Session 3 : Models for determining evapotranspiration
306
In these models the soil and canopy are each treated as a separate source (or sink) of energy
which involves the assignation of temperatures and humidities for each one of the sources (or
sinks) and for the atmosphere. A two layer model can be schemed by figure 1, where T v and T g
are the surface temperature of foliage and ground surface respectively. T a is the air temperature
at screen level and T o is the canopy-air system temperature at level z=z o +d, i.e. the level of the
sources/sinks of sensible heat, or of the hypothetical canopy air flow. H is the sensible heat flux.
If the canopy covers completely the soil a single layer model can be used (Hurtado, 1994a).
To Determination
The total radiation absorbed by the vegetation/soil system is R n =R nv +R ng . The division of R n
between latent and sensible components gives:
R nv = LE v + H v
(1)
R ng = LE g + H g + G
(2)
Looking at figure 1 we can write:
H = H v + H g
(3)
ρ Cp (To - Ta) / ra= ρ Cp (Tv - To) / rv + ρ Cp (Tg - To) / rg
(4)
where r a , r v and r g are the appropriate aerodynamic resistances.
Equation (4) can be written as:
T g -T o = (rg / ra) (To - Ta) - (rg / rv) (Tv - To)
(5)
From equation (5) the expression for the T o calculation can be obtained:
FIGURE 1
zo+d
soil
level z
level
306
In these models the soil and canopy are each treated as a separate source (or sink) of energy
which involves the assignation of temperatures and humidities for each one of the sources (or
sinks) and for the atmosphere. A two layer model can be schemed by figure 1, where T v and T g
are the surface temperature of foliage and ground surface respectively. T a is the air temperature
at screen level and T o is the canopy-air system temperature at level z=z o +d, i.e. the level of the
sources/sinks of sensible heat, or of the hypothetical canopy air flow. H is the sensible heat flux.
If the canopy covers completely the soil a single layer model can be used (Hurtado, 1994a).
To Determination
The total radiation absorbed by the vegetation/soil system is R n =R nv +R ng . The division of R n
between latent and sensible components gives:
R nv = LE v + H v
(1)
R ng = LE g + H g + G
(2)
Looking at figure 1 we can write:
H = H v + H g
(3)
ρ Cp (To - Ta) / ra= ρ Cp (Tv - To) / rv + ρ Cp (Tg - To) / rg
(4)
where r a , r v and r g are the appropriate aerodynamic resistances.
Equation (4) can be written as:
T g -T o = (rg / ra) (To - Ta) - (rg / rv) (Tv - To)
(5)
From equation (5) the expression for the T o calculation can be obtained:
FIGURE 1
zo+d
soil
level z
level