Horses are made up of 65 - 70% water?
And.. an average 500kg horse drinks around 30-50 litres a day. This amount may be higher in hot weather (because the horse will sweat more and use up water reserves in the body) and if working very hard (again the horse will sweat more). A mare with a foal needs more because the milk that she is making to feed the foal requires water. So keep those water troughs filled and clean especially with warmer weather on the way!