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° 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°-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.