10
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
to the cycling of nitrogen and carbon from soil to the hydrosphere, biosphere, and
atmosphere. In climatology and meteorology, the importance of the soil moisture in
the water balance and land surface energy budget has been widely acknowledged,
as it controls the evaporation and the sensible heat fluxes between soil and atmosphere. In digital soil mapping applications, transitory soil moisture measurements
at the field scale may actually provide information about (nearly) time-invariant soil
attributes as soil hydraulic properties, which are dependent on soil structure and
texture. Facing environmental contamination and increasing scarcity of resources,
knowing the spatial variability of soil properties at the field scale at a high resolution
is considerably appealing for designing new agricultural practices, in the framework
of precision agriculture.
As it is exposed to continuously changing atmospheric forcing, soil moisture is
highly variable in space and time. Determining its temporal and spatial variability
is therefore essential for many scientific issues and applications from the field to the
global scale. In that respect, a large number of soil moisture sensing techniques were
used and developed in the last 50 years (Robinson et al. 2008a,b; Vereecken et al.
2008). The only direct soil moisture measurement method is the gravimetric method,
which consists of weighing a soil sample before and after oven-drying it at 105°C.
In the field of hydrogeophysics, numerous indirect methods for soil moisture sensing
exist and rely on the measurement of a physical variable that is a surrogate for soil
moisture. Most of these methods are based on the measurement of the soil response
when it is exposed to electric current or electromagnetic field, depending on the soil
electromagnetic properties. Two main categories of soil moisture measurement techniques are often distinguished: contact-based (or invasive) and contact-free methods
(Vereecken et al. 2008). The contact-based methods require direct contact with the
soil medium and include time-domain reflectometry (TDR) methods (Topp et al.
1980; Robinson et al. 2003), capacitance sensors (e.g., Bogena et al. 2007), electrical resistivity tomography (e.g., Michot et al. 2003), neutron probes (e.g., Hupet and
Vanclooster 2002), heat pulse sensors (Campbell et al. 1991), and fiber optic sensors
(e.g., Garrido et al. 1999). Recently, wireless sensor networks using clusters of invasive sensors have been deployed, offering the potentiality of measuring soil moisture
over a large extent with high temporal resolution (Bogena et al. 2010).
Among the contact-free methods, we may distinguish between spaceborne or
airborne remote sensing and proximal (or ground-based) sensing methods. There
has been a huge development in recent years in remote sensing instruments and
platforms for soil moisture. Methods of remote sensing of soil moisture include passive (radiometer) and active (scatterometer and synthetic aperture radar) microwave
methods that operate at various spatial and temporal resolutions (Wigneron et al.
2003; Wagner et al. 2007). However, remote sensing methods still suffer from several limitations. Measurement capabilities are limited over dense vegetation cover
and by the scattering effect of surface soil roughness (Verhoest et al. 2008; Jonard
et al. in press) because of the relatively high frequencies at which these sensors usually operate. An important drawback is the shallow penetration depth of the remote
sensing instruments (1–5 cm), whereas a deeper characterization of soil moisture is
desirable in many applications (Capehart and Carlson 1997; Vereecken et al. 2008).
Finally, the large-support scale of remote sensing techniques hides the within-pixel
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
to the cycling of nitrogen and carbon from soil to the hydrosphere, biosphere, and
atmosphere. In climatology and meteorology, the importance of the soil moisture in
the water balance and land surface energy budget has been widely acknowledged,
as it controls the evaporation and the sensible heat fluxes between soil and atmosphere. In digital soil mapping applications, transitory soil moisture measurements
at the field scale may actually provide information about (nearly) time-invariant soil
attributes as soil hydraulic properties, which are dependent on soil structure and
texture. Facing environmental contamination and increasing scarcity of resources,
knowing the spatial variability of soil properties at the field scale at a high resolution
is considerably appealing for designing new agricultural practices, in the framework
of precision agriculture.
As it is exposed to continuously changing atmospheric forcing, soil moisture is
highly variable in space and time. Determining its temporal and spatial variability
is therefore essential for many scientific issues and applications from the field to the
global scale. In that respect, a large number of soil moisture sensing techniques were
used and developed in the last 50 years (Robinson et al. 2008a,b; Vereecken et al.
2008). The only direct soil moisture measurement method is the gravimetric method,
which consists of weighing a soil sample before and after oven-drying it at 105°C.
In the field of hydrogeophysics, numerous indirect methods for soil moisture sensing
exist and rely on the measurement of a physical variable that is a surrogate for soil
moisture. Most of these methods are based on the measurement of the soil response
when it is exposed to electric current or electromagnetic field, depending on the soil
electromagnetic properties. Two main categories of soil moisture measurement techniques are often distinguished: contact-based (or invasive) and contact-free methods
(Vereecken et al. 2008). The contact-based methods require direct contact with the
soil medium and include time-domain reflectometry (TDR) methods (Topp et al.
1980; Robinson et al. 2003), capacitance sensors (e.g., Bogena et al. 2007), electrical resistivity tomography (e.g., Michot et al. 2003), neutron probes (e.g., Hupet and
Vanclooster 2002), heat pulse sensors (Campbell et al. 1991), and fiber optic sensors
(e.g., Garrido et al. 1999). Recently, wireless sensor networks using clusters of invasive sensors have been deployed, offering the potentiality of measuring soil moisture
over a large extent with high temporal resolution (Bogena et al. 2010).
Among the contact-free methods, we may distinguish between spaceborne or
airborne remote sensing and proximal (or ground-based) sensing methods. There
has been a huge development in recent years in remote sensing instruments and
platforms for soil moisture. Methods of remote sensing of soil moisture include passive (radiometer) and active (scatterometer and synthetic aperture radar) microwave
methods that operate at various spatial and temporal resolutions (Wigneron et al.
2003; Wagner et al. 2007). However, remote sensing methods still suffer from several limitations. Measurement capabilities are limited over dense vegetation cover
and by the scattering effect of surface soil roughness (Verhoest et al. 2008; Jonard
et al. in press) because of the relatively high frequencies at which these sensors usually operate. An important drawback is the shallow penetration depth of the remote
sensing instruments (1–5 cm), whereas a deeper characterization of soil moisture is
desirable in many applications (Capehart and Carlson 1997; Vereecken et al. 2008).
Finally, the large-support scale of remote sensing techniques hides the within-pixel