♦ DESCRIPTION ♦
The program ONLINE_PENMAN_MONTEITH
calculates potential evapotranspiration (PET) of the reference crop by the PenmanMonteith method.
The reference crop is assumed to be clipped grass of 0.12 m height.

The input is:
(a) the chosen month,
(b) air temperature T_{a} (^{o}C),
(c) net radiation Q_{n} (cal cm^{2} d^{1}),
(d) relative humidity φ (%),
(e) wind velocity [at 2m depth] v_{2} (km d^{1}), and
(f) atmospheric pressure p (mb).
 The output is: (a) daily potential evapotranspiration (cm d^{1}), and (b) monthly potential evapotranspiration (cm).
 Potential evapotranspiration is a function of:
(a) net radiation in evaporation units E_{n},
(b) netradiation weighting factor Δ,
(c) masstransfer evaporation rate E_{a}, and
(d) modified psychrometric constant γ∗.
 The modified psychrometric constant γ∗ is a function of: (a) the psychrometric constant γ, (b) the stomatal (internal)
resistance r_{s}, and (c) the aerodynamic (external) resistance r_{a}.
 The psychrometric constant γ is a function of: (a) specific heat of moist air c_{p},
(b) atmospheric pressure p, and (c) heat of vaporization λ.
 The stomatal resistance of the reference crop is: r_{s} = 200/L, where L = leafarea index.
 The leafarea index L = 24 h_{c}, where h_{c} is the height of the reference crop: h_{c} = 0.12 m.
 The aerodynamic resistance is: r_{a} = 208/v_{2}.
 The netradiation weighting factor Δ is a function of air temperature T_{a}.
 The net radiation in evaporation units E_{n} is a function of: (a) net radiation Q_{n}, (b) mass density of water ρ, and (c)
heat of vaporization λ.
 The masstransfer evaporation rate E_{a} is a function of: (a) vaporpressure difference (e_{s}  e_{a}),
(b) stotamal resistance r_{s}, (c) aerodynamic resistance r_{a}, and (d) a factor K dependent on physical properties [of air and water].
 The vaporpressure difference (e_{s}  e_{a}) is approximated as: (e_{s}  e_{a}) ≅
e_{o} [1  (φ)/100].
 The factor K is a function of: (a) mass density of the air ρ_{a}, (b) specific heat of moist air c_{p},
(c) mass density of water ρ, (d) latent heat of vaporization λ, and (e) psychrometric constant γ.
 The units of E_{n} and E_{a} are cm/d.
 The units of Δ and γ∗ are mb °C^{1}.
 The units of r_{s} and r_{a} are s cm^{1} [the reciprocal of units of conductance, or cm s^{1}].
 The units of K are s d^{1} (mb)^{1}.
 Input allowable ranges: (1) air temperature, 040 ^{o}C; (2) net radiation, 1001000 cal cm^{2} d^{1};
(3) relative humidity 1100%; (4) wind velocity 101000 km d^{1}; (5) atmospheric pressure 5001013.25 mb. Input data
outside of these ranges
will trigger error messages and stop execution.
DISCLAIMER
