Modeling microwave backscatter from tree canopies.

Abstract

Forest ecosystems represent a significant portion of Earth's vegetation cover. While playing an important role in the global carbon cycle, the areal extent of forests, the rate of global deforestation and the amount of forest biomass remain key unknown parameters in understanding atmospheric carbon dioxide flux from these biomes. Spaceborne microwave imaging systems have been proposed as a means of assessing biophysical parameters of vegetation canopies. Such systems can monitor regions of the globe where environmental conditions render optical techniques ineffective. Scattering models that accurately simulate forest canopy backscatter allow measured radar data to be coupled to canopy parameters and significantly aid in applying remotely sensed data to understanding canopy physiological state. The goal of this work is to develop a robust microwave scattering model for forested areas. A tree canopy is characterized as an inhomogeneous medium comprised of discrete scatterers that represent the trunks, branches, stems, needles and leaves. Radiative transfer theory is applied to derive a first-order fully-polarimetric solution for backscatter. The fundamental contribution of this thesis is the development of a model that accounts for backscatter from forest canopies that have discontinuous crown layer geometries. By treating parameters describing the size, shape and location of individual tree crowns as random variables, a statistical approach is taken that defines the expected value of canopy backscatter. Application of the radiative transfer equations to the discontinuous canopy geometry is reviewed. The application of random variables defining the crown geometry and the incorporation of these variables into the radiative transfer solution is discussed. The resulting model is valid for microwave frequencies over a wide range of radar incidence angles. Model simulations are compared to results derived with the continuous canopy model. The effect of the open crown geometry is found to be most significant at shallow incidence angles and at high frequencies for trees with well-developed crowns. The model successfully couples canopy biophysical parameters to radar backscatter measurements. When compared to measured radar data, variations in backscatter that occur because of changing environmental conditions that cause changes in canopy water status are accurately predicted.