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 Ta (oC), (c) net radiation Qn (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 En, (b) net-radiation weighting factor Δ, (c) mass-transfer evaporation rate Ea, 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 rs, and (c) the aerodynamic (external) resistance ra.

  5. The psychrometric constant γ is a function of:   (a) specific heat of moist air cp, (b) atmospheric pressure p, and (c) heat of vaporization λ.

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

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

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

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

  10. The net radiation in evaporation units En is a function of:   (a) net radiation Qn, (b) mass density of water ρ, and (c) heat of vaporization λ.

  11. The mass-transfer evaporation rate Ea is a function of:   (a) vapor-pressure difference (es - ea), (b) stotamal resistance rs, (c) aerodynamic resistance ra, and (d) a factor K dependent on physical properties [of air and water].

  12. The vapor-pressure difference (es - ea) is approximated as:   (es - ea) ≅ eo [1 - (φ)/100].

  13. The factor K is a function of:   (a) mass density of the air ρa, (b) specific heat of moist air cp, (c) mass density of water ρ, (d) latent heat of vaporization λ, and (e) psychrometric constant γ.

  14. The units of En and Ea are cm/d.

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

  16. The units of rs and ra 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.