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Title
Mesures physiques et signatures en télédétection

477
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