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Seasonal Changes in Reflectance of Two Wheat Cultivars¹

Accurate crop identification from remote sensing signals is dependent on a knowledge of differences in characteristics of reflectance from various crop canopies. The objective of the present work was to follow changes in the reflectance of two dissimilar wheat [Triticum aestivum (L.)] cultivars through a growing season to determine which reflectance characteristics were suitable for use in the analysis and interpretation of spectral reflectance curves. Reflectance over the wavelength interval of 0.45 to 2.50 µm was measured with a ground-based spectroradiometer on nine cloud-free days between planting and maturity of wheat planted on Hidalgo sandy clay loam (Typic Calculstols) soil. Seeding rate affected the rate of ground cover, but not the spectral reflectance of either a winter or a spring wheat once about 25% of the area was covered by vegetation. All reflectance curves had the characteristic shape for vegetated surfaces by 4 weeks after emergence. The proportion of the ground covered by plants was more important than development stages of the plants in determining spectral responses, except at the end of the season when the plants senesced and lost pigmentation. Through the season soil was much less reflective than green vegetation at 0.75, 0.90, and 1.10 µm and much more reflective at 1.65 and 2.20 µm, making these wavelengths valuable for distinguishing vegetation from soil background and for assessing vegetation cover or density.

Key Words: Remote sensing • Triticum aestivum • Vegetative growth stages • Spectroradiometer • Spectra

² Soil scientist, physical science technician, and soil scientist, respectively, USDA, Weslaco, Tex.

Received for publication April 8, 1977.

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