!**********************************************************************
!      subroutine sbsfdi
!**********************************************************************
      subroutine sbsfdi (slagm, s0ags, slagn, slagx, sldi, sfdi)
!
!     +++ PURPOSE +++
!     calc soil mass fraction (sfdi) < diameter (sldi)
!     given modified lognormal distribution parameters
!
!     +++ ARGUMENT DECLARATIONS +++
      real*4 slagm, s0ags, slagn, slagx, sldi, sfdi
!
!     +++  ARGUMENT DEFINITIONS +++
!     slagm - aggregate distribution geometric mean diameter (mm).
!     s0ags - aggregate distribution geometric standard deviation.
!     slagn - aggregate distribution lower limit (mm).
!     slagx - aggregate distribution upper limit (mm).
!     sldi  - soil diameter in distribution (mm)
!     sfdi  - soil mass fraction < sldi
!
!     +++ LOCAL VARIABLES +++
      real slt, erf
!
!     +++ FUNCTIONS CALLED+++
!     erf(x)
!
!     +++ END SPECIFICATIONS +++
!
!     calc soil mass < sldi
!
      if (sldi .lt. slagx .and. sldi .gt. slagn) then
        slt = ((sldi - slagn)*(slagx - slagn))/((slagx - sldi)*slagm)
        sfdi = 0.5*(1 + erf(alog(slt)/(sqrt(2.0)*alog(s0ags))))
      elseif (sldi .ge. slagx) then
        sfdi = 1.0
      else
        sfdi = 0.0
      endif
!
      return
      end
!++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++