114
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.
