Passive Microwave Remote Sensing of Soil Moisture and Regional Drought Monitoring - Monitoring and Predicting Agricultural Drought: A Global Study - page 90

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methods have the potential to provide both soil moisture and drought
information. In the past, the options have been limited by the availabil-
ity of satellite systems. Even with these limitations, investigators have ex-
plored the potential of these data in soil moisture studies with some success.
Within the next few years, a wide range of new and significantly improved
satellites will be launched that will offer new opportunities. In this chap-
ter, the basis of passive microwave remote sensing is presented with a de-
scription of alternative techniques for retrieving soil moisture. A review of
current and future satellite systems is presented, along with examples of
soil moisture studies that illustrate how this information can be used for
drought monitoring and assessment.

Passive Microwave Remote Sensing of Surface Soil Moisture

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Ph ysical Basis
Microwave remote sensing provides a direct measurement of the surface
soil moisture for a range of vegetation cover conditions. Two basic ap-
proaches are used, passive and active. In passive methods, the natural ther-
mal emission of the land surface (or brightness temperature) is measured
at microwave wavelengths using very sensitive detectors. Only passive mi-
crowave methods are treated in this chapter. Chapter 8 provides details on
active microwave systems.
The microwave region of the electromagnetic spectrum consists of fre-
quencies between 0.3 and 30 GHz. This region is subdivided into bands,
which are often referred to by a lettering system. Some of the relevant bands
that are used in earth remote sensing are K (18-27 GHz), X (8-12 GHz),
C (4-8 GHz), and L (1-2 GHz). Frequency and wavelength are used in-
terchangeably. Wavelength (in centimeters) approximately equals 30 times
frequency (in GHz). Within these bands only small ranges exist that are
protected for scientific applications, such as radio astronomy and passive
sensing of the earth's surface. A general advantage of microwave sensors,
in contrast to visible and infrared, is that observations can be made through
cloud cover because the atmosphere is nearly transparent, particularly at
frequencies < 10 GHz. In addition, these measurements are not dependent
on solar illumination and can be made at any time of the day or night.
Microwave sensors operating at very low frequencies ( < 6 GHz) provide
the best soil moisture information. At low frequencies, attenuation and
scattering problems associated with the atmosphere and vegetation are less
significant, the instruments respond to a deeper soil layer, and a higher
sensitivity to soil water content is present.
Low-frequency passive sensors provide information on the surface
reflectivity. An examination of relationship between reflectivity and soil
moisture is essential to estimate soil moisture from microwave remote sens-
ing data.

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