The Lake Chad Basin (LCB) is one of the main endorheic basins in the world and has undergone large-level and surface variations during the last decades, particularly during the Sahelian dry period in the 1970s and the 1980s. The Chari–Logone River system covers 25% of the LCB but accounts for up to 82% of the Lake Chad water supply. The aim of this study is to investigate the dissolved phase transported by the Chari–Logone system, in order (1) to elucidate the origin and the behavior of major elements and the weathering processes in the watershed; (2) to estimate the total dissolved flux, its variability over the last decades and the driving factors. To do so, samples were collected monthly between January 2013 and November 2016 at three representative sites of the basin: in the Chari River in “Chagoua,” in the Logone River in “Ngueli” just before the confluence of both rivers, and at a downstream site in “Douguia,” 30 km after the confluence. Concentrations in major elements displayed significant seasonal variations in the Chari and Logone waters. At the seasonal time scale, the comparison between the concentrations of chemical elements and the flow rates showed a hysteresis loop. This hysteresis behavior corresponds to a variable contribution over time of two water bodies, fast surface water, and slow groundwater, the latter carrying higher concentrations and Ca/Na ratio, which may result from the contribution of pedogenic carbonate weathering to the dominant signature of silicate weathering. At the annual time scale, similar average concentrations are observed in the Chari and Logone Rivers, despite contrasted annual runoff. In addition, an interannual stability of ionic concentrations was observed in the Chari–Logone River during the flood regime, both during the years covered by our monitoring (2013–2016) and during the pre-drought period (1969, 1972 and 1973). This situation corresponds to a chemostatic behavior, where the annual river discharge is the main factor controlling the interannual variation of chemical fluxes.

中文翻译：

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剧烈变化的水文气候条件下乍得湖支流的地球化学

乍得湖盆地（LCB）是世界上主要的背胶盆地之一，在过去的几十年中，尤其是在1970年代和1980年代的萨赫勒干旱时期，经历了大范围的地面变化。Chari–Logone河水系统占LCB的25％，但占乍得湖供水的82％。本研究的目的是研究Chari–Logone系统输送的溶解相，以（1）阐明流域中主要元素的成因和行为以及风化过程；（2）估算总溶解通量，过去几十年的变化和驱动因素。为此，在2013年1月至2016年11月期间，每月在流域的三个代表性地点采集样品：位于“ Chagoua，”位于两河汇合之前的“ Ngueli”洛格内河，以及“ Douguia”汇合后30公里处的下游站点。主要元素的浓度在Chari和Logone水域表现出明显的季节性变化。在季节性时间尺度上，化学元素浓度与流速之间的比较显示出磁滞回线。这种滞后行为对应于两个水体随时间变化的贡献，即快速地表水和慢地下水，后者具有较高的浓度和Ca / Na比，这可能是由于成岩碳酸盐风化对硅酸盐的主要特征的贡献风化。在年度时间尺度上，尽管年度径流量相反，但在Chari河和Logone河中观察到了相似的平均浓度。此外，在我们监测的年份（2013-2016年）和干旱前期（1969年，1972年和1973年），在洪水期间，在Chari-Logone河中观察到离子浓度的年际稳定性。这种情况对应于一种化学稳定行为，其中每年的河流流量是控制化学通量年际变化的主要因素。