Here, on the ranch in the western U.S., we have lots of cow ponies and they require a lot of water. Even the smaller ones, weighning perhaps 900 to 1200 pounds require at least 10 to 12 gallons per day.
If you have the availability of electricity, there are "donut" looking devices that actually float on the water and keep even a fairly large trough clear of ice. They don't use a lot of electricity. There are the same kind of devices that have thermostats to regulate the temperature for smaller troughs.
Winter dehydration can cause colic, which left unrecognized can develop serious consequences. We grow up with our horses... i have one roan pony that's 22 years old and still one of the finest cow horses on the place and she drinks 15 gallons a day.
Water is so important that in really cold snaps you may have to carry water two or three times daily if the above remedy isn't available.
Keep an eye on the horse's droppings... if they become really dry and fall apart soon after being dropped, make extra effort to provide water since this is a harbinger of dehydration. Sometimes a little added salt or a salt block will induce the horse to drink since they may not drink readily in cold weather. If you notice the horse "dunking" their hay in the water as they eat, this can be a sign they aren't producing enough saliva. also a sign of dehydration. Be sure to use a little grain daily ... Best of luck!