♦ DESCRIPTION ♦
The program ONLINE_PENMAN_MONTEITH_GENERAL
calculates potential evapotranspiration (PET) for selected ecosystems by the Penman-Monteith method.
The input is:
(1) the chosen month,
(2) air temperature Ta (oC),
(3) net radiation Qn (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).
- 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 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.
- 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.
- 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 En 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 - (RH)/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 (oC)-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 (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) atmospheric pressure 500-1013.25 mb. Input data
outside of these ranges
will trigger error messages and stop execution.