3.3 Optical measurements 
The effects of intnsic parameter of the soil surface (here 
soil moisture) observed under varying extrinsic 
conditions of measurement (spectral band, angle of 
incidence, solar illumination angle) are to be 
discriminated in order to establish the most sensitive 
and optimal conditions to detect soil surface changes 
due to slaking. 
Whatever the configuration of the instrument, it is 
clear, as shown in Figure 5, that the wet condition of 
soil moisture is not appropriated to discriminate stages 
of slaking. Soil moisture decreases reflectance level. Wet 
condition produce unsensitive and quite constant 
measurements. In dry condition, the reflectance globally 
increases with the stage of slaking as the Tortuosity 
index decreases. 
The effect of the solar illumination angle is shown in 
Figure 6. Whatever the spectral bands, it affects the 
reflectance level. At 6.00 am or 6.00 pm, the presence 
of shadows decreases the reflectance level. But it does 
not seem to affect the spectral discrimination of stage of 
slaking although some obtained experimental 
relationships are not continuously decreasing. 
Nevertheless, the most satisfactory relationships are 
obtained at midday whatever the spectral band and the 
XS3 band seems to be more interesting to detect 
changes in stage of slaking. 
The reflectance level is also affected by the chosen angle 
of incidence (0* or 23*). But, the relationships obtained 
at 23 s exhibit quite similar behaviors which do not 
modify conclusions discussed previously. 
4. CONCLUSIONS 
We have observed in controlled conditions that it is 
possible to discriminate stage of slaking on the based of 
multitemporal remote sensed data. 
For practical purposes, the possibility of slaking survey 
using active microwave remote sensing is heavily 
improbable. It is limited to a narrow configuration of 
the instrument only compatible with the use of an 
airborne system which will necessary have a limited 
spatial resolution. Thus, the greatest penetration depth 
of microwave renders the discrimination of slaking more 
dependent on soil moisture conditions in the near 
surface soil layers. 
Optical remote sensing technique seems to be more 
promising. Nevertheless, further studies will be 
necessary to study the effect of the initial soil roughness 
condition. The new measuring programs will also 
include various azimuthal viewing angles for the optical 
measurements. These studies will provide useful 
informations to be incorporated for the explanations of 
multitemporal SPOT images in order to assess soil 
erosion inventories. 
5 REFERENCES 
Bertuzzi P., Bruckler. L., Gabilly, Y. and Gaudu, J.C., 
1987. Calibration, field testing and error analysys of a 
gamma-ray probe for in situ measurement of dry bulk 
density, Soil Science, 144(6): 425-436. 
Bertuzzi, P., Caussignac, J.M., Stengel, P., Morel, G., 
Lorendeau, J.Y., and Pelloux, G., 1990a. An automated 
non contact Laser profile meter for measuring soil 
roughness in situ. Soil Science 149(3), in press, 8pp. 
Bertuzzi, P., Rauws, G. and Courault, D., 1990b. 
Testing roughness indices to estimate soil roughness 
changes due to simulated rainfall. Soil k Till., in press, 
13 pp 
Boiffin, J., 1984. La degradation structurale des couches 
superficielles sous l'action des pluies. Thèse de 
Docteur-Ingénieur, Inst. Nat. Agron., Paris, 128 pp. 
Boiffin, J., 1986. Stages and time - dependency of soil 
crusting in situ. In: Assessment of soil surface sealing 
and crusting, Callebaut, F.; Gabriels, D. and De Boodt, 
M. (Editors), Flanders Research Center for Soil Erosion 
and Soil Conservation, Ghent, 91-98. 
Cierniewski, J., 1987. A model for soil surface roughness 
influence on the spectral response of bare soil in the 
visible and near infrared range. Remote sensing of 
environment, 115: 23-97. 
Curran, P.J., Foody, G.M., Kondratyev, K.Ya., 
Kozoderov, V.V. and Fedchenko, PP., 1990. 
Reflectance of soil in the laboratory and field. In 
Remote Sensing of soil and vegetation in the USSR, 
edited by Taylor k Francis Ltd, London, Great Britain, 
pp 63-82. 
Guyot, G., Hannocq, J. F., Saint, G. and Buis, J. P., 
(1984). Mise au point d'un radiomètre de simulation 
SPOT, m Proceedings of 2 nd Int. Coll, on Spectral 
Signatures of Objects in Remote Sensing, held in 
Bordeaux, France, 12-16 Septembre 1983, edited by 
"Les colloques de l'INRA", INRA Publications, Pans, 
France, 23: 153-157. 
Ulaby, F.T., Batlivala, P.P. and Dobson, M.C., 1978. 
Microwave backscatter dependence on surface 
roughness, soil moisture and soil texture. IEEE Trans. 
Geos. Rem. Sens., 16: 286-295. 
Ulaby, F.T., Moore, R.K. and FUNG, A., 1982. 
Physical mechanisms and empirical models for 
scattering and emission. In Microwave remote sensing, 
Active and Passive, Volume II edited by 
Addison-Wesley Publishing Company, Reading, 
Massachusetts, USA, pp 816-921. 
Van der Heide, G. and Koolen, A.J., 1980. Soil surface 
albedo and multispectral reflectance of short-wave 
radiation as a function of degree of soil slaking. Neth. J. 
Agric. Sci., 28: 252-258. 
119