 levels above the surface.
             The levels k &lt;= kbot are assumed to be above the surface.

             If kbot is present, the interface will function properly
             if  diff = 0 for levels below the surface. Also, to avoid
             division by zero, be sure that the underground 
             input temperatures are non-zero and that the pressure 
             thicknesses of the underground layers are non-zero as well. 

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