:)
             shape must conform to first two dimensions of xi

             For use with models in which different columns
             have different number of 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 