How do I keep a thousand gallon water tank from freezing, cheaply?

I don't know how cold it gets in OR. so I don't know if this will work or not. First thing is paint the tank black. Build that simple structure you were talking about. Have the doors face south and leave them open in the day and close them at night.

Use the solar panel and battery with a timer to power a water fountain pump and circulate the water at night.

Simple. Drain the tank during the winter.

1.You won't have to water any trees in the winter. You don't need that water then.

2. You can't afford to heat the water, and solar heating is dependent on the sun and certainly no guarantee of not having your tank and pipe bursting.

Drain the tank and the water out of the piping. Maybe use some RV/ Marine antifreeze in some of the smaller lines if you can't get all the water out, will keep them from bursting also.

That will be the most sensible and safest, besides there will be plenty of runoff and rain in the spring. If you can't dig or drill a well, why not at least try to drive a well point. Then you could atleast get some water while the tank was empty. You could even be able to lift the water with an old fashioned hand pump just like old times. http://weather.nmsu.edu/hydrology/wellpoint.htm

I say let the trees do their own thing. They obviously had enough water to grow as it is, right? Either that or leave the tank empty in winter and start filling it during Spring. Existing solar heaters MIGHT work, but with the water not moving, this could be hard. A structure won't do unless you can maintain a temperature above 33 degrees...

If they're collecting run-off, won't it be empty in wintertime?

I'd first stick foam insulation on the sides to limit heat loss.

Then perhaps a wind-driven circulating pump & solar panel.

dig a hole and put it in that. make it deep so you dont have to worry about it freezing.