COMPARISONS OF RADIATIVE-TRANSFER MODELS OF VEGETATION CANOPIES AND LABORATORY MEASUREMENTS

Laboratory measurements of the directional reflectance of plant canopi es fit radiative-transfer-based plane-parallel models well when the pl ants are low and leaves are small. Bidirectional reflectance measureme nts were collected at a unique facility in Changchun, China, using an apparatus that simulates solar radiation at zenith angles up to 45 deg rees on a 1-m square target. A curved arm fitted with multiband radiom eters revolves on a circular track around the target, allowing rapid m easurement of multispectral bidirectional reflectance factors (BRFs) o f the target at 10 degrees-zenith and azimuth angles. Because the meas urements are made under controlled conditions, effects of such confoun ding factors as wind and diffuse (sky) irradiance can be avoided. Thre e one-dimensional radiative-transfer canopy models were compared to th e BRF measurements in the near-infrared. The models generally fit the data for a young wheat canopy well. However, young corn and soybean ca nopies showed significant differences that are attributed to the escap e of multiply scattered radiation from the sides of the canopy. (C) El sevier Science Inc., 1997.