Hemispherical Reflectance Variations of Vegetation Canopies and Implications for Global and Regional Energy Budget Studies

Abstract

The variations of spectral hemispherical reflectance (albedo) in vegetation canopies were studied as a function of solar zenith angle, leaf area index, led orientation distribution, and leaf and soil optical, properties. A three dimensional radiative transfer model was used to investigate the radiative transfers that give rise to variations in hemispherical relfectance (?). The results of this model were compared to those derived using an analytical two-stream approximation model which has the advantages of being simple and robust enough to use in real time applications. The hemispherical reflectance in the visible and near-infrared regions can vary as much as 60% with changes in solar zenith angles from 0° to 77°. This variation generally decreases as the leaf orientation distribution approaches a planophile distribution. The probability of gap function through the canopy and the spectral characteristics of the soil and vegetation are key factors in determining the hemispherical reflectance dynamics. The two models showed similar trends in terms of the variation of ? with solar zenith angle, leaf area index and wavelength. Ale two-stream approximation model is the favored approach among modelers of land surface processes in climate studies because of its simplicity. In this study the model is shown to provide a reliable and robust tool in estimating hemispherical reflectance. Many terrestrial energy budget studies require estimates of the daily reflected energy (spectral and total shortwave) of the vegetated surface. Most studies use a hemispherical reflectance value estimated near the solar noon for calculating the daily reflected energy and ignore the diurnal variation of ? as documented in this paper. The results showed that percent errors as high as 18% can result in using this technique to calculate the daily reflected energy of vegetation canopies. Consequently, some knowledge of the daily ? variations is required for studies requiring accuracies of daily reflected energy of less than ?18%.