Lagos, Nigeria is one of the World's fastest-growing cities. The increasing water demand has led to depletion and deterioration of the groundwater resources contained in its highly productive coastal aquifers. Groundwater recharge processes and drivers, which dictate groundwater replenishment, and
their changes associated with rapid urbanization, are yet to be quantified
and understood. We used the spatially distributed WetSpass-M model to
estimate the spatiotemporal patterns and rates of recharge between 2000 and
2020 in the 5440 km2 coastal aquifer system underlying the megacity of Lagos. The influence of expanding urban areas, from 807 km2 in 2000 to 1183 km2 in 2020, coincide with reduction in the simulated recharge by
50 % in 20 years. The study also revealed diffuse and focused recharge
processes associated with, respectively, the urban centres and areas closer
to water bodies signalling strong groundwater-surface water interactions.
The preliminary findings from this modelling work provide directions for
exploring sustainable water management options including Managed Aquifer
Recharge (MAR) in Lagos and other fast-growing coastal megacities in the
world.
Introduction
The human population in Lagos has geometrically grown from 325 000 people in
1950 to more than 15 million human people in recent year 2020, with a
current annual population growth rate of 3.5 %. This, ranks Lagos in the
World's top 10 fastest-growing megacities (Hoornweg and Pope, 2017). Some authors predict Lagos will be the most populous city in the world by 2100
(Desjardins, 2018). Growing human population coupled with insufficiently resourced water governance have aggravated the water availability crisis in the city (Jideonwo, 2014; Balogun et al., 2017).
Currently, the city is struggling with a widening water demand-supply gap of
54.47 million L d-1 (Lagos Water Corporation, 2010). Water supply is highly dependent on groundwater resources which are supporting water needs of the approximately 90 % of the Lagos population (Healy et al., 2020).
Evidence of aquifer depletion, saltwater intrusion, and contamination of
groundwater attributed to groundwater over-abstraction and anthropogenic
activities (wastewater discharge) have been reported
(Ola et al., 2019; Oladapo et al., 2014).
An assessment of the natural replenishment (i.e. recharge) of the coastal
aquifers systems, and its evolution under rapid urbanisation, should be
given close attention in order to assess the capacity of the aquifer to
sustainably support the increasing population. Groundwater recharge is
challenging to measure directly, leading to an inherent uncertainty when
trying to develop aquifer budgets, investigate groundwater vulnerability and
migration of nutrients or contaminants, and determine the impact of changes
in land cover and climate (Smerdon, 2017). In
Lagos, the impacts of climate change and growing human activities on
groundwater recharge in Lagos remain unknown to date.
Location of the Lagos aquifer system on the Southwestern
coast of Nigeria, West Africa.
This study aims at conceptualising and quantifying groundwater recharge
processes and evolution in the coastal aquifer system underlying the growing
Lagos agglomeration (Fig. 1). To achieve this, land-use transitions over
the regional coastal aquifer domain were mapped for the years 2000 and 2020
using remote sensing datasets. Groundwater recharge and its changes as a
results of urban expansion were then assessed using the WetSpass-M model (a
widely used physical-based spatially distributed water balance model)
(Abdollahi et al., 2017) and put in perspective
with the increasing water demand in the megacity.
MethodsLand use transition
Land cover images representative for the years (2000 and 2019), as obtained
from Global Land Cover 1992–2019 (ESA, 2017), were reclassified into three
classes: vegetated cover, open water and built-up areas. The index of all
the land classes was carefully examined. Indices showing related features to
the different land use classes (vegetation, water bodies and the urban
regions) were merged together in the edit tool of ArcGIS environment. The
coverage area (surface area) for the reclassified land-use classes was
computed in the ArcGIS environment.
(a, c) Input Land use maps for 2000 and 2019 and (b, d) simulated recharge for 2000 and 2020 illustrating the rapid increase in urban surfaces at the expense of vegetated surfaces and general decrease of recharge as urban areas expand.
Simulation of groundwater recharge with estimated water demand
in Lagos
The WetSpass-M Model (Vrije Universiteit Brussels, Belgium;
available at https://github.com/WetSpass, last access: 13 December 2022), which is a spatially distributed monthly water balance
model, was utilized to simulate the water balance components between the
years (2000 and 2020). WetSpass-M is a raster-based
model that partitions precipitation into interception, surface runoff,
evapotranspiration, and recharge for each grid cell
(Abdollahi et al., 2017). Groundwater recharge is
obtained as residual from the water balance for each raster cell. Primary
input datasets included spatially distributed/reclassified land use for the
years (2000 and 2019), soil texture, slope, digital elevation model (DEM),
and monthly climatic data (rainfall, potential evapotranspiration, number of
rainy days, wind, and temperature) and groundwater depth for the years 2000
and 2020 (Funk et al.,
2015; Singer et al., 2021). These datasets were sourced from Global Land
Cover 1992–2019 overview, FAO, Shuttle Radar Topography Mission (SRTM),
CHIRPS dataset, ERA5 reanalysed data, and NASA POWER respectively. Four
sub-cell fractions were defined per model grid cell for each of the land-use
classes (vegetated cover, bare soil, open water, and impervious surface).
The differentiation of the grids will address land-use heterogeneity within
the cell (Abdollahi et
al., 2017; Siddik et al., 2022). Because datasets have different spatial
resolutions, this required resampling of the generated maps into 60 m × 60 m
resolution grid cells in ArcGIS, making 809 rows and 2551 columns for the
total of 761 350 raster cells. The model was run for 12 monthly time steps
for each year, yielding the final model outputs: monthly groundwater
recharge, runoff, evapotranspiration, and an interception for the years 2000
and 2020. The model assumed that water bodies have no groundwater
recharge.
