Van,

No the physics are not different, it is the numbers.

Water temperature, -20c, -10c, -5c, 1c. Ground temperature -20c, -10c, 5c. How far does the solar heat penetrate the surface material? Could it always be -10c just a centimeter below the surface even during max heating? What would be the heat transfer rate? Would the cold ground render the solar heating moot?

It is just a lot of stuff we do not know. A lot more than ambient air temp for sure.

Dfrank