The functional dependence of canopy conductance on water vapor pressure deficit revisited

摘要

Current research seeking to relate between ambient water vapor deficit (D) and foliage conductance (g (F) ) derives a canopy conductance (g (W) ) from measured transpiration by inverting the coupled transpiration model to yield g (W) = m - n ln(D) where m and n are fitting parameters. In contrast, this paper demonstrates that the relation between coupled g (W) and D is g (W) = AP/D + B, where P is the barometric pressure, A is the radiative term, and B is the convective term coefficient of the Penman-Monteith equation. A and B are functions of g (F) and of meteorological parameters but are mathematically independent of D. Keeping A and B constant implies constancy of g (F) . With these premises, the derived g (W) is a hyperbolic function of D resembling the logarithmic expression, in contradiction with the pre-set constancy of g (F) . Calculations with random inputs that ensure independence between g (F) and D reproduce published experimental scatter plots that display a dependence between g (W) and D in contradiction with the premises. For this reason, the dependence of g (W) on D is a computational artifact unrelated to any real effect of ambient humidity on stomatal aperture and closure. Data collected in a maize field confirm the inadequacy of the logarithmic function to quantify the relation between canopy conductance and vapor pressure deficit.