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Remote Sensing of Soil Moisture
no projects specifically dedicated to measuring soil moisture globally with
adequate temporal or spatial sampling (Barre, Duesmann, and Kerr, 2008;
Kerr, 2007; Prigent, Aires, and Rossow, 2006).
With the launch of microwave radiometers on the Soil Moisture and
Ocean Salinity (SMOS) and Aqua satellites, soil moisture and sea surface
salinity can now be obtained nearly continuously over a large fraction of
the Earth’s surface. The L-band two-dimensional (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 and to salinity in the surface waters of the oceans
(Font et al., 2010; Kerr et al., 2010; McMullan et al., 2008; Merlin et al.,
2008). 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 (Jackson and Schmugge, 1991; Njoku
and Entekhabi, 1996).
The objectives of this paper are to present the reader with an overview of
remote sensing techniques used for soil moisture mapping and to describe
recently launched satellite systems that are being used to perform frequent
global (including coastal zones) soil moisture and sea-surface salinity measurements.

Figure 1. Calculated values of emissivity at 21 cm wavelength
for a soil surface using several values of the soil roughness parameter h (Choudhury et al., 1979).

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