Classifying the geometry of canopies from time variations of red and near-infrared reflectance

A new method is suggested for classifying the geometry of canopies from red and near-infrared reflectance values measured from a nadir view. The method relies on the fact that the slope of a very strong linear statistical relationship (R2 > 0.95) between two indices varies significantly with the structure of the canopy. The first index (RVIc) is the ratio between reflectance in the near-infrared (NIR) and in the red channels, where intercept (b) of the soil line is substracted from the NIR: RVIc = (NIR - b) / Red. The second index (GEO) in a new index calculated from the indices (PVI and SLI) introduced by Richardson and Wiegand (1977; Wiegand and Richardson, 1982): GEO = (SLI2 + PVI2) / (SLI − PVI). GEO is much more sensitive to canopy architecture than is RVIc. Thus the ratio of RVIc vs GEO, or the slope in such graphs, increases as leaf angle increases. Applied to three wheat cultivars, the smallest slope of the RVIc vs. GEO is for the cultivar that differed from the others by a mean leaf angle of about 15° or less. The significance of the approach is that it helps identify leaf angle differences among observed canopies when leaf area index (LAI) is unknown but has a sufficient space variability. This method is illustrated by the RVIc-GEO relations within a durum wheat field in the south of France observed on two SPOT satellite scenes. An increase of leaf erectness between tillering stage and early grain filling phase (2 weeks after anthesis) is clearly shown by the evolution of the slope of the RVIc vs. GEO relation.