You should always make fresh water available to your dog to drink. Water is the most important substance for your dog to consume. It not only does the obvious, keeps your dog hydrated, but it helps him digest his food, absorb the important nutrients in the food and also carry away the waste products for elimination. A lot of water is especially important to dogs who eat dry food.
It is generally thought that for every pound your dog weighs, he should drink one ounce. A dog who weighs 20 pounds should drink 2 1/2 cups of fresh water each day. Top off the water bowl when you see it getting low. Replace any water that has been sitting stagnant.
Signs that your dog may be dehydrated include: unusually low level of energy, sunken eyes, a loss of interest in eating or drinking, and/or an excessively dry mouth.
Some people use automatic water dispensers so they don’t have to remember to fill the dish. If it is a large vessel, then it is wise to change the water often to keep it fresh.