Agriculture Reference
In-Depth Information
mode lends itself primarily for the direct control of farming operations, whereas
remote sensing is especially suited for tactical inspections of large fields within a
short time. The steadily increasing number of satellites improves the perspective to
perform such remote tactical inspections frequently and hence to sense the occa-
sionally fast changing water situations. This might be valuable for site-specific
irrigation practices.
6.5.2
Sensing Water by Emitted Thermal Infrared Radiation
This method of sensing the water supply of crops relies on the
transpiration
that
takes place at the surface of leaves. The sensing signals are based on the cooling
effect of water transpiration on the canopy. The energy for converting the liquid
water to the gaseous state causes a cooling of the canopy. Because of this, on a very
hot day it can be more comfortable for humans to be inside a dense forest.
The more water is transpired, the more the canopy temperature is below the tem-
perature of the surrounding air. The
crop water stress index
(CWSI)
, developed by
the U.S. Water Conservation Laboratory in Arizona (Jackson et al.
1981
) depends
on this.
The main criterion of the crop water stress index therefore is the
temperature
difference
between the canopy leaves and the air. If a crop has water stress and
therefore cannot transpire, there is hardly any difference between leaf- and air tem-
perature. The red upper baseline in Fig.
6.18
stands for this situation. For the not
water stressed crop, the transpiration depends on the relative humidity of the air.
The lower the relative humidity is, the more the crop transpires. And the more the crop
transpires, the lower the temperatures of the leaves. The green lower baseline
(Fig.
6.18
) represents the case of the fully transpiring, non water stressed crop. The
vertical distances between the upper and lower baseline define the differences of the
temperature span between leaves and air that occur when non transpiring crops on
the one hand with fully transpiring plants on the other hand are compared. It should
be realized that this comparison is about
differences of differences
.
The graphical interpretation of a specific case starts with a point of the actual
situation,
e.g.
point B in Fig.
6.18
. This point has a respective leaf- minus air
temperature. In addition, it is located at a distinct distance to the lower baseline
.
This distance to the lower baseline represents the absolute situation. However,
this absolute situation should be referenced or normalized. The
normalization
is
done by dividing the absolute situation for point B by the total distance between
the upper- and the lower baseline. This quotient is the crop water stress index
(CWSI), which has differences of differences in the numerator as well as in the
denominator (Fig.
6.18
). A CWSI of 0 indicates that a crop is fully transpiring
and hence is well supplied, whereas a value of 1 means maximal water stress.
Generally, watering is recommended when the CWSI value for maize is above 0.22
(Irmak et al.
2000
).
