Remote Sensing of Biological Soil Crusts
The ability of remote sensing to detect and map the distribution of biological soil crusts offers the opportunity to extend site-specific ecological studies of crusts to a regional scale, thus reducing the time and costs associated with ground surveys. Ho
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31.1 Introduction The ability of remote sensing to detect and map the distribution of biological soil crusts offers the opportunity to extend site-specific ecological studies of crusts to a regional scale, thus reducing the time and costs associated with ground surveys. However, despite the global extent of soil crusts and the expanding interest in their ecological roles, there have been relatively few studies published on the use of remote sensing to detect and map their distributions. Wessels and Van Vuuren (1986) were the first to use satellite imagery to detect and map biological soil crusts. Their study of the Namib Desert of SW Africa used Landsat TM to discriminate lichen-covered areas from bare ground and vegetated surfaces. Subsequently, only a relatively small number of publications have either presented the spectral properties of biological soil crusts or applied remote sensing to map them (Clark et al. 1993a,b,c; Kokaly et al. 1994; O’Neill 1994; Karnieli and Tsoar 1995; Karnieli and Sarafis 1996; Karnieli et al. 1996, 1999; Tsoar and Karnieli 1996; Tromp and Steenis 1996; Karnieli 1997). The objective of this chapter is to present the spectral characteristics of biological soil crusts and explore the methods and issues in using remote sensing for their identification. As the reflectance spectra of soil crusts vary among geographical regions, this chapter presents results from three study sites: the Negev Desert in Israel, the semiarid Colorado Plateau of southern Utah, and the rangelands of southeastern Idaho.
31.2 Brief Review of Remote Sensing Remote sensing is used to detect or infer the properties of a substance without being in direct physical contact. Remote sensing instruments measure the electromagnetic energy upwelling from a surface at various parts of the wavelength spectrum. This energy might be an inherent emission from an object due to its temperature, reflected from a natural source such as the sun, Ecological Studies, Vol. 150 J. Belnap and O.L. Lange (eds.) Biological Soil Crusts: Structure, Function, and Management © Springer-Verlag Berlin Heidelberg 2001
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or reflected from an artificial source such as radar. Past studies of biological soil crusts interpreted the reflected sunlight from surfaces to discriminate soil crusts from vegetation and other surface cover. Reflected solar radiation is divided into three main regions: the visible (VIS, 400–700 nm), the near infrared (NIR, 700–1000 nm), and the short-wave infrared (SWIR, 1000– 2500 nm). The VIS region may be further subdivided into three parts: the blue (B, 400–500 nm), the green (G, 500–600 nm), and the red (R, 600–700 nm). These spectral regions are delineated in Fig. 31.1. The relative amounts of energy reflected from surfaces vary as a function of wavelength. Thus, water can be distinguished from other materials by its low reflectance in the VIS and near-zero reflectance in the NIR and SWIR. In contrast, bare soil usually has a high reflectance in the NIR and SWIR. The most important factor affectin
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