band for different values of soil roughness. The microwave radiometer measurement of soil moisture is also affected by the density of any vegetative cover,
including grass, shrubs, and trees.
Radars measure the backscattering coefficient, which is a measure of reflectivity.
Passive and active methods are related through Kirchhoff’s law, e ¼ 1Àr, where e is
the emissivity and r is the reflectance. Since an increase in soil moisture decreases
its emissivity, it simultaneously increases the radar reflectivity or backscatter.
Radar is more sensitive to surface roughness and dense vegetation structure than
optical sensors but is less affected by surface temperature and provides good spatial
resolution.
Early remote sensing of soil moisture was performed from aircraft [29–
31]. More recently satellite sensors are numerous and able to provide information
on surface soil moisture. Remotely sensed surface soil moisture data sets have been
acquired with scatterometer observations of the Active Microwave Instrument
(AMI) on the European Remote Sensing satellites (ERS-AMI) and the Advanced
Scatterometer (ASCAT) on MetOp. Multifrequency radiometers have also been
used, including the Advanced Scanning Microwave Radiometer (AMSR-E), the
Scanning Multichannel Microwave Radiometer (SMMR), and the Microwave
Imager (TMI) on the Tropical Rainfall Measuring Mission (TRMM). Yet despite
the importance of soil moisture information, until recently there have been no
projects specifically dedicated to measuring soil moisture globally with adequate
temporal or spatial sampling [32–34].
With the launch of microwave radiometers on the Soil Moisture and Ocean
Salinity (SMOS) and AQUA satellites, soil moisture (along with other parameters
such as sea surface salinity) can now be obtained nearly continuously over a large
fraction of the Earth’s surface [35, 36]. AQUA is a sister satellite to Terra, the first
of the large Earth observation satellites (EOS), launched in 1999 to monitor the
“health of the planet,” with Terra emphasizing land and AQUA emphasizing water.
AQUA and SMOS data are providing information on the moisture content of the
soil, vegetation conditions (heavily dependent on water), and on many other aspects
of the Earth’s climate system.
The L-band 2-D interferometric radiometer on SMOS receives the radiation
emitted from the Earth’s surface, which then can be related to the moisture content
in the first few centimeters of soil over land [36–39]. Microwave measurements are
largely unaffected by solar illumination and cloud cover, yet accurate soil moisture
estimates are still limited to regions that have either bare soils or low amounts of
vegetation cover. In the absence of significant vegetation cover, soil moisture
dominates the signal received by a microwave radiometer [40, 41]. The
low-frequency microwave range of 1–3 GHz (10–30 cm wavelength) is considered
best for soil moisture sensing due to its sensitivity to soil moisture, reduced
atmospheric attenuation, and greater vegetation penetration at these longer wavelengths. Since mid-July 2010, SMOS has been delivering images of “brightness
temperature” to the science community, which are used to produce global maps of
soil moisture every 3 days (along with maps of ocean salinity) averaged over
30 days.
40
V. Klemas and A. Pieterse
including grass, shrubs, and trees.
Radars measure the backscattering coefficient, which is a measure of reflectivity.
Passive and active methods are related through Kirchhoff’s law, e ¼ 1Àr, where e is
the emissivity and r is the reflectance. Since an increase in soil moisture decreases
its emissivity, it simultaneously increases the radar reflectivity or backscatter.
Radar is more sensitive to surface roughness and dense vegetation structure than
optical sensors but is less affected by surface temperature and provides good spatial
resolution.
Early remote sensing of soil moisture was performed from aircraft [29–
31]. More recently satellite sensors are numerous and able to provide information
on surface soil moisture. Remotely sensed surface soil moisture data sets have been
acquired with scatterometer observations of the Active Microwave Instrument
(AMI) on the European Remote Sensing satellites (ERS-AMI) and the Advanced
Scatterometer (ASCAT) on MetOp. Multifrequency radiometers have also been
used, including the Advanced Scanning Microwave Radiometer (AMSR-E), the
Scanning Multichannel Microwave Radiometer (SMMR), and the Microwave
Imager (TMI) on the Tropical Rainfall Measuring Mission (TRMM). Yet despite
the importance of soil moisture information, until recently there have been no
projects specifically dedicated to measuring soil moisture globally with adequate
temporal or spatial sampling [32–34].
With the launch of microwave radiometers on the Soil Moisture and Ocean
Salinity (SMOS) and AQUA satellites, soil moisture (along with other parameters
such as sea surface salinity) can now be obtained nearly continuously over a large
fraction of the Earth’s surface [35, 36]. AQUA is a sister satellite to Terra, the first
of the large Earth observation satellites (EOS), launched in 1999 to monitor the
“health of the planet,” with Terra emphasizing land and AQUA emphasizing water.
AQUA and SMOS data are providing information on the moisture content of the
soil, vegetation conditions (heavily dependent on water), and on many other aspects
of the Earth’s climate system.
The L-band 2-D interferometric radiometer on SMOS receives the radiation
emitted from the Earth’s surface, which then can be related to the moisture content
in the first few centimeters of soil over land [36–39]. Microwave measurements are
largely unaffected by solar illumination and cloud cover, yet accurate soil moisture
estimates are still limited to regions that have either bare soils or low amounts of
vegetation cover. In the absence of significant vegetation cover, soil moisture
dominates the signal received by a microwave radiometer [40, 41]. The
low-frequency microwave range of 1–3 GHz (10–30 cm wavelength) is considered
best for soil moisture sensing due to its sensitivity to soil moisture, reduced
atmospheric attenuation, and greater vegetation penetration at these longer wavelengths. Since mid-July 2010, SMOS has been delivering images of “brightness
temperature” to the science community, which are used to produce global maps of
soil moisture every 3 days (along with maps of ocean salinity) averaged over
30 days.
40
V. Klemas and A. Pieterse