How often would a buried bucket of water freeze?

I want to build an outdoor fountain, actually a water fountain for my cats. I want to place a 5 gallon bucket in the ground so that the top of the bucket is slightly below grade level with an insulated lid. A water line will feed water into the bucket through a float valve so that water stays at about 5 inches deep in the bucket. A submersible fountain pump will then pump the water to an above ground fountain for the cats to drink. The water will over flow into a collection filter and drain back into the bucket. I want to know if the ground heat and the continuous circulation of the water will keep the water above freezing temperatures. I live in Loris, South Carolina where the frost line is about 2 inches and the winter temperature rarely goes below 30 degrees for more than 8 hours(overnight) at a time.