Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 394

ET 0 =

( R n − G ) + c p ( e a − e d ) ρ/ r a

λ (10.9)

+ γ (1 + r c / r a )

ET 0 is the latent heat of evaporation (kJm 2 /s); where the psychrometric constant

(kPa/ ◦ C); R n the net radiation flux at surface (kJm 2 /s); G the soil heat flux (kJm 2 /s);
c p the specific heat of moist air (kJ/kg/ ◦ C);

λ γ

the atmospheric density (kg/m 3 );

slope of vapor pressure curve (kP a / ◦ C); r a the aerodynamic resistance (s/m); r c the
bulk canopy resistance (s/m); and

ρ

λ

latent heat of vaporization (MJ/kg).
To facilitate the analysis of the combination equation, Smith et al. (1992) defined
the aerodynamic and radiation term as

ET 0 = ET rad + ET aero (10.10)

where
ET 0 =

reference evapotranspiration of standard crop canopy [mm/d]
ET rad = radiation term [mm/d]

ET aero = aerodynamic term [mm/d]

For reference crop, crop canopy resistance, r c = 200 / LAI;

and, LAI = 24 h c

where h c = 0.12 m

From the original Penman-Monteith equation, the aerodynamic term can be
written as

crop height =

86.4
λ

1
+ γ ∗

ρ · c p

r a

ET aero = ( e a − e d ) (10.11)

where

ET aero =

aerodynamic term [mm/d]

86.4 = conversion factor to [mm/d]

γ ∗ =

modified psychrometric constant

The radiation term is written as

1
λ + γ ∗

0.408 ( R n − G )

ET rad = ( R n − G ) = (10.12)

+ γ (1 + 0.34 U 2)

where

ET rad =

radiation term [mm/d]

net radiation [MJ/m 2 /d] R n =

soil heat flux [MJ/m 2 /d]

G =

λ = latent heat of vaporization [MJ/kg] = 2.45

Net radiation, R n = R ns − R nl (10.13)