http://www.petmd.com/dog/nutrition/evr_ ... qTUUsRdUo4
Providing Adequate Water to Dogs
To make sure that your dog always has enough water to keep him/her healthy, you must provide clean, clear water daily that is easily accessible to your pet. Keep your dog’s bowl filled at all times and always refill with fresh, clean water daily.
As a rule of thumb, dogs should drink approximately one ounce of water per pound of body weight each day. There are many factors that can affect how much your dog will drink, however. Depending on the environmental temperature and the amount of exercise your dog performs during the day, more water may be necessary. This is because water is lost due to excessive panting and salivation.
As we mentioned previously, your dog’s food also contains a small amount of moisture. Dogs that are fed a canned food diet will receive quite a bit more moisture as canned food is about 70-80% water. These dogs may drink less than dogs that eat a dry food diet on a daily basis.