An idle horse will drink:
E) 10-12 (gallons of water a day in a mild climate)
Dehydration is one of the most common causes of impaction colic. An idle horse will drink 10 to 12 gallons of water a day in a mild climate. This amount can greatly increase with hot weather, exercise or lactation. A dehydrated horse will appear depressed, refuse feed and may have a rapid respiration rate and elevated temperature. The horse's flanks may also appear sunken and tucked up tight, and the inside of its mouth may be dry. In severe cases, the horse will have an increased heart and respiration rate.
For more information on fluid balance, read the July 2009 e-Tips article Fluid Balance in the Horse.