♦ DESCRIPTION ♦
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.
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).
- 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 En,
(b) net-radiation weighting factor Δ,
(c) mass-transfer evaporation rate Ea, 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 rs, and (c) the aerodynamic (external) resistance ra.
- 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.
- The stomatal resistance of the reference crop is: rs = 200/L, where L = leaf-area index.
- The leaf-area index L = 24 hc, where hc is the height of the reference crop: hc = 0.12 m.
- The aerodynamic resistance is: ra = 208/v2.
- The net-radiation weighting factor Δ is a function of air temperature Ta.
- 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.
- 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].
- The vapor-pressure difference (es - ea) is approximated as: (es - ea) ≅
eo [1 - (φ)/100].
- 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.
- The units of En and Ea are cm/d.
- The units of Δ and γ∗ are mb °C-1.
- The units of rs and ra 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, 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.