PERFORMANCES OF A C-BAND MICROWAVE SCATTEROMETER 
FOR MONOTORING SOIL SURFACE MOISTURE ON BARE SOIL 
P. BERTUZZIl*), L. BRUCKLER«*), A. CFIANZY < * > , 
G. GUYOT<**>, E. CHAPUIS(***> 
(*) Station de Science du Sol, Institut National 
de la Recherche Agronomique (I. N. R. A.), 
Domaine Saint-Paul, BP 91, 84143 Montfavet Cedex FRANCE 
(**) Station de Bioclimatologie, Institut National 
de la Recherche Agronomique (I. N. R. A.), 
Domaine Saint-Paul, BP 91, 84143 Montfavet Cedex FRANCE 
(***) Centre National d’Etudes Spatiales (C.N.E.S.), 
18 Avenue Edouard Bel in, 31055 Toulouse FRANCE 
ABSTRACT 
Microwave backscattering coefficients were measured on 0.1 ha 
bare fields (27.2% clay, 61.7% fine and coarse loam, 11.1% 
sand) using a 5.3 GFIz frequency. Gravimetric soil moisture 
content, dry bulk density, and soil surface roughness were 
recorded concurrently with microwave measurements. 
The effects of the microwave penetration depth, soil surface 
roughness, and spatial heterogeneity of soil moisture 
distribution on the calibration relationship between the 
backscattering coefficient and the volumetric water content 
were evaluated. Results showed that (1) accurate soil surface 
(0-5 cm) moisture content estimates could be derived from 
microwave measurements (standard deviation of 0.02 m 3 • rrr 3 ) , 
(2) microwave penetration depth and soil surface roughness 
effects were negligible, (3) When heterogeneous soil surface 
water contents were considered, mean backscattering 
measurements were related to the mean soil water content. 
Moreover, the physical optics model was tested. It was shown 
that it was able to describe the soil moisture dependence at 
15 and 23 angles of incidence, if near surface volumetric 
water content (0-1 cm depth layer) was used to compute the 
soil dielectric permittivity. 
kEY WORDS ¡Remote sensing,Microwave,C Band,Soil moi sture,Bare 
soi 1 
1 . INTRODUCTION 
Extensive studies have been performed 
by several researchers using active 
microwave remote sensing to test the 
capability of such equipment to measure 
the soil surface water content (Ulaby 
et al., 1978; Bradley and Ulaby, 1981; 
Le Toan et al.. 1981; Bernard et al., 
1982; Jackson and O’Neil, 1985; 
Bruckler et al., 1988) and to monitor 
the water budget at a field or regional 
scale. An optimal sensor configuration 
to measure soil water content (C band, 
polarization HFI, angle of incidence 
between 7 and 17) has been 
established (Ulaby et al., 1978): it 
was shown that using this optimal 
combination of frequency, look angle, 
and polarization, the backscattering 
coefficient was correlated to soil 
volumetric water content (Dobson and 
Ulaby, 1981). This paper is an 
experimental and theoretical 
contribution for evaluating under 
various field conditions on bare soils, 
the effects of different soil 
properties (soil surface water content 
gradients, soil surface roughness, soil 
heterogeneity) on backscattering 
coefficient measurements: 
(1) First, in most studies on 
microwave measurements on bare soils, 
experimental relationships between soil 
moisture content and backscattering 
coefficient were provided by mean 
volumetric water contents measured from 
the soil surface to an arbitrary soil 
depth (generally 0-5 or 0-10 cm). In 
fact, soil moisture gradients are 
sometimes very high in the top thin 
soil layers, which makes it difficult 
to attribute a real physical sense to 
calculated moisture content means. 
(2) Microwave measurements are 
generally performed on more or less 
heterogeneous soil surfaces: Soil 
surface water contents vary in a wide 
range during evaporation phases from 
one location to another for example, 
and thus, the radar is "looking" at 
heterogeneous thin soil layers. 
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