Agriculture Reference
In-Depth Information
30
30
4
5
4
3
3
2
2
5
20
20
1
1
10
10
frequency: 1.4 GHz
wavelength: 21 cm
frequency: 5 GHz
wavelength: 6 cm
0
0
0
10
20
30
40
50
0
10
20
30
40
50
volumetric water content in %
volumetric water content in %
order of decreasing sand content
1 sandy loam
2 loam
3 silt loam
4 silt loam
5 clay
30
30
4
3
5
2
4
20
20
3
1
2
5
1
10
10
frequency: 18 GHz
wavelength: 1.7 cm
frequency: 10 GHz
wavelength: 3 cm
0
0
0
10
20
30
40
50
0
10
20
30
40
50
volumetric water content in %
volumetric water content in %
Fig. 5.16
Real permittivites or dielectric constants of five soils sensed with different frequencies
(Compiled from Hallikainen et al.
1985
)
permittivities presented in Fig.
5.16
are based on experiments in a laboratory and
hence refer to well controlled conditions. The respective volumetric water con-
tents were obtained by adding water to soils that had been completely dehydrated
in a drying oven before. This means that the moistures indicated include also
bound water. And since bound water has lower real permittivities or dielectric
constants than free water (Fig.
5.15
), this should show up when soils that differ in
their texture and thus in the sand- or clay content are compared.
This assumption is supported by the curves for the soils with different textures.
For all frequencies and wavelengths that were used, it shows that the sandy soils had
the highest real permittivities and the clay soils the lowest (Fig.
5.16
).
However, the effect of soil texture depends on the micowave properties. The dif-
ferences between the soil types are quite apparent when low frequencies of 1.4 GHz
and correspondingly long waves were used, but get stepwise smaller with higher
frequencies and hence shorter waves. So in order to account for the bound water -
which is not available to plants - long microwaves are needed.
Very important is the
depth of sensing
. In case the signals are reflected only
from the soil surface, which is the case with visible- and NIR radiation, a few
raindrops or even dew can cause misleading results. Fortunately, microwaves do
