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The calculation methods for the potential evapotranspiration (PET) can be classified into four types: temperature-based, radiation-based, mass-transfer and combination methods. Most require many types of input data, such as air temperature, air pressure, humidity, wind speed and solar radiation. Eight methods for determining PET are integrated into GISMOD (Table 1). As it is widely used, the FAO Penman–Monteith method was selected for the case study.

Table 1

Eight methods for estimating the PET in the GISMOD

No.NameEquation
FAO Penman–Monteith  
Penman  
Kimberly–Penman  
Hargreaves–Samani  
Priestley–Taylor  
Makkink  
Turc  
Doorenboss–Pruitt  
No.NameEquation
FAO Penman–Monteith  
Penman  
Kimberly–Penman  
Hargreaves–Samani  
Priestley–Taylor  
Makkink  
Turc  
Doorenboss–Pruitt  

Rn is the net radiation flux; G is the sensible heat flux into the soil, which can be ignored for the daily estimation; T is the temperature, usually taken as the daily mean air temperature (Tmean); Tmax and Tmin are the daily maximum and minimum air temperatures, respectively; es is the vapour pressure of the air at saturation; ea is the actual vapour pressure; Δ is the slope of the saturation vapour pressure temperature relationship curve; γ is the psychometric constant; U2 is the wind speed at 2 m height; λ is the latent heat of vaporisation; Rs is the solar or shortwave radiation; and Ra is the extra-terrestrial radiation.

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