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Proceedings of the Symposium on Global and Environmental Monitoring

second one is the Tortuosity index (T) which is defined
by the ratio :
L -Lo
T =
where L is the actual length of the profile and Lq is the
projected horizontal length of the profile. One major
interest of the tortuosity index is related to its smaller
sensitivity to field variability compared to other
roughness indices (Bertuzzi et al., 1990b).
Soil surface changes resulting from the sealing processes
was evaluated on three localized 1 m 2 surfaces of
reference using the method proposed by Boiffin (1984,
1986). According to visual cntera, the evolution of the
soil surface structure during slaking can be classified in
five typical stages. (1) So, initial soii surface resulting
from tillage. (2) Si strutural crust. Soil aggregates and
clods disintegrate due to raindrop impacts.Continuous
patches appear and expand due to interstitial infilling.
(3) S\ + local appearance of depositional crusts. (4) S2
depositional crust; the fragmented layer becomes
continuous; depositional areas are formed in small
surface depressions where puddles appear during
rainfall (5) SU the soil surface appears to be completely
close and continuous.
3.1 Ground measurements
Mean values of soil volumetric water content were
estimated from the 10 samples. They were plotted
against the degree of slaking in Figure 3. The vertical
bar corresponds to + /- standard deviation of the 10
samples. In wet condition, soil draining contributes to
decrease soil moisture very quickly during the diurnal
phase of the day. Dry condition corresponded to mean
water contents less than 0.19 cm +3 .cm' 3 . This limited
value may appear high. They must analysed in the
context of the chosen sampling procedure. Whatever
the spectral bands, it is not possible to attribute a real
physical sense to calculated soil moisture. If fact, soil
moisture gradients are generally very high in the top
soil layers particularly in dry conditions. The sampling
depth averaged the effect of water gradients. For optical
measurements, dry conditions always corresponded to
uniform and light soil surface.
Table 2 shows the decrease of the two roughness indices
as functions of the cumulative rainfall and the stage of
slaking. These are mean values computed from the
results of the twelve sampled roughness profiles.
3 2 Microwave measurements
In Figure 4, the backscattering coefficient is plotted
against the Tortuosity index. The vertical bar displays
the magnitude of the three backscatter measurements
obtained during the diurnal phase of the day for each
soil moisture condition and the corresponded stage of
slaking. Whatever the angle of incidence, a change in
soil moisture conditions increases the curve level and
Figure 3 : Mean volumetric water content as
a function of the stage of slaking.
Table 2. Mean roughness indices as functions
of cumulative rainfall and the stage
of slaking.
S 2 +
the variability
of the
measurements. It is always smaller in wet condition
although the volumetric water content decreases rapidly
( Figure 3)
At 50° of angle of incidence, the backscattering
coefficient, exhibits a small increase with the Tortuosity
index. But, the sensitivity of the backscattering
coefficient is the order of magnitude of the observed
variability. At nadir (0°), the backscattering coefficient
is more sensitive to roughness changes due to slaking
whatever the soil moisture conditions.
In term of the standard deviation of height (s), the
decrease of surface roughness (about 4 mm) due to
slaking may appear very small. Consequently, only
backscattering measurements of the coherent
component obtained in the specular direction are
sufficiently sensitive to detect roughness changes due to
slaking. This will be probably verified whatever the
initial roughness conditions.