NOAA/AVHRR bidirectional reflectance: Modeling and application for the monitoring of a temperate forest

In this article, bidirectional effects on NOAA/AVHRR short wavelength data were analyzed for the regional monitoring of the temperate pine (Pinus pina ster) Landes forest. Because of differences in soil hydrologic characterist ics, the forest is described by five ecotypes, which determine the understo rey type and rile potential for pine growth. Bidirectional visible and near -infrared reflectances were analyzed from a daily NOAA-11 archive through y ears 1990-1994 using the bidirectional reflectance function model of Rahman . Rahman's model fits 72% of AVHRR visible reflectance, and 93% of AVHRR ne ar-infrared reflectance. A typical example shows that 1) in the principal p lane, he ratio of hot spot to forward scatter reflectance is 2.9 and 2.1 fo r visible and near-infrared, respectively, 2) bidirectional effects become negligible across the principal plane, and 3) NDVI shows a shape opposite t o that of reflectances. After normalization, the seasonal dynamics of AVHRR reflectances were related to the phenological cycle of pine trees and unde r-storey vegetation. Visible reflectance drops earlier on coastal than on i nland sites, due to an advance in the onset of pine growth. Near-infrared r eflectance increases in summer for annual understory, and remains steady fo e evergreen understorey. Differences in the seasonal dynamics of NDVI conse quently characterize each Landes ecotype. From the modeled reflectances, we estimated the seasonal dynamics of visible and near-infrared albedo. We ca lculated the solar albedo by a combination of the last two. The visible alb edo displays a seasonal dynamic similar ru that normalized visible reflecta nce. The solar albedo was found constant and around 12.7% on each eco-type In both cases, albedo strongly increases with solar zenith angle, consisten t with ground measurements. These results clearly show the usefulness of co arse spatial resolution satellite data for the regional monitoring and mode ling of temperate coniferous ecosystems. (C) Elsevier Science Inc., 1998.