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, Etp, is calculated from Penman’s combination equation in the form given by Monteith (1965):

                                          (3.12)

where Rn is net radiation available for transpiration (i.e. Rna - Rns, 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”), es is the vapour pressure at saturation, ea is the actual vapour pressure, ρa is air density, cp is the specific heat of air at constant pressure, Lν  is the latent heat of vaporisation, is the slope of saturated vapour pressure versus temperature curve, γ is the psychrometer “constant”, rs is an “effective” surface resistance and ra is the aerodynamic resistance. See viewing function Penman-Monteith combination equation.

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

                                      (3.13)

 

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

The slope of this function is given as:

                                 (3.14)