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      subroutine dlwqh1 ( noseg  , notot  , nobnd  , isys   , diag   ,
     &                    delvol , conc   )

!     Deltares Software Centre

!     Function: - sets the diagonal for the steady state option
!               - updates first order term on the diagonal
!               - compresses DERIV for use in SGMRES

!     Created : 10 February 1997 by RJ Vos
!     Modified:  7 June     2010 by Leo Postma  Double precision version
!                                               NSYS fixated at 1

!     File I/O: none

!     Subroutines called  : none

      use timers                         ! WAQ performance timers

      implicit none

!     Arguments           :

!     Kind        Function         Name                   Description

      integer(4), intent(in   ) :: noseg                ! Number of computational volumes
      integer(4), intent(in   ) :: notot                ! Total number of substances
      integer(4), intent(in   ) :: nobnd                ! Number of open boundaries
      integer(4), intent(in   ) :: isys                 ! This substance number
      real   (8), intent(  out) :: diag  (noseg+nobnd)  ! Diagonal vector (1st order term)
      real   (4), intent(in   ) :: delvol(noseg)        ! Closure error correction
      real   (4), intent(in   ) :: conc  (notot,noseg)  ! First order term

!     local variables

      integer(4)  iseg               ! loop counter for computational volumes

      integer(4) ithandl /0/
      if ( timon ) call timstrt ( "dlwqh1", ithandl )

!         set the right hand side and
!         set the diagonal for steady state
!                          first order decay in conc

      do iseg = 1 , noseg
         diag(iseg) = -conc(isys,iseg) + delvol(iseg)
      enddo
      do iseg = noseg+1 , noseg+nobnd
         diag(iseg) = 1.0
      enddo

      if ( timon ) call timstop ( ithandl )
      return
      end