Of the six classes of nutrients required to sustain life in the horse, water is the most important as horses can survive longer without the other nutrients then they can without water. Generally, horses will drink from 5 to 15 gallons of water a day, depending on their size, the environment, their workload and other individual factors. A horse should always have an adequate supply of clean, good quality water available. It is not possible for a horse in winter grazing conditions to meet its water requirement by eating snow.