370
S. Paloscia
water content and also decreases quite rapidly as soil moisture decreases. This means that the
gradients in soil water potential can change significantly in order to transport the same amount
of water through the soil-root medium to the atmosphere. Changes in the water potentials
of soil, roots and atmosphere influence the transpiration stream and therefore the amount of
water released in the atmosphere through the evapotranspirative mechanisms (Denmead, 1976;
Kramer, 1969).
The hydrological cycle is the recirculation of water from ocean to water vapour through the
evaporation processes, the transformation of water vapour in precipitation and its return in
the cycle through infiltration and evapotranspiration again. Although the process is longer and
more complicated than it has been outlined here, we can focus on some simplified aspects. The
water stored in soil and vegetation is an important fraction of the total amount of water involved
in the hydrological cycle and represents a conspicuous quantity subtracted to the rapid loss due
to runoff. The Soil Moisture Content (SMC), besides representing a water resource for plants,
affects the separation of infiltration from runoff and it is therefore an important parameter for
assessing erosion hazards and estimating evapotranspiration. A timely and precise information
concerning water conditions ofthe soil-vegetation system can allow, therefore, a correct schedule
of sowing and of irrigation, limiting wastes or lacks of water, and a reliable forecast of final
yield (Kramer, 1969; Engman and Gurney, 1991).
There are basically two methods available for obtaining a reliable estimate of soil moisture
content: in situ and remote sensing methods. In situ methods allow to retrieve a precise
estimation of SMC in a very concentrated spot and a lot of samples must be collected in order
to have a representative sampling of the examined area. Moreover, such a vast sampling is
time-consuming and needs the work of several people if a reliable measurement of SMC has to
be made in a relatively short time.
The recent development of remote sensing sensors has made it possible to begin the study
of hydrological parameters in a quantitative way. The possibility to monitor a large area in
a very short time and measure integrated values of some geophysical parameters instead of
several sparse points is in fact very interesting. In particular, these methods can significantly
contribute to the estimate of one of the key parameters involved in water balance which is the
moisture content of soils on both a global and a regional scale, whose measurement is of primary
importance in the process of modelling the hydrological cycle. Another advantage of remote
sensing techniques is the possibility of checking the evolution in time of these parameters and
thus improve hydrological models prediction by using repeated satellite passages.
In the following, after a short summary of theoretical statements, the state-of-art applications
of remote sensing methods to soil moisture estimation will be reviewed.
14.2 Remote sensing methods
The electromagnetic spectrum covered by the various sensing methods ranges from very short
wavelengths of gamma and x rays (mA) to very long wavelengths of radio bands (km) and it
is shown in Fig. 14.1. Electromagnetic thermal radiation is described by the Planck law for
which a blackbody at a temperature close to that of the Earth's surface (::::: 300K) reaches a
maximum in the thermal infrared region (10-14 /tm) (Fig. 14.2). In the microwave range the
spectral brightness (i.e. the radiative power per square meter, per unit of solid angle and per
unit bandwidth of frequency) of a blackbody can be approximated by the Rayleigh-Jeans law,
which directly relates the spectral radiance to the surface temperature (e.g. Ulaby et al., 1981).
S. Paloscia
water content and also decreases quite rapidly as soil moisture decreases. This means that the
gradients in soil water potential can change significantly in order to transport the same amount
of water through the soil-root medium to the atmosphere. Changes in the water potentials
of soil, roots and atmosphere influence the transpiration stream and therefore the amount of
water released in the atmosphere through the evapotranspirative mechanisms (Denmead, 1976;
Kramer, 1969).
The hydrological cycle is the recirculation of water from ocean to water vapour through the
evaporation processes, the transformation of water vapour in precipitation and its return in
the cycle through infiltration and evapotranspiration again. Although the process is longer and
more complicated than it has been outlined here, we can focus on some simplified aspects. The
water stored in soil and vegetation is an important fraction of the total amount of water involved
in the hydrological cycle and represents a conspicuous quantity subtracted to the rapid loss due
to runoff. The Soil Moisture Content (SMC), besides representing a water resource for plants,
affects the separation of infiltration from runoff and it is therefore an important parameter for
assessing erosion hazards and estimating evapotranspiration. A timely and precise information
concerning water conditions ofthe soil-vegetation system can allow, therefore, a correct schedule
of sowing and of irrigation, limiting wastes or lacks of water, and a reliable forecast of final
yield (Kramer, 1969; Engman and Gurney, 1991).
There are basically two methods available for obtaining a reliable estimate of soil moisture
content: in situ and remote sensing methods. In situ methods allow to retrieve a precise
estimation of SMC in a very concentrated spot and a lot of samples must be collected in order
to have a representative sampling of the examined area. Moreover, such a vast sampling is
time-consuming and needs the work of several people if a reliable measurement of SMC has to
be made in a relatively short time.
The recent development of remote sensing sensors has made it possible to begin the study
of hydrological parameters in a quantitative way. The possibility to monitor a large area in
a very short time and measure integrated values of some geophysical parameters instead of
several sparse points is in fact very interesting. In particular, these methods can significantly
contribute to the estimate of one of the key parameters involved in water balance which is the
moisture content of soils on both a global and a regional scale, whose measurement is of primary
importance in the process of modelling the hydrological cycle. Another advantage of remote
sensing techniques is the possibility of checking the evolution in time of these parameters and
thus improve hydrological models prediction by using repeated satellite passages.
In the following, after a short summary of theoretical statements, the state-of-art applications
of remote sensing methods to soil moisture estimation will be reviewed.
14.2 Remote sensing methods
The electromagnetic spectrum covered by the various sensing methods ranges from very short
wavelengths of gamma and x rays (mA) to very long wavelengths of radio bands (km) and it
is shown in Fig. 14.1. Electromagnetic thermal radiation is described by the Planck law for
which a blackbody at a temperature close to that of the Earth's surface (::::: 300K) reaches a
maximum in the thermal infrared region (10-14 /tm) (Fig. 14.2). In the microwave range the
spectral brightness (i.e. the radiative power per square meter, per unit of solid angle and per
unit bandwidth of frequency) of a blackbody can be approximated by the Rayleigh-Jeans law,
which directly relates the spectral radiance to the surface temperature (e.g. Ulaby et al., 1981).