Comparisons of radiative transfer models of vegetation canopies and laboratory measurements

Laboratory measurements of the directional reflectance of plant canopies fit radiative-transfer-based plane-parallel models well when the plants are low and leaves are small. Bidirectional reflectance measurements were collected at a unique facility in Changchun, China, using an apparatus that simulates solar radiation at zenith angles up to 45 degrees on a 1-m square target. A curved arm fitted with multiband radiometers revolves on a circular track around the target, allowing rapid measurement of multispectral bidirectional reflectance factors (BRFs) of the target at 10 degrees-zenith and azimuth angles. Because the measurements are made under controlled conditions, effects of such confounding factors as wind and diffuse (sky) irradiance can be avoided. Three one-dimensional radiative-transfer canopy models were compared to the BRF measurements in the near-infrared. The models generally fit the data for a young wheat canopy well. However, young corn and soybean canopies showed significant differences that are attributed to the escape of multiply scattered radiation from the sides of the canopy. (C) Elsevier Science Inc., 1997.