During heavy, strenuous work, a horse can lose 4-5 gallons of water an hour, mostly through sweat.
More water is lost if the temperature is high.
Sweating is an effective method of cooling. However, sweating horses not only lose water but also significant amounts of electrolytes. Electrolytes are principally sodium and chloride with lesser amounts of potassium, calcium and magnesium. The loss of fluid and electrolytes can lead to a vicious circle of decreasing blood volume, weakening pulse, lactic acid build up and decreased performance. If your horse is working hard and losing 4-5 gallons of sweat per hour, he will get electrolytes from the food that you offer him and will replenish water by drinking.