Extending a turbid medium BRDF model to allow sloping terrain with a vertical plant stand

This paper extends the turbid medium approach used for modeling bidirection al reflectance from horizontal plant canopies to sloping terrain with a ver tically oriented plant stand. Previous treatments have accounted for terrai n slope by simple adaptation to an inclined plane of models for horizontal surfaces. However, such treatments implicitly assume that plants grow perpe ndicularly to the surface, despite the fact that plant stems continue to gr ow vertically on slopes. We investigate the differences between our new "ve rtical growth" model and the more usual "perpendicular to the surface growt h" model in terms of the effect on canopy albedo and bidirectional reflecta nce factors. Although the effect of leaf angle distribution on the albedo i s different for both the vertical-growth and perpendicular-growth models, i t appears to be a much smaller effect than that due to terrain slope. For t he bidirectional reflectance factors (BRF's), the magnitude and sign of the differences between the two models varies with the direction of observatio n, the slope, and the leaf angle distribution, and, can exceed 10% for a pl anophile canopy. A comparison between modeled and measured data shows that model predictions under the vertical growth assumption are consistent with measurements, whereas the assumption of perpendicular growth can lead to la rge errors.