Good-quality water is vital to your horse's health and should be available to him at all times. On average, a horse will drink between 8 and 12 gallons a day - more in the summer or after hard work, and less in the winter.
However your horse gets his water (in a bucket or tub, from a stream or pond, or through an automatic waterer), you need to check the source daily.
Containers need to be cleaned and refilled. Ponds can become stagnant, and even running brooks may freeze. Automatic waterers can malfunction, either flooding an area or leaving your horse without needed water.
After strenuous exercise, limit your horse's water intake to a few sips until he is completely cooled down; too much water too soon after exertion can lead to founder. A very warm horse can be sponged or hosed with tepid water to cool him off and wash the sweat from his coat. Cold water is not only uncomfortable for him, but also it can cause his back muscles to stiffen.
See also Facilities