Groundwater flow modelling in the Zambezi river basin to investigate the interaction with surface water bodies

This work is defined within the project, “The African Networks of Centres of Excellence on
Water Sciences PHASE II (ACE WATER 2)” focusing on an overall WEFE (Water-EnergyFood-Ecosystem) nexus assessment over the Zambezi River Basin (ZRB). The project,
whose geographical scope extends over most of the sub-Saharan Africa focusing on three
distinct and complementary networks in Western, Central-Eastern and Southern Africa, is run
under the coordination of the Joint Research Centre of the European Commission (JRC/EC),
responsible for the scientific research, in collaboration with UNESCO in charge of the HCD
(Human Capacity Development) component. Leading scientific experts from ten Southern
Africa Centers of Excellence (CoEs, geographically spanning through South Africa, Botswana,
Namibia, Zimbabwe, Zambia, Malawi and Mozambique), as well as from the Un. of Rhodes
and the Un. of Florida, collaborated at different extent to such a challenge joint effort,
addressing different topics as; the climate variability and climate change; the surface
hydrology; the groundwater hydrology; the hydropower and the agriculture current status and
future developments under the various constraining factors, as water availability reduction and
increasing pressure due to population growth and activities development. Key regional and
basin management Institutions, as the SADC (Southern Africa Development Community),
SADC-GMI (SADC-Groundwater Management Institute) and ZAMCOM (ZAMbezi water
course COMmission) supported the activities, providing guidance with respect to key policies
and access to relevant datasets.
As for the groundwater hydrology, the University of Western Cape (Mengistu, 2018) compiled
an updated geological and hydrogeological map complementing the work from the University
of Zambia (Banda, 2018) and the NUST of Zimbabwe (Chinyama and Makaya, 2018), who
focused on the compilation of detailed databases and the analysis of the hydrogeological and
hydro-chemical status at respective country scale (Zambia and Zimbabwe respectively). The
analysis implemented in the framework of the ACEWATER2 project contributed to identify: (i)
areas relevant to groundwater use, as inferred from wells spatial distribution, characteristics
(e.g. yield, hydrogeological properties estimate after pumping tests, water sampling and water
quality analysis) and any further evidence of groundwater withdrawal, for human supply (e.g.
from population density), irrigation in agriculture (e.g. pivoting systems), cooling of industrial
plants and water use in mining activities; (2) groundwater accessibility, as related to wells and
water table depth (from the ground), and future potential, relevant to expected socio-economic
development (e.g. growing population, expanding irrigated agriculture); (2) few groundwater
vulnerability issues, as due to contamination from surficial or deep origin sources (e.g.
fertilizers and pesticides in agriculture, fecal choliforms from untreated water, leakage from
landfills, upconing of salinized water trapped in deep aquifers, as in western Zambezi, or salt
water intrusion along coastal areas).
Very few quantitative studies and datasets at the basin scale exist, among which include: the
SADC hydrogeological map and atlas, SADC HGM (Pietersen et al., 2010), the groundwater
hydrology and hydrochemistry database from the SADC-GMI, continent quantitative
hydrogeological maps of Africa from the BGS (MacDonald et al., 2012) and global coverage of
potential recharge estimates calculated using WaterGap Model from BGR (Doell and Fiedler
2008). This work focuses on a further specific analysis that was undertaken to investigate the 
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potentials and the bottlenecks in the application of groundwater flow modelling at the basin
scale, with the following objectives, (i). to investigate the reliability of hydrogeological
parameters estimates and the groundwater resources availability; (ii). to support the
assessment of interlinks between the surface water bodies and the aquifer systems. Given the
areal extent, the Zambezi river basin being the fourth largest one in Africa after Nile, Niger and
Congo, and its geological/tectonic complexity, the OS (Open Source) state-of-the-art USGS
codes MODFLOW and MODPATH (Pollock, 2012) were considered not the most suitable
platform, due to the limited discretization flexibility of the finite difference scheme. Instead, the
DHI-WASY finite element code and modelling environment (Diersch, 2009) was adopted;
thanks to the finite element numerical formulation, the high flexibility of triangular meshing
makes possible to capture the relevant features (e.g. drainage network, geological and
tectonic limits), while adopting a rough resolution over more remote and unknown areas.