Remote Sensing of Soil Moisture
375
a unique relationship between plant temperature and soil moisture does not exist and that the
same alterations can be caused by both drought and diseases or temporary water stress due
to, for instance, windy days. Moreover, other parameters such as net radiation and air vapour
deficit are necessary to define the evapotranspirative conditions of vegetation mainly in a humid
environment (Campbell and Norman, 1990).
14.3 Microwave Sensors
Extensive investigations have pointed out that, due to the large difference between the microwave dielectric properties of dry soil and liquid water, microwave sensors are by far the most
efficient SMC detectors, although some limitations still exist on their use from spacecraft (Haefner and Pampaloni, 1992). Two kinds of sensors can be used: (i) radiometers able to measure
thermal emission from soil, and (ii) radars which detect echoes of electromagnetic energy selftransmitted by them and reflected back to the sensor. In both cases the received radiation can
be related to the moisture of bare or vegetated soil, according to the observation frequency,
and for a layer of about 5 cm. As it will be discussed in the following the best frequency from
an operative point of view for soil moisture measurements is around 1.5 GHz (approximately
21 cm wavelength).
14.3.1 Dielectric properties of soils
The two fundamental parameters measured by active and passive microwave sensors are respectively the backscattering coefficient (0'0), which describes the scattering characteristics of
a surface in the direction of the sensor, and the brightness temperature (Tb), which measures
microwave thermal emission. Both these parameters are strongly influenced by the dielectric
properties of the observed media, which are characterized by their complex dielectric constant
or permittivity. The dielectric constant (€) is a measure of the response of the soil to an electromagnetic wave. This response is composed of two parts (real and imaginary) which determine
the wave velocity and the energy losses in the medium, respectively. The square root of € determines the index of refraction for the soil. The dielectric constant of most dielectric natural
media is between 1 and 6 and increases up to 80 for free liquid water. This is the reason for the
very high sensitivity of microwaves to the moisture contained in the observed bodies, depending
on the molecule's ability to align its dipole moment along an applied field; anything that hinders
the molecule's rotation (e.g. freezing, tight binding to a soil particle, etc.) therefore reduces
the € of water. Since the first water quantity added to dry soil is tightly bound to the surface
of the particles, it will cause only a small increase of soil's Eo As more water is added, above a
transition value of moisture (Wt ), the soil's € rapidly increases because the additional molecules
are far from the soil particle surface and free to rotate (Schmugge, 1980). This explains the
effects of soil texture in that the surface area depends on the particle size or texture; clay soils
in fact, with a greater surface area per volume, hold more bound water molecules than sandy
soils.
Soils are heterogeneous systems containing solid particles, bound and free water, air, voids, salts,
etc. and their permittivity depends on that of their constituents but it is not a weighted average.
Many empirical and theoretical models have been suggested to relate the dielectric constant of
the mixture to that of its constituents, but the exact relationship is not yet well understood.
An extensive investigation, which covers laboratory measurements as well as empirical and
theoretical dielectric mixing models in the frequency range between 1.4 and 18 GHz, has been
carried out at the University of Kansas (USA) by Hallikainen et al. (1985). The measurements
375
a unique relationship between plant temperature and soil moisture does not exist and that the
same alterations can be caused by both drought and diseases or temporary water stress due
to, for instance, windy days. Moreover, other parameters such as net radiation and air vapour
deficit are necessary to define the evapotranspirative conditions of vegetation mainly in a humid
environment (Campbell and Norman, 1990).
14.3 Microwave Sensors
Extensive investigations have pointed out that, due to the large difference between the microwave dielectric properties of dry soil and liquid water, microwave sensors are by far the most
efficient SMC detectors, although some limitations still exist on their use from spacecraft (Haefner and Pampaloni, 1992). Two kinds of sensors can be used: (i) radiometers able to measure
thermal emission from soil, and (ii) radars which detect echoes of electromagnetic energy selftransmitted by them and reflected back to the sensor. In both cases the received radiation can
be related to the moisture of bare or vegetated soil, according to the observation frequency,
and for a layer of about 5 cm. As it will be discussed in the following the best frequency from
an operative point of view for soil moisture measurements is around 1.5 GHz (approximately
21 cm wavelength).
14.3.1 Dielectric properties of soils
The two fundamental parameters measured by active and passive microwave sensors are respectively the backscattering coefficient (0'0), which describes the scattering characteristics of
a surface in the direction of the sensor, and the brightness temperature (Tb), which measures
microwave thermal emission. Both these parameters are strongly influenced by the dielectric
properties of the observed media, which are characterized by their complex dielectric constant
or permittivity. The dielectric constant (€) is a measure of the response of the soil to an electromagnetic wave. This response is composed of two parts (real and imaginary) which determine
the wave velocity and the energy losses in the medium, respectively. The square root of € determines the index of refraction for the soil. The dielectric constant of most dielectric natural
media is between 1 and 6 and increases up to 80 for free liquid water. This is the reason for the
very high sensitivity of microwaves to the moisture contained in the observed bodies, depending
on the molecule's ability to align its dipole moment along an applied field; anything that hinders
the molecule's rotation (e.g. freezing, tight binding to a soil particle, etc.) therefore reduces
the € of water. Since the first water quantity added to dry soil is tightly bound to the surface
of the particles, it will cause only a small increase of soil's Eo As more water is added, above a
transition value of moisture (Wt ), the soil's € rapidly increases because the additional molecules
are far from the soil particle surface and free to rotate (Schmugge, 1980). This explains the
effects of soil texture in that the surface area depends on the particle size or texture; clay soils
in fact, with a greater surface area per volume, hold more bound water molecules than sandy
soils.
Soils are heterogeneous systems containing solid particles, bound and free water, air, voids, salts,
etc. and their permittivity depends on that of their constituents but it is not a weighted average.
Many empirical and theoretical models have been suggested to relate the dielectric constant of
the mixture to that of its constituents, but the exact relationship is not yet well understood.
An extensive investigation, which covers laboratory measurements as well as empirical and
theoretical dielectric mixing models in the frequency range between 1.4 and 18 GHz, has been
carried out at the University of Kansas (USA) by Hallikainen et al. (1985). The measurements