Salt distribution in the Senegal middle valley. Analysis of a saline structure on the future irrigation schemes from N'Galenka creek.
Résumé
In the middle Senegal valley, the saline soil distribution is not related to the present faint topography. This lack of logic is one of the major constraints for establishment of new irrigated schemes. The salt distribution is here studied to better understand its variability, and to describe its structure and spatial arrangement. Saline areas are delineated by measuring the electromagnetic soil conductivity (ECm), a rapid technique with a portable instrument (EM38). The results indicate that the saline soils are distributed as stripes. A detailed examination revealed that this major stripe is actually composed of two parallel minor stripes, and the comparison with the aerial photograph shows that one lies in a former creek bed, and the other is fringing it on the southern bank. The stripe is intersected by an actual creek bed, indicating that the salt distribution is ancient, related to the former geomorphology, and does not result from a recent remobilisation of the marine salt deposits incorporated in the soil. The identification of this relationship between the present saline soil distribution and the former geomorphology allows us to survey the whole N'Galenka region (about 6000 ha) using the ECm measurements on selected transects.
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