather, horses should be offered water via a bucket or barrel rather than in the small automatic waterers. Horses like to put their lips and face down into the water to get a drink and the small bowl automatic waterers don't allow the horse this option. Also, small bowl waterers can heat up to more than 100 degrees and burn horse's mouths. Place barrel waterers in pens and soak hay prior to feeding to aid hydration.
Those areas of the country that have good pastures of fresh green grass creates a whole different scenario from hay only. For instance, in the summer when forage is green horses may drink less because there is moisture in the grass. In the winter all the moisture they need comes from the water tank.To help your horse keep his body temperature up in the winter, you must make sure that his digestion is able to function the best it can. And that means making sure he has a readily available source of drinkable water and that does not include snow. A horse would not be taking in enough water if you rely on him eating snow. Also a horse will use up a lot of calories getting that snow from the solid state to the liquid state and so it will be that much harder for your horse to keep his weight on. Although it's not such a huge issue, horses are much less likely to drink cold water as tepid water. Most water tank heaters keep the water a few degrees above freezing, and as long as there isn't any ice floating in the water, it should be fine. But if you have worries about a particular horse getting enough water, you can warm the water up to encourage him to drink more.