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

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.