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[Departement_IRSTEA]GMA [TR1_IRSTEA]11 - VERSEAU / TRANSFEAU; The development of the knowledges in hydrology, and of the accuracy in its analysises, together with the remaining huge complexity of the hydrological processes, leads to a necessity for scientifically based synthesises of such knowledges, including modelization and not only display. Such models, beyond their internal utility, can allow interfacing with neibourghing sciences, and render more easy to use this knowledges for water managers. But for success, some basic adapted concepts of such models or displays have to be defined, understandable for non-hydrologists, and this is not obvious without specific research. When such conditions are satisfied, fructuous things appear, like emerging properties, always too rare in hydrology. Attention must be given to maintain sufficient links betwwen analytical and synthetical approaches, so as to ensure the internal coherency of hydrology.; Le développement des connaissances en hydrologie et de la finesse de ses analyses, associé à l'incontournable complexité des processus hydrologiques, rend nécessaire une approche scientifique rigoureuse des aspects de synthèse vis à vis de ces connaissances, et y compris à un niveau de modélisation et pas seulement d'affichage. De tels modèles, outre leur usage en interne à la discipline, permettent les interfaçages avec les disciplines connexes, et facilitent l'usage des connaissances hydrologiques pour la gestion des eaux et de leurs milieux. Mais il faut que les modes de représentations et de modélisation choisis soient articulables sur ceux de ces autres displines et usagers, ce qui n'est pas accessible a priori, sans concepts adaptés, voire créés, pour cela. Sous ces conditions, ces pistes de recherches de synthèses apparaissent fructueuses, et révèlent des propriétés émergentes fort appréciables, car rares, en hydrologie. Mais il faut veiller à bien interfacer les niveaux d'analyse et de synthèse, pour assurer la cohérence interne en hydrologie.

This study is part of the EQUANIS programme, the objectives of which are (a) estimating inputs from the River Niger to the inner Delta ; (b) computing the hydrological balance ; and (c) monitoring the quality of water in the central lacustrine basin. Eleven sampling locations were selected near gauging stations, both on the Niger River and its main tributary of the Bani River. Those sites have been sampled weekly since July 1990. The inner delta of the Niger River is a system particularly subject to sahelian and sub-desert climatic conditions and characterized by large flood plains. Time series of input water volumes in the inner delta, and of the water losses within it, show that the water losses are high, due to the intense evaporation, and vary from 6 to 40 km3. The water losses reach their maximum during the wettest years - up to 47%, and minimum during the driest years - only 32%, due to the reduction of the flooded area. The surface of the flooded area is inferred from the hydrological balance. The preliminary results of this study indicate that the Niger and Bani rivers have low levels of dissolved element concentrations. The mean conductivity values, ranging from 50-80 microSiemens/cm, increase regularly during the low water stage and decrease drastically with rising water. The pH values are slightly basic, ranging from 7.1 to 8. silica and bicarbonates are the main dissolved species ; they always represent more than 75% of TDS. In May, when the rising water stage begins, the waters are poorly mineralized at the input of the delta, while they have been enriched during the dry season in the delta. A good mass balance is found between inputs and outputs through the delta. However, a disequilibrium appears at the sampling sites within the basin, which could be partially linked to poor mixing between the Niger and Bani river waters... (D'après résumé d'auteur)