Geoscience Reference
In-Depth Information
The danger is considerable because cultivating land till then covered
by sparse vegetation increases evapotranspiration and hence the
concentration of salts, especially if a strongly mineralized irrigation
water is used. For example in Algeria, it has been calculated that the
irrigation of palms can bring to the soil between 40 and 60 tonnes of
various salts per ha annually (Daoud and Halitim 1994). Often, after a
few years of cropping, man is obliged to abandon the lands that he had
developed. This is also the case in Senegal, in Casamance to be more
exact. Moreover, it is thought that salinization of land has led to the
disappearance of many civilizations…
Risk of increase of natural salinity
13.6.3 Example of the Coachella Valley (CVWD 1993)
The Coachella valley in California is a subdesertic region with annual
rainfall of 80 mm. Europeans settled there in 1888 because they could
dig artesian wells from which water gushed out naturally. Then it
became necessary to pump it. Finally, the groundwater being exhausted,
they irrigated with water from outside from 1934 on. At present the
water comes from the Colorado River. The water is led in through a canal
256 km long that it takes five days to traverse. To satisfy the demand for
water, it is necessary to foresee it, for which calculation models of water
balance, remote command systems for sluice gates and a formidable
control room were all developed. At the end of the canal, the water is
stored in a reservoir of 1.9 million cubic metres.
The drainage waters are concentrated in the middle of the valley,
slightly lower, and finally are let into a lake born from a flood in the
Colorado river in 1900 but later artificially maintained by this process of
evacuation: the Salton Sea. Located 69 m below the level of the Pacific
Ocean, it has no outlet. Its level and salinity are rising every year. On
the contrary, the supply points of water from the Colorado are yet higher,
on the edge of the valley (Fig. 13.19).
More than 30,000 ha are irrigated and nearly 200 million m
3
of water
led in annually.
In California nowadays, they are turning to smaller projects. The
water is pumped from the ground and artificial ponds are dug to
dispose of the brines. These are
evaporation ponds.
Their bottom is made
impermeable by a layer of clay. But their storage capacity is limited.
In addition, they occupy more than 10 per cent of the treated area.
Current research consists of examining how the volume of brines could
