Geoscience Reference
In-Depth Information
sodium from the exchange complex. Then, in the long term, we go to
irrigating with a water that, ideally, is slightly saline and calcium-rich.
It is better to eliminate sodium from the exchange complex in several
steps. Less water is required because between leaching irrigations,
equilibrium with the soil solution is established, which means desorption
of the sodium. Then this ion in solution can be eliminated more easily
by the next application of water.
When drainage is not available, when leaching irrigation is practised,
the risk is great of the saline groundwater rising, invading the soil
and rendering it infertile. The groundwater rises quickly because it
represents only the interstitial water in the soil (little water is required
to make a jug previously filled with glass balls to overflow…). Only
some sloping lands are drained naturally and escape this phenomenon,
at least in their upper part. In the floodplain of the Tadla in Morocco,
the groundwater that had been at 13 m rose almost to the surface in five
years (Gallez
et al
. 1981). In Niger, it has come up from 40 m in less than
30 years (Condom 2000). In eastern Australia, irrigated since 1935, the
problems appeared around 1950. Since then it was necessary to drain
on a vast scale. The same dysfunction is manifested in the Euphrates
valley (Dosso 1980), in the valley of the Senegal River (Meyer 1997), in
that of the Syr Daria that flows into the Aral Sea, etc. The risk is higher
when furrow-irrigation is done because much water is lost (Bouteyre
1986). It is a world problem.
In principle, the water table must be maintained at least 1.5 or 2 m
deep. Drainage through buried pipes is desirable. But clays, peptized by
sodium, can pass into the drains and clog them. Wrapping the latter in
casings to act as filter is sometimes recommended. Drainage through
pits is generally less efficient and involves digging of deep channels to
ensure the saline water does not move up by capillarity to the roots of
crops. In permeable alluvium, we can dig wells and establish pumping
stations. The extracted water is then reused for irrigation if it is not too
saline. One method consists of mixing it with water of better quality
and applying it to areas that are then very carefully drained through
buried drains (Wildes 1983).
In all cases, drainage costs a lot and its immediate effect is not easy
for the people concerned to comprehend because they are, first and
foremost, victims of drought!
Risk of rise of water table
