Depletion of groundwater resources under rapid urbanisation in Africa: recent and future trends in the Nairobi Aquifer System, Kenya

Samson Oiro,Jean-Christophe Comte,Canute Mwakamba,Chris Soulsby,Alan Macdonald

doi:10.1007/s10040-020-02236-5

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.

Highlights

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)
This study provided an overview of the historical evolution of the Nairobi volcano-sedimentary regional aquifer system (NAS) groundwater resource since its large-scale development was initiated in the 1950s, along with an assessment of its likely future evolution over the 100 years under two scenarios of (1) continuing groundwater development and (2) development of conjunctive water supply
The increasing water demand in the area is met by groundwater because piped imports of surface-water supply is not reliable and has not been expanded since Kenya gained independence in 1963

Summary

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). Over the last two decades, many studies based on in situ groundwater observations (Scanlon et al 2012; MacDonald et al 2016), regional to global-scale hydrological numerical models (Wada et al 2010, 2014; Knowling et al 2015; Wada 2016; de Graaf et al 2017; de Salis et al Cardoso de Salis et al 2019), and/ or indirect satellite observations using GRACE (Rodell et al 2009; Doell et al 2014; Joodaki et al 2014; Chen et al 2016) have shown that many large sedimentary aquifers in arid and semi-arid regions, most of which being subject to largescale irrigation, are undergoing depletion

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