Phenological influences on the albedo of prairie grassland and crop fields

 The albedo of land surfaces is strongly affected by transient surface conditions. For a vegetated surface, albedo can change with soil moisture, fractional canopy cover, and plant phenology as well as solar zenith angle (θs). In this study, the relationships between albedo and plant phenology in prairie grassland and agricultural crops were examined by removing the effect of variations in θs. Albedos were evaluated at a constant θs, which was assigned to be 20° in this study. For days with a minimum θs larger than 20°, a polynomial function that relates albedos to θs was derived and applied to estimate the albedos at 20°. After the removal of θs variations, the observed albedos of grassland at the Konza Prairie showed a linearly decreasing trend from spring to winter due to prairie phenology. The observed albedo of a maize field showed a clear increase from seedling to peak green stage, except when drought caused a decrease in near-infrared reflectance. The observed albedos of a winter wheat field showed an uninterrupted decrease from peak green to harvesting stage.