Ice free water

Ice free horse water in Northern Montana. . .

What about a little bit from both Earth and Sky?. . . I think John D. is on to something. . .

What if you could heat the water, just enough to keep it from freezing, using Geothermal, and circulate it through the tank using solar? . . .  just a thought. . .

Depending on the terrain, if you had enough fairly level ground you could lay out a simple Geothermal exchange system.  With a simple trencher you could lay out 150’ – 200’ of trench 4’ – 6’ deep (4” wide, just enough for the pipe…), 6’ apart in a connecting loop.
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Using buriable PVC or CPVC 400+PSI 1/2' pipe connect it all up back to the tank - up, over and down into the tank bottem.  You would need a 12VDC circulating pump – I don’t have a lot of info on these from Solar resources but I wonder if something from a Marine application could be adapted?  I refurbished an older sail boat and did a LOT of 12V research.  There are lot of submersible and external 12VDC bilge pumps available in all sizes – what if one of these could be adapted?. . .

http://www.wholesalemarine.com/c/33000002/Bilge+Pumps.html

It wouldn’t take much, and it doesn’t have to lift the water once the system is flooded.  I wonder if there’s a timer that could cycle the pump during the night (50%/50%) to conserve batteries, and then run constant when there is sun. . .  so lets say the pump draws 3-4Amps. . .   you’d need 40+ Ah to run it during the off period, and enough PV panel to run and recharge . . .  I differ to the Solar experts to size all this.  I realize this is a DIY solution, and a lot depends on Andrea Christenot expertise. . .
 
(Extrapolated from Geothermal Heating – Popular Mechanics WEB page)
http://www.popularmechanics.com/how_to_central/home_clinic/1274631.html?page=2
The soil below frost level – 4 ft. to 6 ft. deep – stores the sun's energy at a more or less constant level, with temperatures keyed to latitude.  Subsoil temperatures range from the low 40s in the North to the low 70s in the South.
For purposes of comparison, we'll use the 55 degrees F soil temperature common in much of the Midwest and Central Plains. This area of the country suffers some extreme temperatures, but also has a fair number of mild days, so it's a reasonable choice.

Hi,  We live down in the Wind Rivers.  It's 25 below right now.  We've kept horses here 15 years and I still can't believe they survive.  One thing that helps a lot is plenty of "warm" water all the time.  We're off the grid and the simplest solution has been propane fired tank heaters in metal stock tanks.  The tanks are fitted in insulated plywood boxes.  The boxes cut fuel use by about a factor of four or five times.
     As for any kind of PV powered heater, that would be expensive and unsatisfactory.  These other guys commenting don't grasp.  It takes about a 250 watt electric heater to keep a five gallon rubber bucket thawed in this weather.  That would be about a five thousand dollar PV system for 24 hour use.  I'm sure you could dig a giant hole near your stock tank, put a 200 Gal storage tank 12 foot under ground and circulate to an insulated 10 Gal tank above ground with a simple DC pump and maybe 400 AH worth of batteries.  This would work but it's anything but cheap or simple.  Get a Broyhill tank heater and plumb it to your house tank if you can.