Improving the performance of 3-D radiative transfer model FLIGHT to simulate optical properties of a tree-grass ecosystem

Abstract

The 3-D Radiative Transfer Model (RTM) FLIGHT can represent scattering in open forest or savannas featuring underlying bare soils. However, FLIGHT might not be suitable for multilayered tree-grass ecosystems (TGE), where a grass understory can dominate the reflectance factor (<i>RF</i>) dynamics due to strong seasonal variability and low tree fractional cover. To address this issue, we coupled FLIGHT with the 1-D RTM PROSAIL. The model is evaluated against spectral observations of proximal and remote sensing sensors: the ASD Fieldspec^® 3 spectroradiometer, the Airborne Spectrographic Imager (CASI) and the MultiSpectral Instrument (MSI) onboard Sentinel-2. We tested the capability of both PROSAIL and PROSAIL+FLIGHT to reproduce the variability of different phenological stages determined by 16-year time series analysis of Moderate Resolution Imaging Spectroradiometer-Normalized Difference Vegetation Index (MODIS-<i>NDVI</i>). Then, we combined concomitant observations of biophysical variables and <i>RF</i> to test the capability of the models to reproduce observed <i>RF</i>. PROSAIL achieved a Relative Root Mean Square Error (<i>RRMSE</i>) between 6% to 32% at proximal sensing scale. PROSAIL+FLIGHT <i>RRMSE</i> ranged between 7% to 31% at remote sensing scales. <i>RRMSE</i> increased in periods when large fractions of standing dead material mixed with emergent green grasses —especially in autumn—; suggesting that the model cannot represent the spectral features of this material. PROSAIL+FLIGHT improves <i>RF</i> simulation especially in summer and at mid-high view angles.