ABSTRACT
Risk of increase of natural salinity ƒ 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…
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).
