Water density changes with temperature. Its densest at 2°c (meaning cold water sinks) just until before it freezes at which point again its density changes and becomes less dense and freezes at the top (ice floats).
On a water tank where the water is being heated by the ground; the warm water will circulate by it self to the top because is less dense than the water exposed to the cold wind.
When this system stops working is when the water exposed to the cold outside air is being cooled at a faster rate than the ground heating effect which means 2 things, your hole in the ground is not big enough or the surface of water exposed to the cold is to large.
Hence why most insulate the top and only allow a small square for cows to stick their heads in to drink.
Check this site out. There is more information and Ideas here that will help with the Freezing Water Issue.