Estimation of LAI and FAPAR by constraining the leaf and soil spectral characteristics in a radiative transfer model

Abstract

We evaluated the estimation of the leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (FAPAR) of green plant canopies from top-of-the-canopy (TOC) spectral indices by using the PROSAIL model under the possible constraints of leaf and soil spectra. For the LAI estimation, the ratios (B1 – B3)/(B1 + B3) and B1/B3 provided fewer estimation errors than (B4 – B3)/(B4 + B3) or B4/B3 when the variation in the soil spectral reflectance was small or LAI was large. Here, B1, B3 and B4 denote the blue, red and near-infrared bands of Landsat ETM+, respectively. For the FAPAR estimation, the ratios (B5 – B7)/(B5 + B7) and B5/B7 provided fewer estimation errors than (NIR – R)/(NIR + R) or NIR/R for a FAPAR value of 0.3–0.7 when the variation in the soil spectral reflectance was large. Here, B5 and B7 denote the bands with wavelengths 1.55–1.75 and 2.09–2.35 μm, respectively. These were maintained for various conditions of the solar incident zenith angle (θs), leaf angle distribution (LAD), canopy hotspot parameter (s 1) and clumping index (Ω).