Horses will typically drink between 5 and 10 gallons per day, depending on their environmental conditions like climate, the level of exercise and type of feed. Additional factors like water loss due to diarrhea or sweating also play a role, causing some horses to consume up to 25 gallons of water per day.
Water intake, however, is not just about drinking. In addition to drinking water, horses also get a small amount of water from their grain, fruit treats like carrots and apples as well as forage. Dried hay has a lower water content than fresh, young grass. Fresh grass can be up to 75-80% water, consequently horses will need to drink less fresh water to meet their requirements.

Signs of dehydration
Should a horse not be able to drink the 5-10 gallons a day it needs, dehydration will set in. A healthy horse can go 5-6 days without water; however, signs of dehydration will appear within 2-3 days in form of an elevated heart rate or pink or blue gums. Another sign of dehydration is that the horse appears “tucked up”. The term “tucked up” (or “drawn up”) refers to a horse with a sunken or shrunken flank or belly. Under normal conditions, the large colon full of heavy, wet feed material provides most of the contour of the normal horse's lower abdomen and flank. If that wet feed material dries out and shrinks, the abdomen appears tucked up.

The “skin pinch test”
Dehydration also causes decreased skin elasticity, which can be detected with the „skin pinch test“. Pinch the skin of your horse along the neck in front of the shoulder with your thumb and index finger. The skin should retract back to normal in less than two seconds after being pinched and released. According to the American Association of Equine Practitioners (AAEP), changes to these vital signs occur when the horse is 4-6% dehydrated. Horses typically display visual signs such as sunken eyes and a tucked-up appearance to the abdomen when dehydration levels approach 8-10%.

Ensure your horses are well-hydrated by always offering them clean, quality water regardless of whether they are stabled, turned out, traveling, or competing and consult with your veterinarin for signs of dehydration.

Tanja Schnuderl is the Director of International Services with The Equine Expert LLC, a multi-discipline equine expert witness and consulting firm with expert equestrians offering legal expert witness and consulting services in court cases and legal matters. Tanja is an expert on Barn Management and Horse Behavior. She provides her barn management services at a modern 20-stall boutique barn close to Washington DC and takes excellent care of the resident performance horses. She has established her own equine appraisal company, Sigma Equine LLC. Tanja grew up in Germany and was a paralegal for many years.  For more information on Tanja Schnuderl email tanja@theequineexpert.com, www.theequineexpert.com

Sources:

How Much Water Do Horses Drink? – Cornerstone Equine Academy
Horse Hydration: Your Questions Answered – The Horse
Database Record Viewer | Horse Side Vet Guide