Depletion of groundwater resources under rapid urbanisation in Africa: recent and future trends in the Nairobi Aquifer S
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Depletion of groundwater resources under rapid urbanisation in Africa: recent and future trends in the Nairobi Aquifer System, Kenya Samson Oiro 1,2 & Jean-Christophe Comte 1 & Chris Soulsby 1 & Alan MacDonald 3 & Canute Mwakamba 2 Received: 28 November 2019 / Accepted: 1 September 2020 # The Author(s) 2020
Abstract 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. Keywords Groundwater monitoring . Aquifer depletion . Conceptual model . Numerical modelling . Kenya
Introduction The African population is heavily reliant on groundwater and this dependence is increasing with rapid population growth, especially around large cities (Foster et al. 2018). Rapid groundwater development can lead to falling water tables and aquifer depletion, and this has been observed in several Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10040-020-02236-5) contains supplementary material, which is available to authorized users. * Samson Oiro [email protected] 1
School of Geosciences, University of Aberdeen, Scotland, UK
2
Water Resources Authority, Government of Kenya, Nairobi, Kenya
3
British Geological Survey, Lyell Centre, Edinburgh, Scotland, UK
cities and their well fields, for example, Addis Ababa (Ethiopia), Nairobi (Kenya), and Dar es salaam (Tanzania; Adelana et al.
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