Agriculture Reference
In-Depth Information
the
continuous monitoring leaching volume
control
and estimation of the
substrate
water content
(by means of the dielectric
constant) are other methods that are rapidly
becoming popular in modern greenhouses
using soilless media and hydroponic meth-
ods of production.
The control of leachates and their con-
ductivity allows for ensuring the adequacy
of the scheduled water supplies while main-
taining a fixed leaching percentage. Leaching
control is frequently the only reference for
scheduling irrigation.
When using these methods, the proper
choice of test plants and their location is of
primary importance, so that the measure-
ments are representative of the whole green-
house area, because radiation differences
within the greenhouse can be very influen-
tial (Soriano
et al
., 2004a). When collecting
the drainage, it is recommended to establish
several control points with a minimum of
4 m
2
per sampling point (Urban, 1997b).
The use of models to estimate transpi-
ration, which allows for short-term forecast
of the water requirements of the substrate-
plant system, is becoming more common.
These models are implemented through the
irrigation computer. The simplest types
estimate the ET as a function of the solar
radiation and the most complex models are
based on the crop energy balance (see
Appendix 1 section A.8.3).
When models are used for estimating
the ET, the values are usually increased by
around 20% in winter and 30% in summer,
under Mediterranean conditions, plus any
eventual salinity corrections if any need to
be taken into account.
ET estimation methods based on the
accumulated solar radiation do not consider
the irrigation that takes place at night, which
must be included - at least one in winter
and two in summer (Urban, 1997b).
The methods based on plant parame-
ters are not used widely in commercial
greenhouses. Frequently, two or more of
the described methods are used for compar-
ison purposes and for optimizing irrigation
management.
When closed or semi-closed substrate-
growing systems are used, which recirculate
the drainage water, precision in the manage-
ment of the irrigation doses is not as neces-
sary as in the open systems (free drainage).
If enough water is available, it is essential
in the closed systems to correct the compo-
sition of the nutrient solution, as a function
of the nutrient absorption and the salinity
increase (Sigrimis
et al
., 2003).
The small water inertia of substrate
crops, derived from the limited volume of
water available to the roots, makes it imper-
ative that the required precautions are
taken to avoid failures in the system and
irrigating in excess. Such precautions
include: (i) controlling the drainage;
(ii) inspecting the drippers and filters to
avoid blockages; and (iii) monitoring any
failures in the power supply. Nowadays,
alarm systems are available on the market
for early detection and correction of such
possible failures.
The water use efficiency (WUE) has turned
into an agronomic expression that is
widely used (Howell, 1990; Stewart and
Nielsen, 1990), defining the production
(photosynthetic, biological or economic)
per
water
unit
(transpired,
evapotran-
spirated or applied).
When it refers to transpirated water, the
WUE of the transpirated water is the prod-
uct of the transpiration efficiency (dry mat-
ter fixed per water unit transpirated) by the
crop index.
The transpiration efficiency is deter-
mined, essentially, by genetic factors, being
estimated, in open field conditions, at
around 67 l of water kg
−1
of dry matter in
CAM type plants, 250 l kg
−1
in C4 type
plants and 500 l kg
−1
in C3 plants (Fereres
and Orgaz, 2000; Hsiao and Xu, 2007;
Steduto
et al
., 2007).
In greenhouses, the WUE is considera-
bly higher than in open-field crop produc-
tion due to: (i) the lower evapotranspiration
(derived from the lower radiation and wind
than in open field); (ii) the yield increases
(derived from the better climate control);
