Remote Sensing - a tool for environmental observation

Water content of the leaves and water in the

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Water content of the leaves, and water in the atmosphere, reduce overall leaf reflectance and causes some narrow absorption features described earlier: the water absorption bands. Three major water absorption bands are located near 1.4, 1.9 and 2.7 μm, two minor absorption bands occur near 0.96 and 1.1 μm. The combined effect of pigments and physiological structure give healthy vegetation its typical reflectance properties. Combinations of the visible and near infrared spectral bands (see section on spectral ratioing) enables us to discriminate bare soil surfaces or water bodies from vegetation and to assess different percentages of vegetation coverage as shown in figure 6.3. A thorough description of the optical properties of vegetation and leaves is given by Guyot et al. (1992), Cohen (1991), Elvidge (1990), Goel (1989).
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87 Figure 6.1 Major influences on spectral properties of a leaf (Campbell, 1987). Figure 6.2 The effect of a changing canopy on a remote sensing scene.
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88 Figure 6.3 Normalized Difference Vegetation Index as a function of a spring-wheat canopy for growth (solid line) and senescent (dashed line) periods of the growing season (Asrar, 1989). 6.3 Spectral Behaviour of Soils The spectral behaviour of a soil is considerably different from vegetation: a soil reflectance curve in visible and infrared wavelengths shows less ‘peak and valley’ variation. The factors that influence soil reflectance seems to act over less specific spectral bands than for vegetation. This is the case when soil spectra are collected using conventional broadband sensors with a general band-width of 60 nm or more (SPOT, MSS, TM). Figure 6.4 shows examples of soil reflectance spectra. Notice the overall convex shape. If soil spectra are studied in more detail i.e. at increased spectral resolution (20 nm or less), significant differences between soil spectra become apparent (Goetz et al., 1985). A soil spectrum measured at high spectral resolution shows several diagnostic absorption features. In particular the short-wave infrared spectral region from 2000 to 2500 nm contains diagnostic absorption features of soil constituents. The spectral resolution of Landsat TM and the French SPOT vary from 60 to 270 nm. Consequently, these broadband sensors are only marginally suitable for detecting these absorption features because the absorption bands of most soil materials are too small and are blurred in the broad-bands (20 nm bands are generally adequate
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89 for the detection of absorption features, unfortunately, these kind of systems are not yet available aboard satellites). An example of absorption bands is given in figure 6.5. The most important soil properties regarding reflection in optical wavelengths are: - Moisture content; - Organic matter content; - Texture; - Structure; - Iron content; - Mineral composition; - Type of clay minerals; - Surface conditions of the soil.
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