Science et changements planétaires / Sécheresse


L’alcalinisation/sodisation, un danger pour les périmètres irrigués sahéliens Volume 5, issue 3, Septembre 1994

Cirad-Ca BP 5035 34032 Montpellier cedex 1, France, 1ER SRCVO BP 438 Bamako, Mali
  • Page(s) : 161-71
  • Published in: 1994

The soil of the major sub-Saharan irrigated farmlands is becoming degraded due to two complementary factors: rising water tables and alkalisation/salting up. The water tables in the Niger Office region have risen 50 m in 50 years. This uplift in phreatic water is due to poor control over irrigation, the choice of soils ill-adapted to rice growing, and the design and distribution of water and waste water evacuation. After some fifty years of irrigated farming, alkalisation/salting up has affected 30 to 50% of the land. Foreseeing this phenomenon when the older irrigated farmlands were first established was impossible, since not enough means for assessing water quality were available. It is now known that although the major sub-Saharan rivers are very little mineralised (which led to the belief that they were also of very good quality), their ion imbalance tends towards the sodium end of the scale. When mineral concentration occurs due to water evaporation, the imbalance in-creases. Because the ionic load of clays finds an equilibrium with that of the water in contact with the ground (irrigation and phreatic water), the soil tends to become more alkaline and salt. Under these conditions, the clay defloculates, with the consequent breakdown of structure and highly decreased permeability and porosity. Mineral deficits then appear. This can be seen in the reduced number of possible crop choices, the increasing difficulties in farming itself, the substantial drop in yield and, ultimately, the abandoning of land a lot of money has been put into. Methods of prevention and rehabilitation are discussed. The former requires a certain rethinking of rural planning and land and water management. It may also demand a revision in present-day farming practices. The latter implies money, and the greater the degree of environmental degradation, the greater the expense. Together, they consist in mineral amendments and the implementation of efficient drainage.