Estimating vegetation amount from visible and near infrared reflectances

Abstract

We consider the inference of vegetation status from measurements in the visible and near infrared, in the presence of variable soil reflectance. There are two cases, depending on spatial variability within the instrumental field of view. The first, or "field" case, assumes a spatially uniform vegetation canopy, which is treated in a simple two-stream approximation. In this case, appropriate to Landsat or SPOT data, a value for leaf area index (LAI) may be obtained from reflectance measurements in the visible and near infrared. The second, or "mixed pixel" case, applies for spatial inhomogeneity on a scale larger than individual plant groupings, that is, surface types with varying amounts of vegetation and bare soil. In this case, appropriate to AVHRR observations of agricultural areas, a fractional cover f, corresponding to dense vegetation, may be obtained from the two measurements. A leaf vegetation index VL, having limits 0 nd 1, is introduced for the field case. For a thin canopy ifLAI ⪢ 1 the leaf vegetation index is shown to equal the vegetation fraction f. Analysis results are compared to commonly used vegetation indices.