The Nairobi volcano-sedimentary regional aquifer system (NAS) of Kenya hosts >6 M people, including 4.7 M people in the city of Nairobi. This work combines analysis of multi-decadal in-situ water-level data with numerical groundwater modelling to provide an assessment of the past and likely future evolution of Nairobi’s groundwater resources. Since the mid-1970s, groundwater abstraction has increased 10-fold at a rate similar to urban population growth, groundwater levels have declined at a median rate of 6 m/decade underneath Nairobi since 1950, whilst built-up areas have increased by 70% since 2000. Despite the absence of significant trends in climatic data since the 1970s, more recently, drought conditions have resulted in increased applications for borehole licences. Based on a new conceptual understanding of the NAS (including insights from geophysics and stable isotopes), numerical simulations provide further quantitative estimates of the accelerating negative impact of abstraction and capture the historical groundwater levels quite well. Analysis suggests a groundwater-level decline of 4 m on average over the entire aquifer area and up to 46 m below Nairobi, net groundwater storage loss of 1.5 billion m³ and 9% river baseflow reduction since 1950. Given current practices and trajectories, these figures are predicted to increase six-fold by 2120. Modelled future management scenarios suggest that future groundwater abstraction required to meet Nairobi projected water demand is unsustainable and that the regional anthropogenically-driven depletion trend can be partially mitigated through conjunctive water use. The presented approach can inform groundwater assessment for other major African cities undergoing similar rapid groundwater development.

肯尼亚的内罗毕火山沉积型区域含水层系统（NAS）容纳600万人，其中包括内罗毕市的470万人。这项工作将对多十年原位水位数据的分析与地下水数值模拟相结合，以评估内罗毕地下水资源的过去和未来可能的演变。自1970年代中期以来，地下水抽取量以与城市人口增长相近的速度增加了10倍，自1950年以来，内罗毕下的地下水位以10 m /十年的中位数速率下降，而建成区面积却增加了70％自2000年以来。尽管自1970年代以来气候数据没有显着趋势，但最近，干旱条件导致增加了钻孔许可证的申请。基于对NAS的新概念理解（包括来自地球物理学和稳定同位素的见解），数值模拟可提供对加速的抽象负面影响的进一步定量估计，并能很好地捕获历史地下水位。分析表明，整个含水层区域的地下水位平均下降了4 m，在内罗毕以下下降了46 m，净地下水储量损失为15亿m自1950年以来，河流底流减少了³％和9％。根据当前的做法和轨迹，到2120年，这些数字预计将增加六倍。模拟的未来管理方案表明，满足内罗毕预计的需水量所需的未来地下水抽取是不可持续的，并且该地区通过联合用水可以部分缓解人为驱动的消耗趋势。所提出的方法可以为正在进行类似快速地下水开发的其他非洲主要城市的地下水评估提供依据。