A horse needs at least a gallon of water per 100 pounds of body weight. For your average horse, this equals 10 gallons a day. Water requirements vary greatly according to the weather and the level of work that the horse is doing. For instance, if your horse is exercising in hot, humid weather, he may need two to four times the minimum amount. If your horse does not get enough water he will get dehydrated and lose electrolytes. To prevent this, make sure that your horse has continual access to water. Here's how to make sure he drinks enough:
Horses tend to drink less water in the winter if the water is cold. Studies have shown that horses will drink more water if it is warm or tepid. So, get yourself a water heater!
Some horses are very picky about "foreign" water. Tips from experienced competitors include bringing enough water from home, and getting your horse used to drinking flavored water. Many horses enjoy water flavored with apple juice.
Horses will drink more when it is held up to them after and during competition.
Try to offer your horse water in a quiet area, where he will not be disturbed by all the action around him.
One wet-down flake of hay can absorb 1 to 2 gallons of water. If you feed your horse well-soaked hay, you can make a real impact on his fluid consumption. Endurance riders take advantage of this by feeding horses soaked hay before long rides.