Remote Sensing - a tool for environmental observation

Increasing moisture content generally decreases soil

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Increasing moisture content generally decreases soil reflectance across the entire spectrum. In fact, wet soils also appear darker to the eye than dry soils. Furthermore, reflectance spectra of moist soils include the prominent absorption bands for water and hydroxyl (OH - ion) at 1.9 and 1.4 μm and the bands at 0.97, 1.20 and 1.77 μm. Figure 6.6 shows the effect of soil moisture on the reflectance curve of a soil. Iron is an important constituent of a soil regarding its reflective properties. Many absorption features in soil reflectance are due to the presence of iron in ferric or ferrous form. For example, the general strong decrease of reflectance of soils towards the blue and ultraviolet wavelengths is caused by iron. Absorption features occur around 0.7 μm, 0.9 μm and 1.0-1.1 μm. An increase of the organic matter content of a soil generally causes a decrease of reflectance over the entire spectrum that is similar to moisture. A high organic matter content and hence, a strong decrease of overall reflectance, might even mask other absorption features in the soil spectra. This effect is minimal for soils having organic matter contents below 2.0 to 2.5%. Unfortunately, little knowledge of the influence of different organic soil compounds and the different forms of organic matter on the spectral behaviour of soils is available in the literature. The basic components of soil minerals are silicon, aluminium and oxygen. None of these soil constituents have diagnostic absorption features. Quartz for example, has a large reflectance throughout the short-wave spectrum and absorption features in short-wave quartz spectra are only due to impurities. Soil particle size influences a number of soil properties such as moisture content and soil struc- ture. Both of these properties also influence the reflectance of soils. Consequently, it is very difficult to measure the exact effect of increasing soil particle size on reflectance. Most of the studies concerned with remote sensing and soils follow an empirical approach to distinguish soil types. Vegetation or crop cover always hampers the observation of soils in the optical wavelengths.
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90 Figure 6.4 Some examples of soil reflectance spectra (Asrar, 1989). Figure 6.6 Spectral reflectance curves for wet and dry silty loam and wet and dry clay (Curran, 1986).
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91 Figure 6.5 Reflectance spectra of carbonate minerals showing features at 2280 and 2340 nm and minerals showing OH en H 2 O absorption bands (Asrar, 1989).
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92 6.4 Multi-temporal Remote Sensing & Change Detection Multi-temporal remote sensing techniques refer to the use of images of different dates to study temporal changes of e.g. crop cover or vegetation cover also often referred to as change detection. Sometimes the classification of agricultural crops can significantly be improved by monitoring the different growth stages of the crops. Maize e.g. has a short time span that the soil cover is high, the growing season lasts only from late June to late August. Cereals, in contrast, grow from late April to late July or early August. Hence, if multi-temporal remote sensing
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