c**********************************************************************
c     subroutine sbdzov
c**********************************************************************
      subroutine sbdzov
     i(brlai, brsai, bzht,wzzo,
     o wzzov)
c
c     +++ PURPOSE +++
c     to calculate aerodynamic roughness
c     of vegetation canopy. Ref. Trans ASAE 31(3):769-775
c     Armbrust and Bilbro, 1995
c
c     +++ ARGUMENT DECLATION +++

      real*4 brlai, brsai, bzht, wzzo, wzzov
c
c     +++ ARGUMENT DEFINITIONS +++
c     brlai - total leaf area index (m2/m2)
c     brsai - total stem silhoutte area index (m2/m2)
c     bzht   - maximum height of total biomass (m)
c     wzzo   - aerodynamic roughness of surface below canopy (mm)
c     wzzov - aerodynamic roughness length of canopy (mm)
c
c     +++ LOCAL VARIABLES +++
      real*4 brcd
c
c     +++ END SPECIFICATION +++
c
c     calculate "effective" biomass drag coefficient
      brcd = brlai*(0.2 - 0.15*exp(-8.*brlai)) + brsai
c
c     calculate roughness length of canopy
      wzzov = bzht * 1/(17.27-(1.254*alog(brcd)/brcd) - (3.714/brcd))
c     convert to mm
      wzzov = wzzov*1000
c     choose the maximum of canopy or surface roughness
      wzzov = amax1(wzzov, wzzo)
c
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++