Comparisons of radiative transfer models of vegetation canopies with laboratory measurements

Laboratory measurements of the directional reflectance of three young crop canopies fit radiative-transfer-based plane-parallel models with good accuracy. The 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 diameter target. A curved arm fitted with multiband radiometers revolves on a circular track around the target, allowing rapid measurement of multispectral bidirectional reflectance factors of the target at 10°-zenith and azimuth angle increments. 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 reflectance models were compared to the bidirectional reflectance measurements in the infrared wavelength region, in which multiple scattering treatments differentiate the three models. The models generally fitted the data for young wheat and corn canopies well, with some departures for a young soybean canopy. The escape of multiply-scattered radiation from the sides of the canopy in the case of soybeans and corn produced some deviations from modeled results