[数值问题] 数组计算错误，我看不出来啊

      FUNCTION INTERP (NPAIRS, FUNCT, XVALUE)
C     interpolates between points in data functions
      IMPLICIT NONE
C     input
         INTEGER NPAIRS  ! number of pairs of values to be used
         REAL    FUNCT(*)! array of pairs of values: x1, y1, x2, y2, ...
         REAL    XVALUE  ! x value
C     output
         REAL    INTERP  ! y value
C     local
         INTEGER    I, J ! DO indexes
         REAL XX(1:10) ! Cseries of x values of FUNCT
         REAL YY(1:10) ! Cseries of y values of FUNCT
C
C     put FUNCT into XX and YY
      I = 0
      DO 10 j = 1, 2 * NPAIRS - 1, 2
         I = I + 1
         XX(I) = FUNCT(j)
         YY(I) = FUNCT(j + 1)
   10 CONTINUE
C     interpolate using XX and YY
      DO 20 j = 1, NPAIRS
         IF (XVALUE .EQ. XX(j)) THEN
            INTERP = YY(j)
            RETURN
         ELSEIF (XVALUE .LT. XX(j)) THEN
            INTERP = YY(j - 1) + (XVALUE - XX(j - 1)) * 
     *         (YY(j) - YY(j - 1)) / (XX(j) - XX(j - 1))
            RETURN
         ELSE
         END IF
   20 CONTINUE
      END 
   SUBROUTINE CANOPY (DOY, MAXHT, RELHT, MAXLAI, MXMAI, 
     *   RELLAI,RELMAI,SNOW,SNODEN, MXRTLN, MXKPL, CS, DENSEF,  
     *   HEIGHT,LAI, SAI,MAI, RTLEN,  RPLANT)
C     canopy parameters
      IMPLICIT NONE
C     input
         INTEGER DOY      ! day of year (first day of DFILE and run)"
         REAL    MAXHT    ! maximum height for the year, m, minimum of 0.01 m
         REAL   RELHT(*)  ! ten pairs of DOY and relative canopy height
         REAL   MAXLAI    ! maximum projected leaf area index for the year,m2/m2
         REAL   MXMAI
         REAL   RELLAI(*) ! ten pairs of DOY and relative LAI
         REAL   RELMAI(*)
         REAL   SNOW      ! water equivalent of snow on the ground, mm
         REAL   SNODEN    ! snow density, mm/mm
         REAL   MXRTLN    ! maximum root length per unit land area, m/m2
         REAL   MXKPL     ! maximum plant conductivity, (mm/d)/MPa
         REAL   CS        ! ratio of projected SAI to canopy height, m-1
         REAL   DENSEF    ! density factor
C     output
         REAL   HEIGHT    ! canopy height above any snow, m, minimum of 0.01 m
         REAL   LAI       ! leaf area index, m2/m2, minimum of 0.00001
         REAL   SAI       ! stem area index, m2/m2
         REAL   MAI
         REAL   RTLEN     ! root length per unit land area, m/m2
         REAL   RPLANT    ! plant resistivity to water flow, MPa d/mm
C     local
         REAL   SNODEP    ! snow depth
         REAL   HNOSNO    ! height of canopy without snow
         REAL   HSNO      ! height of canopy above snow
         REAL   RATIO     ! fraction of canopy above snow
         REAL   RELHIT    ! RELHT for DOY
         REAL   KPL       ! plant conductivity, mm d-1 MPa-1
C     intrinsic
C        REAL, MAX
C     external functions needed
         REAL   INTERP
C
      RELHIT = INTERP(10, RELHT(20), REAL(DOY))
      SNODEP = .001 * SNOW / SNODEN
      HNOSNO = MAX(.01, RELHIT * MAXHT)
      HSNO = MAX(0., HNOSNO - SNODEP)
      RATIO = HSNO / HNOSNO
      HEIGHT = MAX(.01, HSNO)
C
      LAI = RATIO * DENSEF * INTERP(10, RELLAI(20), REAL(DOY)) * MAXLAI
      SAI = DENSEF * CS * HEIGHT
      MAI = INTERP(10, RELMAI(20), REAL(DOY)) * MXMAI
      IF (LAI .LT. .00001) LAI = .00001
C
      RTLEN = DENSEF * RELHIT * MXRTLN
      KPL = DENSEF * RELHIT * MXKPL
      IF (KPL .LT. 1E-08) KPL = 1E-08
      RPLANT = 1. / KPL
C
      END