subroutine SubsurfaceDiffusion( xy_DeepSubSurfHeatFlux, xy_SoilHeatCap, xy_SoilHeatDiffCoef, xy_SurfTemp, xyz_SoilTemp, xy_SurfSnowB, xyr_SoilTempTransCoef, xyr_SoilHeatFlux )
    !
    ! 時間変化率の計算を行います. 
    !
    ! Calculate tendencies. 
    !

    ! モジュール引用 ; USE statements
    !

    ! 時刻管理
    ! Time control
    !
    use timeset, only: TimesetClockStart, TimesetClockStop

    ! 座標データ設定
    ! Axes data settings
    !
    use axesset, only: z_SSDepth    ! subsurface grid at midpoint of layer

    ! 雪と海氷の定数の設定
    ! Setting constants of snow and sea ice
    !
    use constants_snowseaice, only: SnowDens, SnowMaxThermDepth, SnowThermCondCoef

    ! 宣言文 ; Declaration statements
    !
    implicit none

    real(DP), intent(in ):: xy_DeepSubSurfHeatFlux (0:imax-1, 1:jmax)
                              ! 地中熱フラックス. 
                              ! "Deep subsurface heat flux"
                              ! Heat flux at the bottom of surface/soil layer.
    real(DP), intent(in ):: xy_SoilHeatCap (0:imax-1, 1:jmax)
                              ! 土壌熱容量 (J K-1 kg-1)
                              ! Specific heat of soil (J K-1 kg-1)
    real(DP), intent(in ):: xy_SoilHeatDiffCoef (0:imax-1, 1:jmax)
                              ! 土壌熱伝導係数 (J m-3 K-1)
                              ! Heat conduction coefficient of soil (J m-3 K-1)
    real(DP), intent(in ):: xy_SurfTemp (0:imax-1, 1:jmax)
                              ! 地表面温度. 
                              ! Surface temperature
    real(DP), intent(in ):: xyz_SoilTemp (0:imax-1, 1:jmax, 1:kslmax)
                              ! 土壌温度 (K)
                              ! Soil temperature (K)
    real(DP), intent(in ):: xy_SurfSnowB (0:imax-1, 1:jmax)
                              ! 積雪量.
                              ! Surface snow amount.
    real(DP), intent(out):: xyr_SoilTempTransCoef (0:imax-1, 1:jmax, 0:kslmax)
                              ! 輸送係数：土壌温度.
                              ! Transfer coefficient: soil temperature
    real(DP), intent(out):: xyr_SoilHeatFlux (0:imax-1, 1:jmax, 0:kslmax)
                              ! 土壌中の熱フラックス. 
                              ! Heat flux in sub-surface soil



    ! 作業変数
    ! Work variables
    !
    real(DP) :: xy_SnowDepth             (0:imax-1, 1:jmax)
    real(DP) :: xy_SurfTransCoefCorFactor(0:imax-1, 1:jmax)

    integer:: k               ! 鉛直方向に回る DO ループ用作業変数
                              ! Work variables for DO loop in vertical direction

    ! 実行文 ; Executable statement
    !

    ! 初期化確認
    ! Initialization check
    !
    if ( .not. subsurface_diffusion_inited ) then
      call MessageNotify( 'E', module_name, 'This module has not been initialized.' )
    end if


    ! 計算時間計測開始
    ! Start measurement of computation time
    !
    call TimesetClockStart( module_name )



    ! 土壌温度計算用の輸送係数の計算
    ! Calculate transfer coefficient for heat diffusion in the soil
    !
    k = 0
    if ( kslmax == 0 ) then
      ! This line is used when kslmax == 0, because z_SSDepth(k+1) does not exist. 
      xyr_SoilTempTransCoef(:,:,k) = 0.0_DP
    else
!!$      xyr_SoilTempTransCoef(:,:,k) = &
!!$        & xy_SoilHeatDiffCoef(:,:) / ( z_SSDepth(k+1) - 0.0_DP )
!!$      xyr_SoilTempTransCoef(:,:,k) =                                          &
!!$        & (   ( z_SSDepth(k+1) - 0.0_DP ) / xy_SoilHeatDiffCoef               &
!!$        &   - min( max( xy_SurfSnowB / SnowDens, 0.0_DP ), SnowMaxThermDepth )&
!!$        &     / SnowThermCondCoef )**(-1)
      xyr_SoilTempTransCoef(:,:,k) = xy_SoilHeatDiffCoef(:,:) / ( z_SSDepth(k+1) - 0.0_DP )
      xy_SnowDepth = min( max( xy_SurfSnowB / SnowDens, 0.0_DP ), SnowMaxThermDepth                       )
      xy_SurfTransCoefCorFactor = SnowThermCondCoef * ( z_SSDepth(k+1) - 0.0_DP ) / (   SnowThermCondCoef   * ( z_SSDepth(k+1) - 0.0_DP ) + xy_SoilHeatDiffCoef * ( 0.0_DP - xy_SnowDepth   ) )
      xyr_SoilTempTransCoef(:,:,k) = xyr_SoilTempTransCoef(:,:,k) * xy_SurfTransCoefCorFactor

    end if
    do k = 1, kslmax-1
      xyr_SoilTempTransCoef(:,:,k) = xy_SoilHeatDiffCoef(:,:) / ( z_SSDepth(k+1) - z_SSDepth(k) )
    end do
    k = kslmax
    xyr_SoilTempTransCoef(:,:,k) = 0.0_DP


