es contribute to 
                the tendency before, and which after, this call

     optional input:

          kbot -- integer,dimension(:,:)
             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 module will function properly
             if  diff = 0 for levels 