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Eléments d'hydrologie isotopique dans le bassin du lac Tchad

International Atomic Energy Agency
Publication Date
  • Evaporation
  • Nappe Phreatique
  • Pluie
  • Tracage Isotopique
  • Earth Science
  • Geography


Eléments d'hydrologie isotopique dans le bassin du lac Tchad I - *! ELEMENTS D'HYDROLOGIE ISOTOPIQUE DANS LE BASSIN DU LAC TCHAD J.-Ch. FONTES Laboratoire d e gkologie dynamique, Faculte des sciences, Paris, France et G. MAGLIONE, M.A. ROCHE Centre de Fort-Lamy (ORSTOM), Tchad Abstract - Resumé USE OF ISOTOPE HYDROLOGY IN THE LAKE CHAD BASIN. This study relates to precipitations, surface waters (Chari and Logone, secondary tributaries and Lake Chad), isolated evapcration basins and underground waters in the area north-east of Lake Chad. The method is based on isotope trace measurements after evaporation. The oxygen-18 content of rain storms is generally within the range +4% to -12700 vs SMOW. The weighted annual means from four stations distributed from the north to the south of the country vary from -7.84 to -5.29700. The mean contribution to the lake is estimated at approximately -5.3700. The variation in the isotopic composition of the Chari and the Logone is specified in relation to the hydrograph reading. The mean contribution of these two rivers to Lake Chad is estimated at 6l80 = -3.37m vs SMOW and that of all the tribu- taries and the rainwaters at -3.4700. The evaporative nature of Lake Chad is brought out by the variations in the oxygen-18 and deuterium contents of the waters. The enrichment which begins from the Chari delta is mainly a function of the relative humidity and may reach 20% oxygen-18 and 100% deuterium at the extreme end of certain river branches. The isotopic composition of the aquifers ( 6 ' * 0 = -570. i s SMOW) is evidence of meteoric feeding. Participation of the lake in the feeding of the aquifers cannot be detected beyond a limited littoral zone. Enrichment due to upward capillary movement of the aquifers in the clay beds of the Kanem evaporation basins reaches +20%0 oxygen-13 and +54700 deuterium at the moment of salt precipitation. These salts are carried by the infiltrating rainwater which has not underg

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