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E.T. Engman
For the active microwave approach over a bare soil, the measured radar backscatter, ss, can be related directly to soil moisture by
(9.7)
where R is a surface roughness term, a is a soil moisture sensitivity term, and Mv
is the volumetric soil moisture. Although R and a are known to vary with wavelength, polarization, and incidence angle, there is no satisfactory theoretical model
suitable for estimating these terms independently. Thus, as is the case for the passive microwave approach, the relationship between measured backscatter and soil
moisture requires an empirical relationship with ground data, even for bare soils.
An additional approach for using soil moisture data derived with microwave approaches is through change detection. This approach can be used for both passive
or active microwave data. The change detection method minimizes the impact of
target variables such as soil texture, roughness, and vegetation because these tend
to change slowly, if at all, with time.
9.3 Sensor-Target Interactions
As discussed above, microwave techniques for measuring soil moisture have a
strong theoretical basis. In addition, they are not limited to cloud-free and bare-soil
conditions because the microwave approach can sense through cloud cover and, in
many cases, through a vegetation canopy. Each of the two basic approaches, passive and active, offer different but distinct advantages. The differences being in
their instrument characteristics and their interaction with the characteristics of the
target.
There are a number of target and target-sensor characteristics that affect the
measurement of soil moisture. These include the effects of soil texture, the depth
of measurement, surface roughness, vegetation effects, and instrument parameters
such as incidence angle and frequency. Each of these are discussed in more detail
below.
Soil Texture. Soil texture affects the microwave sensing of soil moisture in the
way that the dielectric constant changes with the relative amounts of sand, silt, and
clay in the soil. Figure 9.2 shows this effect with laboratory data and an empirical
model developed by Wang and Schmugge (1980). However, it can be seen that
this effect is relatively small and given the overall accuracy of the methods and
uncertainty in other factors, texture effects can be neglected for practical purposes.
Measurement Depth. The same principles control the depth of soil that is being
measured by the microwave technique, whether it is passive, as discussed above,
or active. In a series of careful field experiments with a C-band, HH polarization
radar, Bruckler et al. (1988) showed experimental results of penetration depth
compared with soil moisture that followed very closely to the theoretical curve for
a uniform profile.
The relationship between emissivity and soil moisture depends upon the dielectric contrast across the air-soil interface. Consequently, this results in some uncertainty as to exactly how thick the soil layer is for determining the dielectric con-
E.T. Engman
For the active microwave approach over a bare soil, the measured radar backscatter, ss, can be related directly to soil moisture by
(9.7)
where R is a surface roughness term, a is a soil moisture sensitivity term, and Mv
is the volumetric soil moisture. Although R and a are known to vary with wavelength, polarization, and incidence angle, there is no satisfactory theoretical model
suitable for estimating these terms independently. Thus, as is the case for the passive microwave approach, the relationship between measured backscatter and soil
moisture requires an empirical relationship with ground data, even for bare soils.
An additional approach for using soil moisture data derived with microwave approaches is through change detection. This approach can be used for both passive
or active microwave data. The change detection method minimizes the impact of
target variables such as soil texture, roughness, and vegetation because these tend
to change slowly, if at all, with time.
9.3 Sensor-Target Interactions
As discussed above, microwave techniques for measuring soil moisture have a
strong theoretical basis. In addition, they are not limited to cloud-free and bare-soil
conditions because the microwave approach can sense through cloud cover and, in
many cases, through a vegetation canopy. Each of the two basic approaches, passive and active, offer different but distinct advantages. The differences being in
their instrument characteristics and their interaction with the characteristics of the
target.
There are a number of target and target-sensor characteristics that affect the
measurement of soil moisture. These include the effects of soil texture, the depth
of measurement, surface roughness, vegetation effects, and instrument parameters
such as incidence angle and frequency. Each of these are discussed in more detail
below.
Soil Texture. Soil texture affects the microwave sensing of soil moisture in the
way that the dielectric constant changes with the relative amounts of sand, silt, and
clay in the soil. Figure 9.2 shows this effect with laboratory data and an empirical
model developed by Wang and Schmugge (1980). However, it can be seen that
this effect is relatively small and given the overall accuracy of the methods and
uncertainty in other factors, texture effects can be neglected for practical purposes.
Measurement Depth. The same principles control the depth of soil that is being
measured by the microwave technique, whether it is passive, as discussed above,
or active. In a series of careful field experiments with a C-band, HH polarization
radar, Bruckler et al. (1988) showed experimental results of penetration depth
compared with soil moisture that followed very closely to the theoretical curve for
a uniform profile.
The relationship between emissivity and soil moisture depends upon the dielectric contrast across the air-soil interface. Consequently, this results in some uncertainty as to exactly how thick the soil layer is for determining the dielectric con-