Soil Spectra Contributions to Grass Canopy Spectral Reflectance

The soil or background spectra contribution to grass canopy spectral reflectance for the 0.35 to 0.80 fJ-m region was investigated using in situ collected spectral reflectance data. Regression analysis was used to estimate accurately the unexposed soil spectral reflectance and to quantify maxima and minima for soil-green vegetation reflection contrasts. generally be impractical to take detailed soil spectra measurements of the area in question because the vegetation canopy obscures the soil surface and the time involved for these in situ measurements is usually prohibitive. The work reported in this article was part of a larger effort sponsored by the u.S. International Biological Program's Grassland Biome. The data were collected with the objective of evaluating non-destructive "National Academy of Sciences-National Academy of Engineering Research Fellow. ally a soil-litter background) has a characteristic spectra for a given particular area (Figure 1). The soil spectra for the field study site was a monotonically increasing function with wavelength over the spectral range of 0.35 to 1.00 fJ-m. The dry soil surface was more highly reflective than the wet soil surface. Dry refers only to the uppermost layer as determined by visual inspection. The plant canopy on the grassland soil surface will be viewed as some statistical ensemble of foliage elements superimposed over the soil-litter background. The density of the ensemble offoliage elements will be a PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING, Vol. 43, No.6, June 1977, pp. 721-726. 721 722 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING, 1977 function of biomass. The incoming spectral irradiance will interact with the grass canopy and, depending upon the vegetational density or biomass, can also interact with the soil background. The interaction(s) with the soil background become less and less as the vegetational density or biomass increases until the asymptotic spectral radiance or reflectance is reached (Tucker, 1977). Increases in the vegetational density or biomass effect no change in the canopy spectra when the asymptotic spectral radiance or reflectance has been reached. This can be explained because the canopy is of sufficient density and thickness to prevent the penetration of the incident spectral irradiance to lower biomass levels of the canopy. Hence, the incident spectral irradiance does not interact with additional (and lower level) biomass. As the vegetational density increases to the point where the spectral reflectance begins to asymptote at a given wavelength, the soil spectra contribution to the canopy spectra is minimal at that wavelength. When the canopy is of sufficient density or biomass to result in the asymptotic spectral reflectance, there is no soil spectral reflectance contribution to the composite canopy spectral reflectance. Thus the relative contribution of the soil spectra to the composite canopy spectra is inversely related to the biomass or vegetation density. The asymptotic spectra for green grass canopies were quite different from those for the soil surface at the study site (Figure 1). As plant growth and development result in increasing amounts of green plant material above the dry soil surface, the canopy spectra changes. In regions of the spectrum where absorption occurs, the composite canopy spectra decreases and approaches the asymptotic green reflectance spectra. In OO.r-----------------, ... GREEN VEGETATION PLOT 20074 (TOTAL DRY BIOMASS =530 g/m2) \ ----_...--------_...... --_.--_._----~.36 OAO 0.45 0.50 0.56 O.eo 0.86 0.70 0.75 0.80 0.85 0.90 0.95 1.00