c**********************************************************************
c     subroutine sbdzov
c**********************************************************************
      subroutine sbdzov
     i(brcd,  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 brcd, bzht, wzzo, wzzov
c
c     +++ ARGUMENT DEFINITIONS +++
c     brcd - "effective" silhouette area (m^2/m^2)
c             (combination of SAI and LAI values)
c     bzht   - weighted average 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 +++
c
c     +++ END SPECIFICATION +++
c
c     calculate "effective" biomass drag coefficient
c     brcrd now computed in "sumbio.for"
c
c     calculate roughness length of canopy
      wzzov = bzht * 1.0/(17.27-(1.254*alog(brcd)/brcd) - (3.714/brcd))
c     convert to mm
      wzzov = wzzov*1000.0
c     choose the maximum of canopy or surface roughness
      wzzov = amax1(wzzov, wzzo)
c
      return
      end
c++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++