Remote Sensing of Soil Moisture
377
parameters to fit experimentally measured data. The model uses two components: the bound
water fraction and the free water fraction, according to the pore-size distribution calculated
from the particle-size distribution.
Theoretical studies on the permittivity of dielectric mixtures have been carried out in The
Netherlands (de Loor, 1982) and at the Helsinki University of Technology as well (Shivola,
1989). Shutko and Reutov (1982) compared several current models with experimental data
and concluded that, in the frequency range between 1 and 10 GHz, a two component refractive
formula which considers only the volumetric fractions of dry matter and free water, is a good
working model for most soil types. Dielectric measurements of soils in the 0.5-1.0 GHz range
have been performed at the University of Bern (Stotzer et al., 1986) with a surface and a volume
sensor.
14.3.2 Passive sensor measurements
Microwave radiation from soils can be expressed through the Rayleigh Jeans law, by means of
an equivalent ("brightness") temperature n which, for homogeneous media, is related to the
thermodynamic temperature T of the emitting surface through a factor e (emissivity) which
takes into account the loss of energy at a surface and depends on the surface and dielectric characteristics of soils. Radiation collected by an antenna observing terrain (TA) can be expressed
as follows:
TA = t(H) * [rTsky + (1 - r)Tsoill + Tatm
(14.3)
where t(H) is the atmospheric transmittance for a radiometer at a height H above the soil,
r is the smooth surface reflectance (I-e), Tsoil and Tatm are the thermometric temperatures of
soil and atmosphere, respectively and Tsky is the contribution from the reflected sky brightness.
The atmospheric transmission (for wavelengths higher than 5 cm which are typically used for
soil moisture measurements) is around 99%, while Tsoil and Tatm are usually both lower than
5K. Thus these terms can be neglected and n can be expressed as follows:
n = (1 - r)Tsoil = eTsoil
(14.4)
Since e is dependent on the dielectric char.acterisitcs of soil, in the usual range of soil moisture a
decrease of e from 0.95 to 0.60 can be expected with a consequent change of n of about lOOK.
As a first approximation, the soil emissivity of smooth surfaces can be computed from the
Fresnel relations (e.g. Ulaby et al., 1981). In Fig. 14.5 observed and predicted relationships
between emissivity and volumetric soil moisture content for a bare smooth homogeneous soil
at 2.25 cm and 18 cm of wavelength are shown. In the diagram variations of emissivity from ""
0.95 to "" 0.60 as the volumetric SMC changes from 0 to "" 40% are appreciable (Shutko, 1982).
When soil temperature and absorption coefficient change with depth, microwave emission from
soil is due to contributions from all depths with appropriate weights and can be predicted by
means of theoretical models, which are mainly based on the radiative transfer theory (Schmugge
and Choudhury, 1981). According to the models, the fraction of radiation upwelling from soil
is determined by the dielectric properties of a surface layer whose thickness is in the order of
a few tenths of a wavelength (Basharinov et al., 1979). Consequently, sensors operating at the
longer wavelengths of the microwave region are the most suitable for collecting information
on thick soil layers. An example of this sensitivity is shown in the diagram of Fig. 14.6,
where a profile of Tb recorded using a 21 cm airborne radiometer is represented together with
a profile of surface temperature measured with a thermal infrared radiometer. Whereas the
infrared temperature does not change very much, the fluctuations of brightness temperature of
377
parameters to fit experimentally measured data. The model uses two components: the bound
water fraction and the free water fraction, according to the pore-size distribution calculated
from the particle-size distribution.
Theoretical studies on the permittivity of dielectric mixtures have been carried out in The
Netherlands (de Loor, 1982) and at the Helsinki University of Technology as well (Shivola,
1989). Shutko and Reutov (1982) compared several current models with experimental data
and concluded that, in the frequency range between 1 and 10 GHz, a two component refractive
formula which considers only the volumetric fractions of dry matter and free water, is a good
working model for most soil types. Dielectric measurements of soils in the 0.5-1.0 GHz range
have been performed at the University of Bern (Stotzer et al., 1986) with a surface and a volume
sensor.
14.3.2 Passive sensor measurements
Microwave radiation from soils can be expressed through the Rayleigh Jeans law, by means of
an equivalent ("brightness") temperature n which, for homogeneous media, is related to the
thermodynamic temperature T of the emitting surface through a factor e (emissivity) which
takes into account the loss of energy at a surface and depends on the surface and dielectric characteristics of soils. Radiation collected by an antenna observing terrain (TA) can be expressed
as follows:
TA = t(H) * [rTsky + (1 - r)Tsoill + Tatm
(14.3)
where t(H) is the atmospheric transmittance for a radiometer at a height H above the soil,
r is the smooth surface reflectance (I-e), Tsoil and Tatm are the thermometric temperatures of
soil and atmosphere, respectively and Tsky is the contribution from the reflected sky brightness.
The atmospheric transmission (for wavelengths higher than 5 cm which are typically used for
soil moisture measurements) is around 99%, while Tsoil and Tatm are usually both lower than
5K. Thus these terms can be neglected and n can be expressed as follows:
n = (1 - r)Tsoil = eTsoil
(14.4)
Since e is dependent on the dielectric char.acterisitcs of soil, in the usual range of soil moisture a
decrease of e from 0.95 to 0.60 can be expected with a consequent change of n of about lOOK.
As a first approximation, the soil emissivity of smooth surfaces can be computed from the
Fresnel relations (e.g. Ulaby et al., 1981). In Fig. 14.5 observed and predicted relationships
between emissivity and volumetric soil moisture content for a bare smooth homogeneous soil
at 2.25 cm and 18 cm of wavelength are shown. In the diagram variations of emissivity from ""
0.95 to "" 0.60 as the volumetric SMC changes from 0 to "" 40% are appreciable (Shutko, 1982).
When soil temperature and absorption coefficient change with depth, microwave emission from
soil is due to contributions from all depths with appropriate weights and can be predicted by
means of theoretical models, which are mainly based on the radiative transfer theory (Schmugge
and Choudhury, 1981). According to the models, the fraction of radiation upwelling from soil
is determined by the dielectric properties of a surface layer whose thickness is in the order of
a few tenths of a wavelength (Basharinov et al., 1979). Consequently, sensors operating at the
longer wavelengths of the microwave region are the most suitable for collecting information
on thick soil layers. An example of this sensitivity is shown in the diagram of Fig. 14.6,
where a profile of Tb recorded using a 21 cm airborne radiometer is represented together with
a profile of surface temperature measured with a thermal infrared radiometer. Whereas the
infrared temperature does not change very much, the fluctuations of brightness temperature of