ffects of diffusion
             Because the algorithm is implicit, the final tendency
                depends on which other processes have contributed to 
                the tendency before, and which after, this call

             Units of dt_xi are [xi]/s where [xi] are units of xi

     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