The program ONLINE_PENMAN_MONTEITH_GENERAL calculates potential evapotranspiration (PET) for selected ecosystems by the Penman-Monteith method.

1. The input is:   (1) the chosen month, (2) air temperature T_a (^oC), (3) net radiation Q_n (cal cm^-2 d^-1), (4) relative humidity RH (%), (5) atmospheric pressure p (mb), (6) stomatal resistance (s/cm), and (7) aerodynamic resistance (s/cm).

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 is either chosen for one of several vegetation types (input 6A) or specified as a value (input 6B). For input 6A, in s/cm: tropical rain forests, 0.8; deciduous, 0.85; general conifers, 0.85; savannah, 1.; citrus 2.; upland vegetation, 1.2; bare soil, 1.; water, 0.; short crop (daily) [ASCE], 0.7; tall crop (daily) [ASCE], 0.45.

7. The aerodynamic resistance is either chosen for one of several vegetation types (input 7A) or specified as a value (input 7B). For input 7A, in s/cm: Indonesia forest, 0.5; wheat, 0.5; eucalyptus, 0.2.

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

9. 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.

10. The mass-transfer evaporation rate E_n 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].

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

12. 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.

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

14. The units of Δ and γ∗ are mb (^oC)^-1.

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

16. The units of K are (mb)^-1.

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