Full text: Mesures physiques et signatures en télédétection

L. Laguerre (1), Suresh Raju (2), A. Chanzy (2), Y.H. Kerr (1), J.C. Calvet (3), JP. Wigneron (4) 
(1) LERTS Unité mixte CNES-CNRS, 18 Av. E. Belin, 31055 TOULOUSE Cedex, FRANCE. 
(2) INRA - Science du Sol - 84143 MONTFAVET, BP91, Cedex, FRANCE. 
(3) INRA - Bioclimatologie - 84143 MONTFAVET, BP91, Cedex, FRANCE. 
(4) CNRM, 42 Av. G. Coriolis, 31057 TOULOUSE Cedex, FRANCE. 
A comparison between model predictions of the microwave emission from bare soils and multifrequency, both 
polarized experimental data from bare soils (using Portos ground-based radiometer) is presented in this paper. 
The study is based on two different approaches, one considering the effect of soil moisture and temperature 
gradients from a smooth surface (radiative transfer approach), the other considering the effect of the soil 
surface roughness with averaged dielectric properties (surface scattering models). 
KEYWORDS: Microwave radiometry, soil moisture, surface roughness, multifrequency, radiative transfer 
model, surface scattering model. 
The emission of microwave radiation from bare soils is influenced by many surface characteristics parameters 
such as soil moisture and temperature profiles, surface roughness, soil type and bulk density (Schmugge et al. 
1986, Jackson and Schmugge 1989). Previous studies have demonstrated the capability of microwave 
radiometry to provide soil moisture information. By using the lower frequency region of microwave spectrum, 
it is possible to recover soil moisture, from surface to a few centimeters of depth (Newton et al. 1982, Wang 
1987). However, the soil surface roughness affects the sensitivity of microwave measurements to the soil 
moisture determination (Choudhury et al 1979, Wang el al. 1983). Nevertheless, despite these numerous 
works, few confrontations between theoritical models predicting soil microwave brightness temperature from 
bare soils and a wide range of experimental data were attempted (Schmugge and Choudhury 1981, Tsang and 
Newton 1982, Mo et al. 1987, Coppo et al. 1991, Saatchi and Wegmuller 1992). The aim of this study is to 
understand the physical phenomenons involved in the microwave emission through the analysis of: (1) soil 
moisture profile influence using the radiative transfer approach, (2) surface roughness effects using the theory 
of scattering from rough surfaces. The objective is the evaluation of these models using experimental data 
combining different surface roughness and soil moisture conditions. 
Radiative transfer models 
The radiative transfer models commonly used are based on a coherent approach developped by N’joku and 
Kong (1977), and Wilheit (1978); and a non-coherent approach (Burke et al. 1979). Schmugge and Choudhury 
(1981) performed a comparative study and concluded that coherent models provide more accurate predictions, 
particularly for moisture conditions with large gradients. In this study, the Wilheit model is considered for the 
brightness temperature simulation (TB). It considers the soil medium as series of N dielectric layers with first 
and last layers semi-infinite. The soil moisture and temperature are taken homogeneous in each layer. 
Assuming a propagation of the electromagnetic radiation through the soil medium, the net electromagnetic 
radiation in each layer (Sj) is obtained by the Poynting theorem and boundary equations at each interface. 
Hence the net energy flux (Fj) absorbed by the jth layer and observed at the first layer is the difference 
between the energy radiation entered into the jth layer at the interface j-1 and the energy radiated from jth 
layer into (j+l)th layer at the jth interface. If the jth layer is assumed to be in a thermally equilibrium state at a

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