example, the measured neutron intensity can be affected by variations in the atmospheric water vapour that, when not corrected for, ultimately can lead to bias in the
derived soil moisture [36]. Therefore correction procedures must be carried out such
as those studies by [33, 36–38].
2.2 Satellite Remote Sensing
Compared with in situ methods, satellite remote sensing provides soil moisture
observations globally and at larger footprints, so it is more suitable for hydrological
usages. A considerable number of studies have shown that near-surface soil moisture
(~5 cm) can be measured by many remote sensing techniques including optical,
thermal infrared and microwave [39, 40]. The major differences among them are
the region of electromagnetic spectrum employed, the power of the corresponding
electromagnetic energy, the signal received by the sensors and the relationship
between the retrieved signal and the soil moisture [10, 11, 40]. Table 1 lists the
advantages and limitations of each technique for surface soil moisture measurement
and their characteristics [44].
Only a brief description of each technique is introduced as follows, and interested
readers are encouraged to read further details from the references provided.
2.2.1 Optical
Optical satellites measure the reflected radiation of the Sun from Earth’s surface,
known as the reflectance [45]. Its correlation with the soil moisture has long
been recognised [46]. Although there are a large number of optical sensors currently
serving in orbit, relatively fewer studies have been carried out regarding their
application in soil moisture assessment [47]. This is partially because the optical
sensors can only detect the reflectance or emittance at the top few millimetres of
Earth surface. Compared with the longer microwave wavelength, the optical signal
is highly affected by cloud contamination and vegetation cover. Furthermore the
received soil reflectance is not solely affected by the soil moisture but also influenced
by mineral composition, organic matter, soil texture and observation conditions,
which makes this technique less popular for soil moisture estimation [42, 48]. Therefore the optical technique is normally applicable only under restricted conditions
for soil moisture determination (e.g. with specific soil types, bare soil and climate
dominated by clear sky) [47, 49].
2.2.2 Thermal Infrared
Thermal infrared satellites measure Earth radiative temperature, which is then
converted to soil moisture either singularly or by combination with the vegetation
index information obtained from the optical wavebands (e.g. Normalised Difference
Satellite Remote Sensing of Soil Moisture for Hydrological Applications. . .
263
derived soil moisture [36]. Therefore correction procedures must be carried out such
as those studies by [33, 36–38].
2.2 Satellite Remote Sensing
Compared with in situ methods, satellite remote sensing provides soil moisture
observations globally and at larger footprints, so it is more suitable for hydrological
usages. A considerable number of studies have shown that near-surface soil moisture
(~5 cm) can be measured by many remote sensing techniques including optical,
thermal infrared and microwave [39, 40]. The major differences among them are
the region of electromagnetic spectrum employed, the power of the corresponding
electromagnetic energy, the signal received by the sensors and the relationship
between the retrieved signal and the soil moisture [10, 11, 40]. Table 1 lists the
advantages and limitations of each technique for surface soil moisture measurement
and their characteristics [44].
Only a brief description of each technique is introduced as follows, and interested
readers are encouraged to read further details from the references provided.
2.2.1 Optical
Optical satellites measure the reflected radiation of the Sun from Earth’s surface,
known as the reflectance [45]. Its correlation with the soil moisture has long
been recognised [46]. Although there are a large number of optical sensors currently
serving in orbit, relatively fewer studies have been carried out regarding their
application in soil moisture assessment [47]. This is partially because the optical
sensors can only detect the reflectance or emittance at the top few millimetres of
Earth surface. Compared with the longer microwave wavelength, the optical signal
is highly affected by cloud contamination and vegetation cover. Furthermore the
received soil reflectance is not solely affected by the soil moisture but also influenced
by mineral composition, organic matter, soil texture and observation conditions,
which makes this technique less popular for soil moisture estimation [42, 48]. Therefore the optical technique is normally applicable only under restricted conditions
for soil moisture determination (e.g. with specific soil types, bare soil and climate
dominated by clear sky) [47, 49].
2.2.2 Thermal Infrared
Thermal infrared satellites measure Earth radiative temperature, which is then
converted to soil moisture either singularly or by combination with the vegetation
index information obtained from the optical wavebands (e.g. Normalised Difference
Satellite Remote Sensing of Soil Moisture for Hydrological Applications. . .
263