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VEGETATION MODELS USED TO RETRIEVE GEOPHYSICAL PARAMETERS 
FROM PASSIVE MICROWAVE MEASUREMENTS 
Jean-Pierre WIGNERON 1 , Yann KERR 2 , Frédéric BIARD 1 , Nadine BRUGUIER 1 
1 INRA, Bioclimatologie, BP91, 84143 MONTFAVET cedex, FRANCE, Phone 
90316094, Fax 90899810 
2 LERTS UMR CNES-CNRS, 18 Av Ed BELIN, 31055 TOULOUSE cedex, FRANCE, 
Phone 61274472, Fax 61281410 
ABSTRACT: 
Passive microwave measurements are sensitive to several geophysical parameters of interest to assess 
global energy and water balance of vegetation-covered land surfaces: surface temperature, soil 
moisture and vegetation water content. Depending on the conditions of the study (sensor performance, 
spatial and temporal resolution of the microwave observations, observed vegetation canopy, ground 
measurements availability, etc.), numerous approaches have been used to retrieve surface parameters 
from passive microwave signatures. In this paper a survey of current retrieval methods together with 
examples of potential applications of passive microwave measurements over vegetation cover are 
presented. Considering the large number of input parameters contributing to the microwave emission, 
efforts have to be made to integrate a priori information and synthetic description of vegetation effects 
into the inversion algorithms. 
KEY WORDS: retrieval, algorithm, microwave, emission, vegetation, soil, inversion, model 
1. INTRODUCTION 
Passive and active microwave measurements are very attractive since microwave radiations are weakly 
sensitive to solar illumination effects and to the atmosphere at low frequencies. Numerous studies 
showed that passive microwave measurements can be efficiently used to monitor soil and vegetation 
characteristics. The methods are either based on satellite measurements (England et al., 1992; 
Choudhury and Wang, 1990; Van de Griend and Owe, 1993), or at ground level (Matzler, 1990; 
Paloscia and Pampaloni, 1992; Wigneron et al., 1993a). The surface microwave emission intensity 
(expressed as a brightness temperature T Bp ) can be written as the product of an effective emissivity 
(e p ) and an effective temperature (T eff ). Due to the high sensitivity of soil dielectric permittivity to 
water content, bare soil microwave emissivity is a good indicator of soil moisture content. A linear 
relationship between soil emissivity and soil water content can often be established. The slope of 
regression is mainly dependent upon surface roughness effects (Wang et al., 1983). Under vegetation 
cover, the global microwave emission is a combination of soil emission attenuated by the vegetation 
layer, and of vegetation emission. As biomass and frequency increase, the contribution of vegetation 
emission becomes predominant. The vegetation layer microwave emissivity is mainly determined by 
the dielectric constant of the vegetation material (usually related to its volumetric water content) and 
by the vegetation structure (in terms of leaves and branches density, size, shape, orientation and spatial 
distribution). Accurate estimation of vegetation effects is therefore necessary to monitor both 
vegetation and underlying soil characteristics (Jackson and Schmugge, 1989). Surface effective 
temperature T eff depends on the moisture and temperature profiles, but in most applications it is well