Geoscience Reference
In-Depth Information
The same phenomenon is reproduced at the scale of ridges and
furrows from ploughing. The salts accumulate on the higher points.
Farmers often adopt strategies for establishing their plants at about a
decimetre in suitable areas. On the other hand, in sandy areas, capillary
rise is limited.
The height above the saline water table is important to consider.
In the Camargue, when it is 50 cm or less, only adapted plants survive.
If it is as much as 1.5 m, some crops are possible (vineyards). If it is
greater than 2 m, trees establish without difficulty.
13.4.2 Simplified Modelling of Transport
Two mechanisms are responsible for transport in the soil:
Molecular diffusion
within a solution at rest according to a
concentration gradient of salts. This mechanism acts mostly in
moist soil.
Hydrodynamic dispersion
, also called
convective
or
advective transport
.
This mechanism is dominant in saturated soils when the speed
of flow of the soil solution is high. The displacement takes place
downward, or even upward by capillary rise of water.
Modelling of transport is useful for predicting salinization of the
soil under the effect of irrigation water of poor quality or for predicting
desalinization of soil under the effect of pure water.
The piston effect is a very simple way of modelling the leaching
of salts (Fig. 13.10a). Laboratory tests are done with chlorinated water
because Cl
-
does not interact with the solid phase. This water is
presumed to push the soil water in front of it and replace the latter
totally. But the reality is often more complex (Fig. 13.10b).
a. EXPERIMENT
b. RESULTS
Continuous addition
of water containing
chlorine
1
Theoretical
c
urve for a
perfect
piston
effect
Soil + salts not
containing Cl
-
-
Actual
response
curve
Continuous
determination of
chloride in the
percolate
2
1
Number of pore volumes
Fig. 13.10
Experiment showing the limitations of a piston-effect approach (Condom 2000).
