Modeling canopy transpiration and stomatal conductance of young apples using a parameterized Penman-Monteith equation

The parameterization process to use the Penman-Monteith (P-M) transpiration equation is usually based on daily mean values of main environmental factors, namely radiation (R) and vapor pressure difference (VPD). We have re-arranged the P-M equation for stomatal conductance (g_s) and then found the best fit to R and VPD using diurnal data of variables instead of commonly used daily means.
Instead of using the standard rearranging of P-M equation for g_s calculation and following discrete regression analyses to R and VPD we wrote a complex P-M equation which already includes the canopy conductance (g_c) as a function of R and VPD. Analysis suggested a missing degree of freedom in VPD part of the Lohammar equation.
The formula we used is more complex but it gives more flexibility to stomata behavior, which does not mimic stomata closure so sharply under low VPD conditions.
Transpiration was calculated from sap flow data from two-year-old apple trees measured by sensors installed on the trunks of eight trees.
Using these calculated parameters when applied to the “canopy conductance” equation, stomata response to both important environmental variables could be analyzed and when introduced to the P-M equation they allowed calculating canopy transpiration, which fit measured canopy transpiration measured with sap flow gauges.

International Symposium on Physiological Principles and Their Application to Fruit Production

E. Kullaj, V. Avdiu, L. Lepaja, F. Thomaj, J. Kucera

Malus × domestica, vapor pressure deficit

https://doi.org/10.17660/ActaHortic.2017.1177.58