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Keywords:

  • boundary layer;
  • decoupling coefficient;
  • stomatal conductance;
  • transpiration

ABSTRACT

Stomatal conductance and transpiration were measured concurrently in an irrigated Eucalyptus globulus Labill. plantation. Canopy stomatal conductance, canopy boundary layer conductance and the dimensionless decoupling coefficient (Ω) were calculated (a) summing the conductance of three canopy layers (gc) and (b) weighting the contribution of foliage according to the amount of radiation received (gc′). Canopy transpiration was then calculated from gc and gc′ for Ω = 1 (Eeq), Ω = 0 (Eimp) and by weighting Eeq and Eimp using Ω (EΩ). Eeq, Eimp and EΩ were compared to transpiration estimated from measurements of heat pulse velocity. The mean value of Ω was 0·63. Transpiration calculated using gc and assuming perfect coupling (12·5 ± 0·9 mmol m−2 s−1) significantly overestimated measured values (8·7 ± 0·8 mmol m−2 s−1). Good estimates of canopy transpiration were obtained either (a) calculating EΩ separately for the individual canopy layers or (b) treating the canopy as a single layer and using gc′ in a calculation of Eimp (Ω = 0). The latter approach only required measurement of stomatal conductance at a single canopy position but would be unsuitable for use in combined models of canopy transpiration and assimilation. It should however, be suitable for estimating transpiration in forests regardless of the degree of coupling.