Agriculture Reference
In-Depth Information
•
to define the
lower baseline
by using the temperature of an artificial wet refer-
ence surface - such as a small wet foam mat floating in water - instead of the leaf
temperature. The air temperature is then subtracted from the temperature of this
artificial wet reference surface. The latter provides for more stable temperatures
than moist leaves do.
Provided the baselines are well defined, the crop water stress index can be
regarded as a prime choice for estimating the water situation of crops (Meron et al.
2010
). However, there are still limitations for its use in practice. An important limi-
tation is that very erroneous signals result if the radiation hits soil or plant residues
instead of active leaves. Hence closed canopies offer ideal conditions. But these
might not exist in early growing seasons or with widely spaced crops. Correctives
that can provide a remedy are either narrowing the sensors view to rows of crops in
case of proximal sensing or alternatively sorting out soil data via post-processing by
means of reflectance signals. For details to this see Sect.
6.3
.
The crop water stress index or similar thermal indices are occasionally used for
the control of large area irrigation in dry regions, yet presently barely ever employed
in regions with humid climate. Of course, in humid regions there is less water stress
of crops. But it is also more difficult there to get precise signals. This is because the
lower the vapor pressure deficit is, the smaller the differences of the temperature
spans are, which are the basis of the crop water stress index (Fig.
6.18
). Advances
in measurement techniques may help to get precise signals even under more humid
conditions.
The water situation should be sensed under uniform conditions of illumination.
In order to get signals from the time when the maximal expression of water stress
exists, the sensing should be done around noon and possibly when the sky is clear.
Remote sensing facilitates getting signals that are based precisely on the same time
within the whole field, but the delivery of the results might delay the application.
With proximal sensing the situation is
vice versa
.
A challenging perspective would be
site-specific irrigation
based on either the
crop water stress index or on water sensing via reflectance. This could save water or
improve yields in many cases. Real-time control should be aimed at since irrigation
is a timely matter.
Provided the signals were available, the site-specific irrigation control for sys-
tems with center pivoting or linearly moving booms could be realized via nozzles
thats either pulse or which have variable orifices. With
pulsing nozzles
, the “on” to
“off” time ratio controls the water supply. In case of nozzles with
variable orifices,
,
moving a coned pin towards the direction of the orifice varies the flow rate. For
actuating the site-specific irrigation by these devices, a sophisticated telemetric con-
trol network is feasible. Details to this have been dealt with by Camp et al.
2006
;
Pierce et al.
2006
and Sadler et al.
2000
.
However, there still are obstacles to real time site-specific irrigation. At least this
holds for site-specific irrigation that relies on sprinkler systems and is based on
sensing the water supply via radiation. The path of the radiation that senses the moisture
status should not be obstructed by water drops. These would cause large sensing
