400
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
limited to discrete observations at point locations, and such sparse measurements
cannot be simply extrapolated to represent the highly variable soil moisture at the
regional or global scale.
Therefore, satellite remote sensing is desired to retrieve global soil moisture
information that can potentially improve numerical weather prediction, flood risk
assessment, and agricultural and water management efficiency (Betts et al. 1996;
Entekhabi et al. 2010). This chapter overviews the remote sensing techniques and
satellite missions for soil moisture estimations, with particular focus on commonly
used microwave remote sensing. Due to page limits, the list of the sensors covered in
this chapter is in no way a complete representation of the numerous remote sensingbased soil moisture retrieval methods.
17.2 MICROWAVE REMOTE SENSING OF SOIL MOISTURE
Satellite remote sensing instruments used for soil moisture estimations include
the multispectral scanner, thermal infrared scanner, thematic mapper, synthetic
aperture radar (SAR), and microwave radiometers (Walker 1999; Wang and Qu
2009). The main differences among these sensors and techniques are the wavelength used from the electromagnetic spectrum, the source of the electromagnetic
energy, the response measured by the sensor, and the physical relation between the
response and the soil moisture content. The sensitivity of microwave responses to
soil moisture variations and the relative transparency of microwaves to the atmosphere make microwave sensors especially well suited for remote sensing of soil
moisture (Schmugge et al. 1974). More importantly, microwave signals can penetrate, to a certain extent, the vegetation canopy and retrieve information from
the subsurface (Brown et al. 1992; Engman 1991; Schmullius and Furrer 1992).
Currently, microwave radiometers are being actively investigated for soil moisture
estimation; therefore, in this chapter, we briefly discuss microwave techniques, followed by a discussion of possible applications at the global scale.
17.2.1 active MicRowave ReMote SenSing of Soil MoiStuRe
Active microwave sensors are based on the technique of Radio Detection and
Ranging (RaDaR). This type of method consists of a transmitter that emits the radiation toward the earth’s surface and an antenna that measures the returning backscattered radiation. The strength of the backscattered signal, measured to discriminate
between dry and wet soils and the time delay between the transmitted and reflected
signals, determines the distance to the surface. The ratio of the strength of the emitted to transmitted signals, termed the backscattering coefficient, depends on the surface reflectivity and the antenna characteristics (Behari 2005).
Active microwave sensors have shown great potential in high-spatial-resolution
soil moisture estimation for catchment-based hydrologic applications (Makkeasorn
et al. 2006). Spaceborne active microwave sensors such as SAR are able to provide
high spatial resolution (up to 1 m) but have relatively low temporal resolution compared with passive microwave sensors (an important difference between the two
types of sensors). For example, passive microwave sensors generally provide low
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
limited to discrete observations at point locations, and such sparse measurements
cannot be simply extrapolated to represent the highly variable soil moisture at the
regional or global scale.
Therefore, satellite remote sensing is desired to retrieve global soil moisture
information that can potentially improve numerical weather prediction, flood risk
assessment, and agricultural and water management efficiency (Betts et al. 1996;
Entekhabi et al. 2010). This chapter overviews the remote sensing techniques and
satellite missions for soil moisture estimations, with particular focus on commonly
used microwave remote sensing. Due to page limits, the list of the sensors covered in
this chapter is in no way a complete representation of the numerous remote sensingbased soil moisture retrieval methods.
17.2 MICROWAVE REMOTE SENSING OF SOIL MOISTURE
Satellite remote sensing instruments used for soil moisture estimations include
the multispectral scanner, thermal infrared scanner, thematic mapper, synthetic
aperture radar (SAR), and microwave radiometers (Walker 1999; Wang and Qu
2009). The main differences among these sensors and techniques are the wavelength used from the electromagnetic spectrum, the source of the electromagnetic
energy, the response measured by the sensor, and the physical relation between the
response and the soil moisture content. The sensitivity of microwave responses to
soil moisture variations and the relative transparency of microwaves to the atmosphere make microwave sensors especially well suited for remote sensing of soil
moisture (Schmugge et al. 1974). More importantly, microwave signals can penetrate, to a certain extent, the vegetation canopy and retrieve information from
the subsurface (Brown et al. 1992; Engman 1991; Schmullius and Furrer 1992).
Currently, microwave radiometers are being actively investigated for soil moisture
estimation; therefore, in this chapter, we briefly discuss microwave techniques, followed by a discussion of possible applications at the global scale.
17.2.1 active MicRowave ReMote SenSing of Soil MoiStuRe
Active microwave sensors are based on the technique of Radio Detection and
Ranging (RaDaR). This type of method consists of a transmitter that emits the radiation toward the earth’s surface and an antenna that measures the returning backscattered radiation. The strength of the backscattered signal, measured to discriminate
between dry and wet soils and the time delay between the transmitted and reflected
signals, determines the distance to the surface. The ratio of the strength of the emitted to transmitted signals, termed the backscattering coefficient, depends on the surface reflectivity and the antenna characteristics (Behari 2005).
Active microwave sensors have shown great potential in high-spatial-resolution
soil moisture estimation for catchment-based hydrologic applications (Makkeasorn
et al. 2006). Spaceborne active microwave sensors such as SAR are able to provide
high spatial resolution (up to 1 m) but have relatively low temporal resolution compared with passive microwave sensors (an important difference between the two
types of sensors). For example, passive microwave sensors generally provide low