2:2.2.1 Penman-Monteith Method

The Penman-Monteith equation combines components that account for energy needed to sustain evaporation, the strength of the mechanism required to remove the water vapor and aerodynamic and surface resistance terms. The Penman-Monteith equation is:

λE=Δ(HnetG)+ρaircp[ezoez]/raΔ+γ(1+rc/ra)\lambda E=\frac{\Delta*(H_{net}-G)+\rho_{air}*c_p*[e^o_z-e_z]/r_a}{\Delta+\gamma*(1+r_c/r_a)} 2:2.2.1

where λE\lambda E is the latent heat flux density (MJ m2^{-2} d1^{-1}), EE is the depth rate evaporation (mm d1^{-1}), Δ\Delta is the slope of the saturation vapor pressure-temperature curve, de/dTde/dT (kPa ˚C1^{-1}), HnetH_{net} is the net radiation (MJ m2^{-2} d1^{-1}), GG is the heat flux density to the ground (MJ m2^{-2} d1^{-1}), ρair\rho_{air} is the air density (kg m3^{-3}), cpc_p is the specific heat at constant pressure (MJ kg1^{-1} ˚C1^{-1}), is the saturation vapor pressure of air at height zz (kPa), eze_z is the water vapor pressure of air at height zz (kPa), γ\gamma is the psychrometric constant (kPa ˚C1^{-1}), rcr_c is the plant canopy resistance (s m1^{-1}), and rar_a is the diffusion resistance of the air layer (aerodynamic resistance) (s m1^{-1}).

For well-watered plants under neutral atmospheric stability and assuming logarithmic wind profiles, the Penman-Monteith equation may be written (Jensen et al., 1990):

λEt=Δ(HnetG)+γK1(0.622γρair/P)(ezoez)/raΔ+γ(1+rc/ra)\lambda E_t=\frac{\Delta*(H_{net}-G)+\gamma*K_1*(0.622*\gamma*\rho_{air}/P)*(e^o_z-e_z)/r_a}{\Delta+\gamma*(1+r_c/r_a)} 2:2.2.2

where λ\lambda is the latent heat of vaporization (MJ kg1^{-1}), EtE_t is the maximum transpiration rate (mm d1^{-1}), K1K_1 is a dimension coefficient needed to ensure the two terms in the numerator have the same units (for uzu_z in m s1^{-1}, K1K_1 = 8.64 x 104), and PP is the atmospheric pressure (kPa).

The calculation of net radiation, HnetH_{net}, is reviewed in Chapter 1:1. The calculations for the latent heat of vaporization, λ\lambda, the slope of the saturation vapor pressure-temperature curve, Δ\Delta, the psychrometric constant, γ\gamma, and the saturation and actual vapor pressure, ezoe^o_zand eze_z, are reviewed in Chapter 1:2. The remaining undefined terms are the soil heat flux, GG, the combined term K10.622λρ/PK_1 0.622 \lambda\rho/P, the aerodynamic resistance, rar_a, and the canopy resistance, rcr_c.

Last updated