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

1. The input is:   (a) the chosen month, (b) air temperature T_a (^oC), (c) net radiation Q_n (cal cm^-2 d^-1), (d) relative humidity φ (%), (e) wind velocity [at 2-m depth] v₂ (km d^-1), and (f) atmospheric pressure p (mb).

2. The output is: (a) daily potential evapotranspiration (cm d^-1), and (b) monthly potential evapotranspiration (cm).

3. Potential evapotranspiration is a function of:   (a) net radiation in evaporation units E_n, (b) net-radiation weighting factor Δ, (c) mass-transfer evaporation rate E_a, and (d) modified psychrometric constant γ∗.

4. 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.

5. 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.

6. The stomatal resistance of the reference crop is:   r_s = 200/L, where L = leaf-area index.

7. The leaf-area index L = 24 h_c, where h_c is the height of the reference crop: h_c = 0.12 m.

8. The aerodynamic resistance is:   r_a = 208/v₂.

9. The net-radiation weighting factor Δ is a function of air temperature T_a.

10. 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.

11. The mass-transfer evaporation rate E_a is a function of:   (a) vapor-pressure 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].

12. The vapor-pressure difference (e_s - e_a) is approximated as:   (e_s - e_a) ≅ e_o [1 - (φ)/100].

13. 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.

14. The units of E_n and E_a are cm/d.

15. The units of Δ and γ∗ are mb °C^-1.

16. The units of r_s and r_a are s cm^-1 [the reciprocal of units of conductance, or cm s^-1].

17. The units of K are s d^-1 (mb)^-1.

18. Input allowable ranges: (1) air temperature, 0-40 ^oC; (2) net radiation, 100-1000 cal cm² d^-1; (3) relative humidity 1-100%; (4) wind velocity 10-1000 km d^-1; (5) atmospheric pressure 500-1013.25 mb. Input data outside of these ranges will trigger error messages and stop execution.