Agriculture Reference
In-Depth Information
whether the ET rate of a crop is below its
maximum level (Idso
et al
., 1981):
( ) ( )
( ) ( )
concentrations of calcium and magnesium,
expressed in milliequivalents per litre
(meq l
−1
).
There are several ways to evaluate the
quality of irrigation water. Table 11.9 (in
Chapter 11) summarizes the guidelines pro-
posed by the FAO (Ayers and Westcot, 1987).
TT TT
TT TT
−−−
c a c a
min
CWSI
=
− −−
c a c a
max
min
(11.11)
where:
(
T
c
−
T
a
) is the difference between the crop
temperature (
T
c
) and the air temperature (
T
a
)
in the moment of the measurement.
(
T
c
−
T
a
)
min
corresponds to a crop without water
stress and (
T
c
−
T
a
)
max
to the maximum value of
the difference, when the stress is maximum.
The critical value of CWSI at which
irrigation should be started is 0.25 (Villalobos
et al
., 2002).
The radiation method estimates the
ET
0
as a
function of the global solar radiation
R
s
(Doorenbos and Pruitt, 1976):
ET
0
= a + b
×
R
s
(11.13)
where
a
and
b
are coefficients that depend
on the climatic conditions.
For greenhouses, an adaptation of the
Penman-Monteith method, proposed by
Baille
et al
. (1994) and used in soilless
crops, is based on the solar radiation, the
VPD (vapour pressure deficit) and the leaf
development to calculate the evapotranspi-
ration of the crop:
(
( )
( )
A.8.2
Irrigation water quality
The electrical conductivity of the water
(
EC
w
) and the sodium adsorption ratio (SAR)
are the most common indices used to evalu-
ate the salinity and alkalinity of water.
The conductivity increases with the
dissolved salt content; by convention, it is
measured at 25°C.
The SAR is:
−⋅
=−
K
LAI
ET A
1
e
×
R
C
s
(11.14)
+×
B
f
LAI
×
VPD
−
1/2
Ca Mg
+
[ ]
(11.12)
SAR
=
Na
×
where
A
and
B
are two coefficients,
K
is the
radiation extinction coefficient and
f
(LAI) a
function dependent of the LAI (leaf area
index).
2
where [
Na
] is the sodium concentration
and
[
Ca
+
Mg
]
is
the
sum
of
the
