The combination equation for potential transpiration

Transpiration is defined as a potential rate when neither soil water deficits nor low soil temperatures influence the water loss. The potential transpiration, E_tp, is calculated from Penman’s combination equation in the form given by Monteith (1965):

                                          (3.12)

where R_n is net radiation available for transpiration (i.e. R_na - R_ns, see “Partitioning of net radiation”, for multiple plants the fraction of radiation to each plant is calculated in the radiation section, see “Partitioning of radiation between plants”), e_s is the vapour pressure at saturation, e_a is the actual vapour pressure, ρ_a is air density, c_p is the specific heat of air at constant pressure, L_ν  is the latent heat of vaporisation, ∆ is the slope of saturated vapour pressure versus temperature curve, γ is the psychrometer “constant”, r_s is an “effective” surface resistance and r_a is the aerodynamic resistance. See viewing function Penman-Monteith combination equation.

The saturated vapour pressure function, e_s(T), is defined by:

                                      (3.13)

 

where e_s is calculated in Pa and T in °C.

The ∆ slope of this function is given as:

                                 (3.14)