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Proceedings of the Symposium on Global and Environmental Monitoring

2.1 Location and Site Description
The experiments were conducted at the Soil
Science Laboratory of the Institut National de la
Recherche Agronomique (I.N.R.A.) of Montfavet
(France) during July 1988. The soil was a clay loam
with about 27 % of clay and 62 % of silt.
The field (0.1 ha) was initially tilled with a rotary
digging machine. Soil clods were arranged in an
apparently random manner and resulted from soil break
up tillage implements (no tools marks or tillage
directions). The general soil surface was flat.
Rainfall was simulated using a sprinkler irrigation
equipment composed of an aluminium frame (20 m
long) with regularly distributed sprinklers and
supported by two pneumatic tyred wheels on each side.
The irrigation system was moved across the
experimental field on two tramlines located on each side
of the field. Rainfall was applied during 4 successive
periods to simulate five stages of slaking (including the
initial stage). The rainfall intensity was approximately
40 mmh' 1 . The cumulative rainfall precipitation was
At each stage of slaking, ground observations
concurrent with remote sensing measurements were
performed on the experimental field under wet and dry
surface soil moisture conditions. Wet condition is
obtained after completely surface draining (no puddles).
Dry condition is reached by natural evaporation during
two or three days.
2.2 Remote sensed data.
Table 1 : Spectral band of the SPOT
simulation radiometer.
Wavelength (nm)
500 - 590
600 - 690
790 - 900
green band
red band
near infrared band
This instrument has two heads, (l)the first is equipped
with diffusing screens and measures irradiance. It was
fixed at the bottom of the crane. (2)the second is
equipped with a 12° field of view and measures field
target radiance. It was fixed at the mobile platform.
The plot position, in respect to viewing and
illumination directions are seen in Figure 2. The
radiometer is enable to simulate SPOT sattelite data
taken in nadir viewing angle (0* from nadir) and
oblique viewing angle, here 23° (from to nadir). The
choosen angle of incidence was different but quite near
the oblique viewing angle of the SPOT satellite (27‘
relative to nadir). One cloud free day, thirty
radiometric measurements in the three spectral bands
were recorded during the travel of the platform and
averaged. Radiometric measurements were performed at
three approximately constant solar illumination angle at
6.00 am, 12.00 and 6.00 pm G.M.T.
The two instruments were mounted an a mobile
platform of a crane-boom. They were at about 19.70 m
height above the soil surface. Remote sensing
measurements were performed on about one third of the
experimental field (Figure 1).
2.2.1 Microwave measurements. We used the
scatterometer RAMSES designed by the Centre
National d'Etudes Spatiales (C.N.E.S.) of Toulouse
(France). It is a frequency-modulated continuous wave,
multifrequency and multipolarization system.
Microwave measurements were made at 5 3 GHz (C
band) and HH polarization. According to theoretical
and experimental results (Ulaby et al., 1978; Ulaby et
al., 1982), we used only small or large angles of
incidence to increase the sensitivity of microwave
measurements to roughness changes. The two angles of
incidence were fixed at 0‘ (nadir j and 50' from nadir.
The surface foot print varied from 3 m 2 at nadir to 15
m 2 at 50*. Microwave measurements were expressed in
term of the backscattermg coefficient ( in units of decibels. Spatial averaging of the measurements was achieved by recording 200 during the travel of the platform along the crane-boom.
2.2.2 Optical measurements. They were performed in
three different spectral bands (Table 1.) using a SPOT
simulation radiometer (Guyot et al, 1984).
2.3 Ground measurements
Depending on soil moisture conditions, ground
observations of soil moisture, soil roughness and
determination of stage of slaking were performed on the
the field area not seen by the intruments (Figure 1).
Ten gravimetric water content samples were collected
approximately at the same location to measure soil
moisture (Figure 1). They were taken from 0 to 2 cm
depth. Dry bulk densities were measured at 3 cm depth
using a gamma transmission probe (Bertuzzi et al.,
1987). They were extrapolated at the 0-2 cm soil layer
to compute volumetric water contents by combining
gravimetric water contents and dry bulk densities.
An automated non-contact Laser profile meter was
used to measure changes in soil surface roughness
(Bertuzzi et al., 1990a). Each profile was 2 m long with
1000 elevation data recorded at 2 mm sampling
interval. The accuracy on elevation data was less than
0.25 mm. After each rainfall, twelve profiles, regularly
distributed on the experimental field were recorded
always at the same location (Figure 1). Six profiles were
recorded in the main longitudinal eastern -» western
direction of the field, parallel to the tillage direction.
The six other profiles were recorded in the nothern ->
southern direction perpendicular to the tillage direction.
At each stage of slaking, roughness profiles were used to
compute two roughness indices. The first one is the
most common standard deviation of height (s). The