Soil Moisture

Recent advances in remote sensing have shown that soil moisture can be measured by a variety of techniques. However, only microwave technology has demonstrated a quantitative ability to measure soil moisture under a variety of topographic and vegetation cover conditions so that it could be extended to routine measurements from a satellite system. Both active and passive microwave techniques have been applied by researchers for the estimation of soil moisture. Two material properties provide clues about composition and surface state by the manner in which these attributes interact with the microwave radiation. One property is the dielectric constant (its symbol is the small Greek letter, k), which is the ratio of the capacitance of a material to that of a vacuum. It is a dimensionless number that is set at 1.00. This electrical property describes a material's capability (capacity) to hold a charge, which also measures its ability to polarize when subjected to an electric field. Microwave radiation penetrates deeper into materials with low dielectric constants and reflect more efficiently from those with high constants. Values for k range from 3 to 16 for most dry rocks and soils, and up to 80 for water with impurities. Moist soils have values typically between 30 and 60. Thus, variation in emitted microwave radiances (in case of passive microwave observations e.g. by radiometers) or reflected-pulse intensities (in case of active microwave observations e.g. by radar) may indicate differences in soil moisture, other factors being constant. Dry soil has a high emittance; water surfaces have low emittance in microwave region. If one adds water to the soil, the emittance falls and becomes polarized. With the knowledge of "normal" emittance at a particular location (which depends upon soil type and vegetation), microwave observations can be used to detect changes in emittance and therefore of soil moisture. Since the soil moisture is changed by precipitation, these emittance changes between two satellite passes can serve as a proxy of precipitation, known as Antecedent Precipitation Index (API).

The second material property is roughness that can be used to define the soil texture. Materials differ from one another in their natural or cultivated state of surface roughness. Roughness, in this sense, refers to minute irregularities that relate either to textures of the surfaces or of objects on them (such as, closely-spaced vegetation that may have a variety of shapes). Examples include the textural character of pitted materials, granular soils, gravel, grass blades, and other covering objects whose surfaces have dimensional variability on the order of millimeters to centimeters. The height of an irregularity, together with radar wavelength and grazing angle at the point of contact, determines the behavior of a surface as smooth (specular reflector), intermediate, or rough (diffuse reflector). A surface with an irregularity height averaging 0.5 cm will reflect Ka band (l= 0.85 cm), X band (l= 3 cm), and L band (l= 25 cm) radar waves as if it were a smooth, intermediate, and rough surface, respectively. Other average heights produce different responses, from combinations of "all smooth" to "all rough" for the several bands used. This situation means radar, broadcasting three bands simultaneously in a quasi-multi-spectral mode, can produce color composites, if we assign a color to each band. Patterns of relative intensities for images made from different bands may serve as diagnostic tonal signatures for diverse materials whose surfaces show contrasted roughness.

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