Geoscience Reference
In-Depth Information
were laid (Fig. 8.15). It is therefore, relatively porous if the work had been
done properly, i.e., in the dry season. Secondly, the poorly permeable
layer, despite everything, has a hydraulic conductivity of 0.3 to 0.6 mm
per day. This is not negligible, however, according to the authors we
have cited. Indeed, a permeability of 1.3 or 1.6 mm per day is sufficient
to evacuate the monthly surplus of water in winter (P - PET), which
rarely exceeds 50 mm in the temperate zone.
Poorly permeable layer
Drainage trench: turned-over soil
Movement
of water
Silty A horizon
50 cm
Cross-section
of drain
Clayey B horizon
120 cm
To an
outlet
Silty clay BC horizon
Fig. 8.15
Positioning of drains at 80-90 cm within the impermeable horizon.
The scientist is, therefore, required to closely examine the structure
of the horizons of the soils for which he wishes to plan the drainage.
Obtaining a
coefficient of restitution
of 33 per cent at the level of the drains
means that the water evacuated will be one-third of the precipitation.
We cannot hope for any better.
CONCLUSION
Many arguments have led us to take a fresh look at the classic model
of clay illuviation in which the clay missing from the E horizon is
presumed to be that found in the B. Actually, in Luvisols, most of the
clay of the B horizon does not come from the A but is the product of
weathering
in situ
. The depletion of the E is a complex phenomenon
involving simultaneously vertical translocation, lateral transport and
degradation of clays, the last process probably being preponderant. At
the same time, this transfer of ferric clay is evident at the base of the
soil profile in the early stages of development, whereas the translocation
of deferrified clay remains a hypothesis not fully consistent with careful
observations made in the tongues. This, therefore, is contrary to what
is often described (blocking of the clay by cations)! But it is true that
argilluviation starts only when calcium carbonate is lacking.
