Agriculture Reference
In-Depth Information
emitters and their density, but also to opti-
mize their management (Bressler, 1977;
Fereres, 1981; Castilla, 1985).
R
n
= Net water supplied by irrigation (the
part of the irrigation water which remains
stored in the root volume and is available
for the crop)
P
e
= Effective rain (part of the rain that
remains stored by the root volume and is
available for the crop)
AC
= Water that enters the root volume by
capillary ascension
ET
c
= Water evapotranspired by the crop
The soil water balance method is often
used to calculate how much water to apply
in surface or sprinkler irrigation.
Where HFLI is practised, considering
the time interval as the time passed between
the end of two consecutive irrigation epi-
sodes, the initial water content and the final
water content are almost the same (D
q
= 0)
and the net amount of water that must be
added by irrigation,
R
n
, becomes:
R
n
=
ET
c
− (
P
e
+
AC
) (11.3)
This is an equation that can be used to carry
on the previously mentioned accounting,
starting from the moment at which the soil
stores all the retainable water.
The effective rain (
P
e
) is non-existent in
a greenhouse and the capillary ascension
(
AC
) is negligible in the Mediterranean area,
because the water table is deep. Therefore,
R
n
=
ET
c
.
In HFLI, when the density of drippers
is high, such as in greenhouse horticultural
crops, the volume of wet soil in many cases
is close to 100%.
In low frequency irrigation systems, the
time for irrigation comes when the 'allowable
soil water depletion' is achieved in the soil
(section 11.4.2). When irrigating, water is re-
placed up to field capacity, providing the soil
with the maximum amount of useful water.
In HFLI the irrigation frequency is
much higher and is fixed as a function of
other parameters, mainly by matrix tension
in practice (as described later).
scheduling (soil-grown crops)
As water is a limiting resource in many agri-
cultural areas, it must be a basic objective of
its management to optimize its productivity
by means of adequate (i.e. avoiding water
deficits in the root zone) and efficient irri-
gation (i.e. maximizing the fraction of the
applied water that remains stored in the rooted
soil profile and is used later by the crop) to
obtain maximum yields.
Two questions are essential in irriga-
tion scheduling:
1.
When to irrigate? (frequency)
2.
How much water to apply?
The amount of water to apply must compen-
sate for the evapotranspired water corrected
as a function of the application efficiency
(assuming that the water content of the soil
is quite stable under drip irrigation given
its high frequency). Where saline water is
used, the supply must be increased to cover
the leaching requirements as described by
several authors (Ayers and Westcot, 1976;
Doorenbos and Pruitt, 1976; Vermeiren and
Jobling, 1980; Veschambre and Vaysse,
1980). Other components of the water bal-
ance are irrelevant for greenhouse drip irri-
gation (Castilla, 1987), except for rain in the
case of a perforated greenhouse cover.
Various methods can be used to calcu-
late the irrigation schedule in a greenhouse.
Method based on calculating the water
balance in the soil
The most simple expression for soil water
balance is:
Determination of the ET
c
The ET depends on: (i) the climate para-
meters; (ii) the availability of water in the
soil; and (iii) the crop. When the ET
requirements are not fulfilled, the crop can
q
1
−
q
2
= D
q
=
R
n
+
P
e
+
AC
−
ET
c
(11.2)
where:
D
q
=
q
2
-
q
1
= The difference of moisture
content at the beginning (1) and the end (2)
of the considered period
