405
Global Soil Moisture Estimation Using Microwave Remote Sensing
generate images of relatively higher azimuthal resolution, even using a physically
small antenna at longer radar wavelength (i.e., 24-cm L-band). Examples of spaceborne active microwave sensors for soil moisture measurements include Radarsat 2
SAR, ENVISAT ASAR, and ALOS/PARSAR. Table 17.2 lists the system parameters of current commonly used SAR-based radar imaging systems.
As shown in Table 17.2, active microwave sensors for soil moisture estimation generally suffer from low temporal resolution, an important difference from passive microwave sensors, so there are no SAR-based soil moisture products suitable for daily or
subdaily hydrological applications on a global basis. However, active microwave sensors have great potential in high-spatial-resolution soil moisture estimation (up to a 1-m
grid scale) for catchment-based hydrological applications (Makkeasorn et al. 2006).
Additionally, active sensors are more sensitive to surface characteristics, implying
potentially higher accuracy of soil moisture estimations (Baghdadi et al. 2008).
17.4 HYDROLOGICAL APPLICATIONS OF
SOIL MOISTURE PRODUCTS
Satellite-based soil moisture estimates with global coverage can be useful for many
research and practical applications across several disciplines, including hydrology,
meteorology, ecology, and climatology. For example, knowledge of antecedent soil
moisture conditions provides a key source of predictability for hydrologic modeling.
Such information can be retrieved from combined active and passive microwave
instruments aboard spaceborne satellites. In a previous global hydrological runoff
simulation, the satellite TRMM rainfall product was used as a proxy of antecedent
moisture conditions by using an antecedent precipitation index (API; Hong et al.
2007) for regions lacking soil moisture observations. Figure 17.3 shows the AMSR-E
soil moisture daily data availability in 2005 and also indicates soil moisture generally
correlates with API, but the coefficient becomes saturated after 9-day antecedence.
TABLE 17.2
System Parameters of Different SAR-Based Active Microwave Sensor
Platforms for Soil Moisture Estimation
System Parameters
Radarsat 2 SAR
ENVISAT ASAR
ALOS PALSAR
Incidence angle
20°–50°
23°
10°–51°
SAR band
C
C
L
Wavelength (cm)
5.7
5.7
23
Polarization
HH
VV
HH, VV, VH, HV
Resolution (m)
3–100
30
10–100
Revisit (day)
24
35
46
Source: Modified from Hossain, A. K. M. A. and Easson, G., Microwave Remote Sensing of Soil Moisture
in Semi-arid Environment, Geoscience and Remote Sensing, Pei-Gee Peter Ho (Ed.), ISBN: 978953-307-003-2, InTech, Available from http://www.intechopen.com/articles/show/title/micro waveremote-sensing-of-soil-moisture-in-semi-arid-environment, 2009.
Global Soil Moisture Estimation Using Microwave Remote Sensing
generate images of relatively higher azimuthal resolution, even using a physically
small antenna at longer radar wavelength (i.e., 24-cm L-band). Examples of spaceborne active microwave sensors for soil moisture measurements include Radarsat 2
SAR, ENVISAT ASAR, and ALOS/PARSAR. Table 17.2 lists the system parameters of current commonly used SAR-based radar imaging systems.
As shown in Table 17.2, active microwave sensors for soil moisture estimation generally suffer from low temporal resolution, an important difference from passive microwave sensors, so there are no SAR-based soil moisture products suitable for daily or
subdaily hydrological applications on a global basis. However, active microwave sensors have great potential in high-spatial-resolution soil moisture estimation (up to a 1-m
grid scale) for catchment-based hydrological applications (Makkeasorn et al. 2006).
Additionally, active sensors are more sensitive to surface characteristics, implying
potentially higher accuracy of soil moisture estimations (Baghdadi et al. 2008).
17.4 HYDROLOGICAL APPLICATIONS OF
SOIL MOISTURE PRODUCTS
Satellite-based soil moisture estimates with global coverage can be useful for many
research and practical applications across several disciplines, including hydrology,
meteorology, ecology, and climatology. For example, knowledge of antecedent soil
moisture conditions provides a key source of predictability for hydrologic modeling.
Such information can be retrieved from combined active and passive microwave
instruments aboard spaceborne satellites. In a previous global hydrological runoff
simulation, the satellite TRMM rainfall product was used as a proxy of antecedent
moisture conditions by using an antecedent precipitation index (API; Hong et al.
2007) for regions lacking soil moisture observations. Figure 17.3 shows the AMSR-E
soil moisture daily data availability in 2005 and also indicates soil moisture generally
correlates with API, but the coefficient becomes saturated after 9-day antecedence.
TABLE 17.2
System Parameters of Different SAR-Based Active Microwave Sensor
Platforms for Soil Moisture Estimation
System Parameters
Radarsat 2 SAR
ENVISAT ASAR
ALOS PALSAR
Incidence angle
20°–50°
23°
10°–51°
SAR band
C
C
L
Wavelength (cm)
5.7
5.7
23
Polarization
HH
VV
HH, VV, VH, HV
Resolution (m)
3–100
30
10–100
Revisit (day)
24
35
46
Source: Modified from Hossain, A. K. M. A. and Easson, G., Microwave Remote Sensing of Soil Moisture
in Semi-arid Environment, Geoscience and Remote Sensing, Pei-Gee Peter Ho (Ed.), ISBN: 978953-307-003-2, InTech, Available from http://www.intechopen.com/articles/show/title/micro waveremote-sensing-of-soil-moisture-in-semi-arid-environment, 2009.