subroutine ddx(a,ka)
c
c finds da/dx 
c
c BD
c
      implicit none
      include 'itg.par'
      include 'itg.cmn'

c arguments:
      real a(lz,mz,nz,nspecz),ka(lz,mz,nz,nspecz)
      integer l,m,n,i
c
c-----------------------------------------
c
c
c Centered differencing:
c (This is what we had before)
c
      do 1 i=1,nspecies
      do 1 n=1,nd
      do 1 m=1,md
      ka(1,m,n,i)=(a(2,m,n,i)-a(1,m,n,i))/(0.5*dr(1))
      ka(ld,m,n,i)=(a(ld,m,n,i)-a(ldb,m,n,i))/(0.5*dr(ld))
      do 1 l=2,ldb
      ka(l,m,n,i)=(a(l+1,m,n,i)-a(l-1,m,n,i))/dr(l)
 1    continue
c
      return
      end