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REFLECTANCE IN SPOT BANDS AND MICROWAVE 
BACKSCATTERING COEFFICIENT IN C BAND 
AS A FUNCTION OF STAGE OF SLAKING OF BARE SOIL 
Patrick BERTUZZI 1 ,Dominique COURAULT 2 
Gérard GUYOT 3 and Emile CHAPUIS 4 
1 I.N.R.A., Station de Science du Sol, Domaine Saint Paul, BP 91, 
84143 Montfavet Cedex, France. 
2 O R S T O M., L I A. de Télédétection, 70-74 route d'Aulnay, 
93163 Bondy Cedex, France 
3 I.N.R.A., Station de Bioclimatologie, Domaine Saint Paul, BP 91 
84143 Montfavet Cedex, France. 
4 C.N.E.S, 18 avenue Edouard Belin, 31055 Toulouse Cedex, France. 
ABSTACT 
Slaking process results from the effects of rainfall impacts on soil surface. It contributes to 
decrease soil surface roughness. It may result in the formation of a thin and dense top soil 
layer which affects vertical heat, air and water fluxes. 
A field study was carried out to determine both : 
(1) the reflectance behavior in visible (XS1,XS2 SPOT bands) and near infrared (XS3 
SPOT band) 
(2) the microwave backscatter behavior in C band and polarization HH. 
from bare soil as a function of degree of slaking. 
The experimental field (0.1 ha) was a clay loam soil initially tilled with a rotary digging 
machine.The tillage treatment was performed to provide a seed-bed. Artificial rainfalls 
was used to simulate five steps of slaking. Remote sensed data were obtained under wet 
and dry surface soil moisture conditions for various angles of incidence and illumination 
angles in the case of the optical measurements. Optimal microwave and optical 
configurations of the two instruments were discussed as a function of the stage of slaking. 
KEY WORDS: Remote sensing; Visible, Near Infrared, Microwave bands, Bare Soil, 
Slaking. 
1. INTRODUCTION 
Water erosion and runoff concentration are serious 
factor causing soil degradation in loamy soil. Mam 
damages are related to the accumulation of excess 
surface water and the concentration of runoff in nils 
and gullies during rainfalls. They involve as the result 
of the sensitivity of loamy soil to the slaking that 
contributes to the structural degradation of the soil 
surface. Even in moderate climatic conditions, 
cumulative rainfall impacting involves fairly rapid and 
extensive degradation of the tilled soil surface causing 
the decrease soil roughness and infiltration capacity of 
soil. It may result in the formation of a thin dense top 
layer which limit infiltration of rain and also heat and 
water transfer and seedling emergence. 
Since several years, with the advent of airborne and 
spaceborne remote sensing, studies of spectral behavior 
of bare soil in various spectral bands have been 
conducted. The influence of surface roughness on the 
soil spectral behavior has been recongnized whatever 
the spectral band by many authors (Ulaby et al., 1978; 
Ulaby et al., 1982; Cierniewski, 1987; Curran et al., 
1990). Slaking induces sensitive changes in the 
organization of the soil surface roughness which may 
produce significant changes in the spectral behavior of 
bare soil. Laboratory studies reported by Van der Heide 
and Koolen (1980) have demonstrated this for optical 
measurements. In this paper, we investigated under 
field conditions mapping feasability of stage of soil 
slaking using either (1) optical measurements obtained 
from a SPOT simulation radiometer and (2) active 
microwave measurements in C band. Nevertheless, 
whatever the spectral band, (1) mtnsic factor like the 
surface water content for optical and microwave 
measurements or (2) extrinsic factor like the solar 
illumination for optical measurements have an influence 
on the spectral response of bare soil. Consequently, to 
demonstrate the ability of remote sensing technique to 
monitor stage of slaking implies to identify the optimal 
configuration of measurement which gives the most 
sensitive variations in spectral response due to changes 
in soil roughness.