Processus de minéralisation et impact de l’irrigation sur les ressources en eau souterraine au Sahel : Cas du périmètre irrigué de Birni N’Konni (Sud-Niger)
Abstract
Au Sahel, en raison de la forte variabilité interannuelle des eaux de surface, les ressources en eau souterraine représentent la ressource la plus fiable pour l’irrigation. Toutefois, une évaluation préalable de leurs disponibilité, qualité et dynamique pluriannuelle s’avère nécessaire afin de garantir l’efficacité et la durabilité des systèmes de productions. Une étude associant les données piézométriques et géochimiques a été réalisée à postériori dans le périmètre irrigué de Birni N’Konni (Sud Niger) afin d’évaluer l’impact de l’irrigation sur les ressources en eau souterraine. Les résultats montrent que les eaux sont principalement sulfatées calciques et magnésiennes (75%) et bicarbonatées calciques et magnésiennes (25%). Le diagramme de Gibbs, l’analyse en composante principale (ACP) et les relations ioniques montrent que la minéralisation des eaux est principalement contrôlée par le processus d’interaction eau-roche et dans une moindre mesure par l’évaporation de la nappe. L’utilisation de différents indicateurs conventionnels de la qualité de l’eau (EC, SAR, %Na, RSC, MAR et le IP) pour l’irrigation suggère que l’eau souterraine dans la zone est classée dans une large gamme entre excellente et impropre et présente, à court et à long terme, de forts risques de salinisation qui suggéreraient un recours aux cultures de plantes plus tolérantes au sel. Les données piézométriques montrent que le développement de l’irrigation et le faible taux de renouvellement ont entraîné une baisse généralisée du niveau de la nappe de plus de 4 m depuis les années 1960. Cette étude montre que le risque de salinisation à court ou à long terme demeure permanent mais reste moins préoccupant que la baisse du niveau de la nappe.
In the Sahel, due to the high inter-annual variability of surface water, groundwater is the most reliable resource for irrigation. However, a prior assessment of their availability, quality, and multi-year dynamics is necessary to guarantee the efficiency and sustainability of production systems. A study combining piezometric and geochemical data was carried out posteriori in the Birni N'Konni irrigated perimeter (southern Niger) to assess the impact of irrigation on groundwater resources. The results show that the water is mainly calcium-magnesium sulfate (75%) and calcium-magnesium bicarbonate (25%). The Gibbs diagram, principal component analysis (PCA), and ionic relationships show that water mineralization is mainly controlled by the water-rock interaction process, and to a lesser extent by groundwater evaporation. The use of various conventional water quality indicators (EC, SAR, %Na, RSC, MAR and the PI) for irrigation suggests that groundwater in the area is classified in a wide range between excellent and unsuitable and presents, in the short and long term, high risks of salinization, which would suggest the use of more salt-tolerant crops. Piezometric data show that the development of irrigation and the low renewal rate have led to a generalized drop in the water table of over 4 m since the 1960s. This study shows that the risk of salinization in the short or long term remains permanent, but is less of a concern than the drop in the water table.
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