Effects of Vegetation on Spectral Irradiance at the Soil Surface¹

Abstract

Abstract Spectral discrimination of vegetation is affected by changes in irradiance at the soil surface produced by the overlying canopy. This study was conducted to quantify effects of vegetation on spectral and spatial variations of irradiance at the soil surface. Direct and diffuse spectral irradiance were measured beneath cotton ( Gossypium hirsutum L.) with a spectroradiometer for three soils differing in reflectance. At intermediate and high canopy cover, the cotton altered the spectral composition of diffuse irradiance at the soil surface in a manner that reflected the optical properties of the canopy. At shaded locations, irradiance at visible wavelengths was low due to the high absorptance of the canopy, while the near infrared irradiance was high (nearly 40% of that measured above the canopy) because of the high degree of transmission and scattering by the canopy. At sunlit locations between rows, the total irradiance (direct beam and diffuse) was lower in visible wavelengths and higher in the near infrared than that incident at the top of the canopy. Midway between rows, visible diffuse irradiance was higher and the near infrared diffuse irradiance was lower than beneath the plants. At less than 40% cover, insufficient vegetation was present to significantly affect soil irradiance. The results demonstrate that at intermediate and high canopy cover, irradiance at the soil surface contains spectral contributions from the overlying canopy. The canopy produces significant spectral and spatial variations in diffuse irradiance that should be considered in the analysis of spectral measurements.

Keywords

Affiliated Institutions

• Texas A&M University US
• United States Army Corps of Engineers US

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