Niamey, the capital of Niger, is particularly prone to floods, since it is on the banks of the Niger River, which in its middle basin has two flood peaks: one in summer (the red flood) and one in winter (the black flood). In 2020, the Niger River in Niamey reached its all-time highest levels following an abundant rainy season. On the other hand, the floods in Niamey have been particularly frequent in the last decade, a symptom of a change in hydroclimatic behaviour already observed since the end of the great droughts of the 1970s and 1980s and which is identified with the name of Sahelian Paradox. This study, starting from the analysis of the 2020 flood and from the update of the rating curve of the Niamey hydrometric station, analyses the rainfall–runoff relationship on the Sahelian basins of the Medium Niger River Basin (MNRB) that are at the origin of the local flood. The comparative analysis of runoffs, annual maximum flows (AMAX) and runoff coefficients with various rainfall indices calculated on gridded datasets allowed to hydroclimatically characterise the last decade as a different period from the wet one before the drought, the dry one and the post-drought one. Compared to the last one, the current period is characterised by a sustained increase in hydrological indicators (AMAX +27%) consistent with the increase in both the accumulation of precipitation (+11%) and the number (+51%) and magnitude (+54%) of extreme events in the MNRB. Furthermore, a greater concentration of rainfall and extremes (+78%) in August contributes to reinforcing the red flood’s positive anomalies (+2.23 st.dev in 2020). The study indicates that under these conditions the frequency of extreme hydrological events in Niamey will tend to increase further also because of the concurrence of drivers such as river-bed silting and levee effects. Consequently, the study concludes with the need for a comprehensive flood-risk assessment on the Niamey city that considers both recent hydroclimatic trends and urbanisation dynamics in flood zones hence defining the most appropriate risk-reduction strategies.

中文翻译：

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2020 年尼亚美（尼日尔）洪水发源时尼日尔中河流域水文气候模式的最新变化

尼日尔的首都尼亚美特别容易发生洪水，因为它位于尼日尔河岸边，在其中部流域有两个洪水高峰：一个在夏季（红色洪水），一个在冬季（黑色洪水） ）。2020 年，尼亚美的尼日尔河在经历了丰富的雨季后达到了历史最高水位。另一方面，尼亚美的洪水在过去十年特别频繁，这是自 1970 年代和 1980 年代大干旱结束以来已经观察到的水文气候行为变化的症状，并被称为萨赫勒悖论. 本次研究，从2020年洪水的分析出发，从尼亚美水文站等级曲线的更新出发，分析了中尼日尔河流域 (MNRB) 萨赫勒流域的降雨-径流关系，这些流域是当地洪水的起源。径流、年最大流量 (AMAX) 和径流系数与在网格数据集上计算的各种降雨指数的比较分析允许将过去十年的水文气候特征描述为与干旱前的湿润时期、干旱时期和干旱后时期不同的时期一。与上一期相比，本期水文指标持续上升（AMAX+27%），与降水累积量（+11%）、数量（+51%）和量级的增加相一致（ +54%) MNRB 中的极端事件。此外，8 月更集中的降雨和极端事件 (+78%) 加剧了红色洪水的正异常（2020 年+2.23 st.dev）。研究表明，在这些条件下，尼亚美极端水文事件的频率将趋于进一步增加，因为河床淤积和堤坝效应等驱动因素的共同作用。因此，该研究的结论是需要对尼亚美市进行全面的洪水风险评估，同时考虑洪水区的近期水文气候趋势和城市化动态，从而确定最合适的降低风险策略。研究表明，在这些条件下，尼亚美极端水文事件的频率将趋于进一步增加，因为河床淤积和堤坝效应等驱动因素的共同作用。因此，研究的结论是需要对尼亚美市进行全面的洪水风险评估，同时考虑洪水区的近期水文气候趋势和城市化动态，从而确定最合适的降低风险策略。研究表明，在这些条件下，尼亚美极端水文事件的频率将趋于进一步增加，因为河床淤积和堤坝效应等驱动因素的共同作用。因此，该研究的结论是需要对尼亚美市进行全面的洪水风险评估，同时考虑洪水区的近期水文气候趋势和城市化动态，从而确定最合适的降低风险策略。