   dflux_datmos -- real, dimension(:,:)
             shape must conform to first two dimensions of xi.
             Set = 0.0 if the surface flux is treated explicitly.
             If dependence of flux on lowest level of atmosphere is
             treated implicitly, set equal to derivative of flux 
             with respect to value of xi in lowest model layer.
             Units = [factor]*kg/(m**2 s)

    
    input/output:

         flux -- real, dimension(:,:)
             Surface flux (positive upwa