Agriculture Reference
In-Depth Information
6.5
Sensing the Water Supply of Crops by Infrared
Radiation
The perspectives of crop development depend not only on the chlorophyll situation
but equally well on the water supply. Hence information about the water supply is
needed too for the control of several farming operations such as irrigation or the
application of farm chemicals. And getting this information directly from the crop
instead of from the soil has distinct advantages. This automatically takes into
account the site-specific influence of water-tension within the soil on the supply of
the plants. Depending on the respective soil texture, the water tension varies consid-
erably and thus the amount of water available. Furthermore, when sensing the supply
of the crop instead of the soil, the influence of the root development on the situation
is taken care of without any attention. And the root development varies during the
growing season as well as on a site-specific basis.
Signals can be derived either from reflected radiation or from emitted thermal
radiation .
6.5.1
Sensing Water by Near- and Shortwave-Infrared
Relectance
The visible wavelengths can be left out for water sensing from crop canopies.
Instead, the reflected infrared domain can be considered as basis for signals. As with
many other applications for spectral sensing, generally the best results are not
obtained with wide bands that extend over long wavelength ranges within the spec-
trum, but instead from narrow bands or indices that use them. Such hyperspectral
bands ranging from 1 to 10 nm width allow to select the most sensible region within
the spectrum and to avoid the weakening or averaging effect that inevitably is asso-
ciated with wide bands.
Soil- and canopy sensing differ not only in the region of the spectrum that is used
(Fig. 6.2 ). With soil sensing via reflectance, a fundamental problem is the fact that
it is based solely on the top-surface that is hit by the radiation. In order to get infor-
mation from below the surface, sensing of vertical soil profiles is necessary.
This can only be done via proximal sensing from terrestrial vehicles or from farm
machines that cut a slit into the soil. This limitation does not exist for sensing of
crops by infrared radiation because this penetrates well into the vegetation and is
reflected back from various layers below the canopy surface. Consequently, canopy
sensing - contrary to soil sensing - lends itself for proximal sensing as well as for
remote sensing from satellites or from aerial platforms.
Which wavelengths of narrow band reflectance can indicate the crop water sup-
ply? Table 6.2 shows results of experiments, for which the coefficients of determi-
nation (r 2 ) were above 0.60. The bandwidths of the wavelengths listed were between
1.5 and 10.0 nm with one exception, for which the range is indicated.
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