VEGETATION BACKGROUND Spectral data: The spectral properties of individual leaves have been well- understood for quite a long time (e.g., Gates et al., 1965), especially in the VNIR and SWIR. Until recently, laboratory instrumentation was not available to make equivalent measurements in the thermal infrared, but recent spectroscopic studies have provided confirmation of general information derived from earlier broad-band measurements (Salisbury and Milton, 1988). Although leaf spectra are readily available, good canopy spectra are not, because of the technical difficulty of making such measurements. In the reflective region, difficulties arise particularly in the SWIR from the strong water vapor absorption bands in solar radiation illuminating the canopy, which leave large gaps in the spectrum where they absorb completely, and introduce observational difficulties even where they do not (Biehl et al., 1984). Atmospheric absorption of emitted radiation is a problem in the thermal infrared region, along with an historical limitation on the availability of portable field spectrometers. These difficulties have been at least partially remedied by spectral measurements of leaf piles and canopy parts in the laboratory to provide simulated canopy spectra relatively untroubled by water vapor absorption (e.g.., Salisbury and Milton, 1988). BRDF data: Ever since the pioneering measurements of directional scattering properties of individual leaves by Breece and Holmes (1971), gradually more sophisticated models of canopy scattering have been developed, as best summarized by Kimes (1984). Such models are not simple, because canopy scattering is complicated by the fact that individual leaf reflectances vary with wavelength, from predominantly surface scattering in the visible and TIR regions, to predominantly volume scattering in the SWIR and MWIR regions; and typical leaf orientation varies during different growth stages for a given species, and from one species to another. To provide real data input to such models, Goddard Space Flight Center developed a sphere-scanning radiometer, called the PARABOLA, for field measurements of the BRDF of natural surfaces (Deering and Leone, 1984). This field instrument typically measures BRDF in three narrow bandpasses in the visible, near-infrared, and shortwave infrared. Typical scattering data for soils and vegetation have been summarized by Deering (1989), and have been made available by Don Deering in digital form. Other field measurements have been made in the VNIR and SWIR regions of the spectrum by Ranson et al. (1985). Few measurements of directional emittance have been made because of the unavailability, until recently, of appropriate field instruments. We have made field measurements that show that conifers are Lambertian emitters because of the strong canopy scattering produced by randomly-oriented needles. However, some preliminary measurements by others appear to show small, but inconsistent, directional effects on grass canopy emissivities (Norman and Balick, 1992), which may be due to the quasi-parallel surfaces produced by the bent tips of long grass. Such directional effects could be even greater for deciduous leaf canopies, where leaf orientations tend to be more horizontal (depending on species). NATURE OF GENERIC BACKGROUND SPECTRA GENERATED HERE VEGETATION Spectral data: Spectra were assembled from two segments; the VNIR and SWIR comprising segment one, and the MWIR and TIR comprising segment two. The first segment for trees used simulated canopy spectra measured by Barry Rock of the University of New Hampshire on leaf piles and canopy parts using a GER IRIS Mark IV field spectrometer in the laboratory. The tree leaves or branches were illuminated from directly above and measured at a reflectance angle of about 30 degrees (Barry Rock, personal communication). The grass VNIR/SWIR spectra were measured in the laboratory at JHU, also with a GER IRIS Mark IV, using large pieces of fresh sod. The grass was illuminated from directly above and measured at a reflectance angle of 60 degrees to avoid viewing the thatch. The artificial illumination sources used by Barry Rock and by us emit much less radiation in the blue region of the spectrum than does the sun. This results in an instrumental artifact in GER IRIS Mark IV spectra, characterized by an apparent increase in reflectance of the sample from the blue into the UV (the so-called "blue tail"). Spectra of vegetation measured outdoors are not affected in the blue region by atmospheric water vapor absorption, and so have been used to check the true reflectance spectra of vegetation, which actually decline through the blue and into the UV. Thus, the VNIR/SWIR segments of the vegetation spectra were corrected by hand to remove the blue tail before being joined with the thermal infrared segments. The thermal infrared segments were generated by us from 10-degree directional hemispherical reflectance spectra of needle and leaf piles, or, in the case of grass, from sod. Conifer needles, deciduous tree leaves and grass blades all have a very low reflectance (high emissivity) throughout the thermal infrared range, although the conifer needles are consistently lower in reflectance than the other two. There are subtle spectral features associated with differences in cuticular waxes that could be diagnostic of deciduous species in the laboratory (Salisbury and Milton, 1988). The diagnostic differences in these features vary, however, typically only about 2%, and canopy scattering will further reduce this spectral contrast by a factor of at least two. Thus, spectral features are of interest for laboratory applications, but not usually for remote sensing. Because spectral features are so subdued, we selected one typical deciduous leaf spectrum to represent all deciduous species, one conifer to represent all conifers, and one grass species to represent all grasses. Each simulated tree canopy thermal infrared spectrum was reduced in reflectance by a factor of two to conservatively account for canopy scattering (conifers, in particular, should undergo more intense scattering, and field measurements show conifers to exhibit black body behavior within measurement error of about 1%). The thermal infrared segments were then joined with the VNIR/SWIR segment of the appropriate species by making a straight-line interpolation between 2.5 and 3.0 µm. 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