The total water consumption was calculated by summing the daily water
quantities required for various water usages such as industrial use,
domestic or public use, fire use, waste of water. The estimated total water
consumption stand as 320 L d-1 in Lagos. The Wetspass-M
model outputs, i.e. monthly recharge were subsequently summed up to total
annual values and plotted alongside estimated water demand in 2000 and 2020
for comparisons.
Results and discussion
The transition of different land-use categories (vegetation, open water, and
built-up areas/urban areas) for years between 2000 and 2019 are shown in
Fig. 2a and c. Between 2000 and 2019, the surfaces of water bodies and
vegetation have decreased significantly from 982 to 926 km2
(-6 %) and 3650 to 3330 km2 (-9 %), respectively. Built-up
areas have recorded an increase from 807 to 1183 km2
(+50 %) due to the increasing human population, industrialization and
expansion of Lagos. The rapid urbanization experienced in Lagos appears
similar in several cities such as Nairobi, Dhaka, Moscow, Tokyo, Chengdu etc
(Oiro et al., 2020; Sun et al., 2020).
Urbanisation has been accompanied by drilling boreholes/wells to meet the
increasing human population, which has led to acute water shortage arising
from over-abstraction of groundwater from the coastal aquifers system, with
looming problems such as saline water intrusion, wastewater discharge, and
leakage of contaminants (Adiat,
2019; Ajani et al., 2021; Akoteyon et al., 2018; Balogun et al., 2017;
Oloruntola et al., 2017; Tijani et al., 2018).
Spatially, the central part of the Lagos region received a lower recharge
than the western and eastern, due to extensive built-up areas associated
with altered soils and constructed surfaces (compacted gravels, compacted
laterite, tar, concrete, etc.), whereas vegetated surface elsewhere are
associated with well drained sandy soils. Diffuse groundwater recharge
processes characterize the urban centers while focused groundwater recharge
associated with the hydrological drainage network (rivers and streams)
prevails in areas close to the water bodies. These findings agree with
previous isotopic studies suggesting recent recharge from precipitation
(Yusuf et al., 2018) and strong groundwater-surface water interaction in the vicinity of the lagoon areas in Lagos (Ayolabi et al., 2013). The
model results show an average 30 % of precipitation potentially
infiltrates and recharges the coastal aquifer of Lagos.
Over time, i.e. over the two decades period between 2000 and 2020, models
results show an overall reduction in infiltrating water alongside and
increase in runoff waters and flood water in the Lagos region, which is
consistent with the results obtained from land use/land classes transitions
(Fig. 2b, d). When comparing with water demand in Lagos agglomeration
(Fig. 3), water demand in 2000 represented about a quarter of the total
aquifer recharge where in 2020 the demand has rised to about half of the
modelled recharge. When spatially limiting the analysis to the urban
recharge, where in 2020 it exceeds the urban recharge by about 20 %. At
these rates of increasing demand and decreasing recharge, it can be
extrapolated that in a couple of decade groundwater resources will no longer
be able to meet demand sustainable, and confirms that depletion which is
already occurring in Lagos agglomeratoion may accelerate and extend to the
entire aquifer. In this context, artificial aquifer replenishment, such as
managed aquifer recharge using the increasing runoff could be a solution to
slow down or stabilise these trends and contribute to more sustainable
groundwater use.
Comparison of simulated recharge over the entire aquifer domain
and over the Lagos-urban with water demand for the years 2000 and 2020.
Conclusion
In most (mega)cities, rapid urban transformation has occurred in the past
decades due to increasing human population and industrial activities. This
is likely to continue over the next decades particularly in developing
countries when demographic growth rates are highest. In Lagos, groundwater
resources have become the most dependable resource in meeting the megacity's
growing water needs; however to date no consideration was given to the
groundwater resources renewability required to sustainably meet the
increased demand. The recharge estimates and trends provided from this study
show that over the last two decades, water demand has overtaken the recharge
rate over the urban agglomeration and is now rapidly converging towards the
total aquifer recharge rate. These results are important to inform and guide
a more sustainable groundwater development and water management decisions.
It is recommended in particular to explore the potential for Managed Aquifer
Recharge to address recharge decline and increase groundwater availability
as a sustainable water management option in Lagos megacity. Groundwater
numerical modelling is urgently needed to better understand flow and
transport in the coastal aquifer system and explore management scenarios
involving managed aquifer recharge and abstraction optimisation.
Code and data availability
WetSpass-M code is publicly available at: https://github.com/WetSpass/WetSpass-M (last access: 13 December 2022; Abdollahi et al., 2017; 10.1007/s12665-017-6512-1). There is no publicly available data for this paper.
Author contributions
OFO and JCC conceptualized the research and defined the methodology. OFO implemented the modelling. OFO and JCC analysed and validated the results. OFO wrote the original draft of the paper. JCC contributed to the writing, reviewing and editing of the paper. OFO obtained the scholarship which funded this research.
Competing interests
The contact author has declared that neither of the authors has any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Quality and quantity issues in urban hydrogeology (EGU2022 HS8.2.8 session)”. It is a result of the EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022.
Acknowledgements
This research has been supported by the Nigerian Tertiary Education Trust
Fund (TETFUND) which sponsored the first author's PhD studies at the
University of Aberdeen, Scotland UK. Authors thank Syed Mustafa for advices
during WetSpass-M installation and learning process.
Financial support
This research has been supported by the Tertiary Education Trust Fund (grant no. TETF/ES/UNIV/ONDO/TSAS/2018).
Review statement
This paper was edited by Estanislao Pujades and reviewed by Anna Jurado Elices and Sonia Valdivielso.
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