Agriculture Reference
In-Depth Information
900
0.408 (
∆−+
R G
)
γ
ue e
(
−
)
N
s
a
(
T
+
273)
(10.2)
ET
0
=
mean
∆+ +
γ
(1 0.34 )
u
where
u
is air velocity measured at 2 m above the ground. Details for calculations
of daily values of the other parameters in Eq. 10.2 are given in Allen et al. (
1998
).
Obviously the conditions of most protected crops are significantly different from
those of the reference grass for which Eq. 10.2 was derived. Therefore adjustments
should be made according to actual climatic conditions and canopy properties in
order to obtain the actual crop evapotranspiration (ETc). For mild climates, von
Zabeltitz (
2011
) suggested increasing daily mean maximum and minimum tempera-
tures by 4 and 2 °C, respectively, using mean relative humidity of about 75-80 %,
and assuming transmittance of the global radiation by the cladding material (mostly
for plastic greenhouses) of about 0.6-0.7. In addition, recommendations for actual
crop coefficient (Kc), relative area of crop to ground and irrigation loss are given
(von Zabeltitz
2011
).
Fernandez et al. (
2009
) demonstrated the use of the ET0 approach (Eq. 10.2) for
an unheated plastic greenhouse in Almeria, Spain using two sub-models: one for
radiation and the other for the crop coefficient for pepper cultivation. They con-
ducted experiments in a naturally ventilated greenhouse in the Almeria region in
which pepper was grown. The measurements included inside and outside climatic
variables, and ETc was measured by lysimeters. Good agreement was obtained re-
garding estimated and measured values of ET0, ETc and Kc.
In an attempt to optimize water use in greenhouses, several studies measured
crop ET, mainly by the lysimeter technique, and compared the measurements with
theoretical models, adapted to the greenhouse conditions. For example, Jolliet and
Bailey (
1992
) examined the effects of inside climatic conditions on tomato transpi-
ration and compared their transpiration measurements with five transpiration mod-
els. Their results showed that transpiration rate increased linearly with solar radia-
tion, VPD and air velocity; however air temperature and CO
2
concentrations had
no significant influence on crop transpiration. Among the five transpiration models
they investigated, they found two (Stanghellini
1987
and Jolliet and Bailey
1992
) to
be in best agreement with the measurements as these two models represented most
accurately the solar radiation and VPD effects on the stomatal conductance.
A different approach in simulating the ETc in Mediterranean plastic greenhouses
was examined, by Boulard and Wang (
2000
). They suggested that unlike north-Eu-
ropean glasshouses, Mediterranean plastic greenhouses are strongly coupled with
the external environment, such that ET modeling can be based on the relationships
between the external and internal greenhouse conditions. Their results showed that
when the greenhouse was closed, i.e., strongly decoupled from the external environ-
ment, ET predictions based on external conditions deteriorated in comparison with
periods when the greenhouse was well ventilated. This was due to strong interac-
tion between inside and outside in the ventilated greenhouse. In their model, Bou-
lard and Wang (
2000
) applied the general Penman-Monteith equation (Eq. 10.1)