    ! 土壌中の熱フラックスの計算
    ! Calculate heat flux in sub-surface soil
    !
    k = 0
    if ( kslmax == 0 ) then
      ! This line is used when kslmax == 0, because xyz_SoilTemp(:,:,k+1) does not exist. 
      xyr_SoilHeatFlux(:,:,k) = 0.0_DP
    else
      xyr_SoilHeatFlux(:,:,k) = - xyr_SoilTempTransCoef(:,:,k) * ( xyz_SoilTemp(:,:,1) - xy_SurfTemp(:,:) )
    end if
    do k = 1, kslmax-1
      xyr_SoilHeatFlux(:,:,k) = - xyr_SoilTempTransCoef(:,:,k) * ( xyz_SoilTemp(:,:,k+1) - xyz_SoilTemp(:,:,k) )
    end do
    k = kslmax
    xyr_SoilHeatFlux(:,:,k) = xy_DeepSubSurfHeatFlux


    ! 計算時間計測一時停止
    ! Pause measurement of computation time
    !
    call TimesetClockStop( module_name )

  end subroutine SubsurfaceDiffusion

  subroutine SubsurfaceDiffusionInit
    !
    ! subsurface_diffusion_heat モジュールの初期化を行います. 
    ! NAMELIST#subsurface_diffusion_heat_nml の読み込みはこの手続きで行われます. 
    !
    ! "subsurface_diffusion_heat" module is initialized. 
    ! "NAMELIST#subsurface_diffusion_heat_nml" is loaded in this procedure. 
    !

    ! モジュール引用 ; USE statements
    !

    ! NAMELIST ファイル入力に関するユーティリティ
    ! Utilities for NAMELIST file input
    !
!!$    use namelist_util, only: namelist_filename, NmlutilMsg, NmlutilAryValid

    ! ファイル入出力補助
    ! File I/O support
    !
    use dc_iounit, only: FileOpen

    ! 種別型パラメタ
    ! Kind type parameter
    !
    use dc_types, only: STDOUT ! 標準出力の装置番号. Unit number of standard output

    ! 文字列操作
    ! Character handling
    !
    use dc_string, only: StoA

    ! ヒストリデータ出力
    ! History data output
    !
    use gtool_historyauto, only: HistoryAutoAddVariable

    ! メッセージ出力
    ! Message output
    !
    use dc_message, only: MessageNotify

    ! 宣言文 ; Declaration statements
    !
    implicit none

    ! 作業変数
    ! Work variables
    !
!!$    integer:: unit_nml        ! NAMELIST ファイルオープン用装置番号. 
!!$                              ! Unit number for NAMELIST file open
!!$    integer:: iostat_nml      ! NAMELIST 読み込み時の IOSTAT. 
!!$                              ! IOSTAT of NAMELIST read

!!$    real(DP) :: SoilHeatCap
!!$    real(DP) :: SoilHeatDiffCoef

    ! NAMELIST 変数群
    ! NAMELIST group name
    !
!!$    namelist /subsurface_diffusion_heat_nml/ &
!!$      & SoilHeatCap, SoilHeatDiffCoef
!!$          !
!!$          ! デフォルト値については初期化手続 "subsurface_diffusion_heat#SubSurfaceDiffusionInit" 
!!$          ! のソースコードを参照のこと. 
!!$          !
!!$          ! Refer to source codes in the initialization procedure
!!$          ! "subsurface_diffusion_heat#SubsurfaceDiffusionInit" for the default values. 
!!$          !

    ! 実行文 ; Executable statement
    !

    if ( subsurface_diffusion_inited ) return

    ! デフォルト値の設定
    ! Default values settings
    !


    ! NAMELIST の読み込み
    ! NAMELIST is input
    !
!!$    if ( trim(namelist_filename) /= '' ) then
!!$      call FileOpen( unit_nml, &          ! (out)
!!$        & namelist_filename, mode = 'r' ) ! (in)
!!$
!!$      rewind( unit_nml )
!!$      read( unit_nml, &           ! (in)
!!$        & nml = subsurface_diffusion_heat_nml, &  ! (out)
!!$        & iostat = iostat_nml )   ! (out)
!!$      close( unit_nml )
!!$
!!$      call NmlutilMsg( iostat_nml, module_name ) ! (in)
!!$    end if


    ! 印字 ; Print
    !
    call MessageNotify( 'M', module_name, '----- Initialization Messages -----' )
    call MessageNotify( 'M', module_name, '-- version = %c', c1 = trim(version) )

    subsurface_diffusion_inited = .true.

  end subroutine SubsurfaceDiffusionInit