How well can we estimate the Directional Area Scattering Factor from Directional Reflectance Measurements of Vegetation Canopies?

Abstract:

The Directional Area Scattering Factor (DASF), corresponding to surface directional reflectance for foliage single scattering albedo of 1 and a boundary reflectance of 0, has demonstrated potential for quantify canopy structure. Knyazhikin showed it can be consistently estimated without prior knowledge using directional reflectance between 710nm-790nm. Here, the uncertainty of their estimator is quantified using theory, simulations and measurements. Established statistical theory is used to directly quantify the precision of the DASF. To assess bias, reference DASF estimates are produced over a range of canopies using: i) simulations using the FLIGHT ray tracing model, ii) unbiased one point extrapolation of reflectance versus foliage single scattering albedo to a single scattering albedo of 1 using measurements of both quantities as well as understory reflectance at 6 field sites. The precision error of DASF generally falls below reflectance measurement error (<5%). The bias of DASF varies with LAI: i) for LAI>3, Knyazhikin's method underestimates DASF by up to 15%, in inverse proportion to foliage albedo; ii) for LAI<1, it overestimates DASF by up to 100%, in proportion to understory reflectance ; iii) for intermediate canopies, D is generally unbiased due to cancellation of these effects . In contrast, exhibits a precision error up to 15% but with bias below 5% across tested conditions. These findings indicate the DASF can be estimated with uncertainty of under 20% for moderate to dense canopies (LAI>1) using Knyazhikin’s approach although a bias correction using a prior estimate of foliage single scattering albedo may be useful to improve high DASF estimates.

Figure 1. Comparison of DASF estimated by Knyazhikin's method (x-axis) and reference method (y-axis) based on measurements (coloured symbols) and FLIGHT simulations for canopies with different foliage cholophyll concentration, Cab, (greyscale symbols).