Aerodynamic resistance of coppiced poplar

The aerodynamic resistance to the transfer of heat and water vapour, of a canopy of coppiced poplar (Populustrichocarpa×deltoides) was estimated as the sum of three components; the bulk leaf boundary layer, within-canopy, and roughness-sublayer resistances. These components were calculated from measurements of wind speed and leaf area distribution. Account was taken of enhanced transfer of heat and water vapour over momentum within the roughness sublayer. The resulting estimates of aerodynamic resistance, which are in agreement with values determined more directly from the flux–gradient relationship for sensible heat, are less than for momentum transfer calculated from the classical semi-logarithmic formulae. Consequently, the excess resistance for coppiced poplar at this site was negative.