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Luther Lazaro
Africa is particularly vulnerable to climate change impacts, which threatens food security, ecosystem protection and restoration initiatives, and fresh water resources availability and quality. Groundwater largely contributes to the mitigation of climate change effects by offering short- to long-term transient water storage. However, groundwater storage remains extremely difficult to monitor. In this paper, we review the strengths and weaknesses of satellite remote sensing techniques for addressing groundwater quantity issues with a focus on GRACE space gravimetry, as well as concepts to combine satellite observations with numerical models and ground observations. One particular focus is the quantification of changes in groundwater resources in the different climatic regions of Africa and the discussion of possible climatic and anthropogenic drivers. We include a thorough literature review on studies that use satellite observations for groundwater research in Africa. Finally, we identify gaps in research and possible future directions for employing satellite remote sensing to groundwater monitoring and management on the African continent.
Worldwide renewable freshwater resources are estimated to have a magnitude of \(\sim\)43, 750 \(\hbox km^3\) \(\hbox year^-1\) where Africa possesses only 9% of this amount (e.g., FAO 2003; Xu et al. 2019). This percentage hides the fact that, at its continental scale, freshwater supply presents a paradox (Naik 2017). Indeed, even if Africa is considered as one of the driest continents of the world, there are abundant large surface water bodies including (1) rivers, e.g., Congo, Nile, Niger, Zambezi, etc., (2) lakes, such as the second-largest lake in the world, Lake Victoria, and the second-deepest lake in the world, Lake Tanganyika, and (3) wetlands like the Inner Niger Delta, the Cuvette Centrale of Congo, and the Caprivi wetlands in Namibia (Bernacsek et al. 1992). The continent also possesses some of the largest transboundary aquifer (TBA) systems such as the Nubian Sandstone Aquifer System (NSAS) and the North Western Sahara Aquifer System (NWSAS) (Nijsten et al. 2018).
Although groundwater (GW) depletion is small compared to global recharge (Aeschbach-Hertig and Gleeson 2012), regionally GW depletion has severe impacts for society, economy and environment (Famiglietti 2014). Over Africa, Bonsor et al. (2018) identified no significant regional long-term depletion of sedimentary aquifers, but local hot spots exist, such as the Nairobi aquifer system (Oiro et al. 2020). Generally, depletion of GW resources comes along with lowering of water tables, which increases the costs for pumping, leads to drying up of wells (Konikow and Kendy 2005), and might even cause land subsidence accompanied with damaged infrastructure (Chen et al. 2016). Furthermore, lowering of the water table impacts the ecosystems by reduced discharge to rivers, lakes, ponds, and wetlands (Sophocleous 2000). In some basins, excessive human water extraction can lead to aquifer salinization by upwelling of underlying saline water, e.g., in Southern Tunisia (Zammouri et al. 2007), while in coastal regions the danger of salinization by sea saltwater intrusion increases, e.g., in Lybia (Alfarrah and Walraevens 2018). Further water quality issues arise mainly from agricultural areas due to increasing potential of widespread contamination from nutrients and pesticides, and from fecal contaminants due to proximity to sanitation facilities (Upton and Danert 2019).
In view of the strategic significance of GW for water use and management as well as for food security the impact of climate change on GW has moved to the fore of GW research during the last decade. Climate change is expected to further modify the hydrological cycle, temperature balance, rainfall patterns, and to alter basin biodiversity and water productivity across Africa, thus leading to limited access and management of GW (Al-Gamal 2021). Generally, climate related impact through natural and anthropogenic induced processes are distinguished as well as GW related feedbacks on regional and global climatic conditions (Taylor et al. 2013; Serdeczny et al. 2017). Direct effects on GW storage are related to: (1) changes in the infiltration of rainfall water due to changes in precipitation patterns and changes of land use/land cover (LU/LC), (2) interaction with surface water, and (3) GW pumping (Bierkens and Wada 2019). In the future, longer droughts and more intense rainfall events may accelerate changes in recharge (Dll 2009; Taylor et al. 2013) and discharge. Reduction in surface water availability might also further increase the pressure on GW resources.
Comprehensive regional strategies are necessary to adapt GW management to the specific local societal and environmental needs under changing climate conditions in a sustainable way (Aeschbach-Hertig and Gleeson 2012). In this respect, one important challenge is the lack of consistent and reliable observations of GW storage, exchange fluxes with surface waters, and residence time (Wada and Bierkens 2014; Joseph et al. 2020). Due to the limited availability of ground-based GW monitoring, particularly in Africa, remote sensing observations are of tremendous importance. In this article, we review past and future contributions of remote sensing observations to monitor GW resources in Africa. We also summarize findings on the impact of different GW drivers. Furthermore, we provide a comprehensive overview on available data sets and processing strategies to promote the use of remote sensing observations for GW research in Africa.
We will introduce the distribution and hydrogeological properties of African GW resources in Sect. 2. In Sect. 3, relevant remote sensing techniques will be explained including (1) satellite gravimetry, (2) radar altimetry (RA), (3) thermal infrared (TIR) remote sensing, and (4) interferometry synthetic aperture radar (InSAR). Then, in Sect. 4 we will, on the one hand, discuss the link between ground and satellite observations and, on the other hand, provide a systematic summary of analysis techniques for an integrated evaluation of different remote sensing and complementary data sets. In Sect. 5, we will provide an overview on studies that address the status and evolution of African GW resources using remote-sensing observations. In particular, we will emphasis the impact of human influences and climate change on GW resources. We will also highlight examples that show how the combination of different remote-sensing techniques helps to obtain a detailed picture of GW resources and interactions with other water storage compartments and fluxes. Finally, we will conclude on gaps in research and provide recommendations for future directions in Sect. 6.
African GW resources are unevenly distributed and have different characteristics regarding (1) extent and depth/thickness of aquifers, (2) hydrogeological properties, (3) climatic conditions that affect recharge processes, (4) GW use, and (5) management strategies (Fig. 1). In total, MacDonald et al. (2012) estimated the volume of GW in Africa to 0.66 million km\(^3\), which is 20 times the freshwater stored in African lakes and 100 times the amount of annual renewable African freshwater resources. Regarding the geographic distribution, the volume of GW storage in North Africa is one to two magnitudes higher than in Sub-Saharan Africa (SSA) (MacDonald et al. 2012).
In North Africa, large and deep sedimentary aquifers dominate, such as the NWSAS located in the Meso-Cenozoic sedimentary and the NSAS located in the Nubian sandstone (e.g., MacDonald et al. 2012; Margat 2007; Petersen et al. 2018). On the contrary, shallow aquifers are mainly found in alluvial deposits and sand dunes. Besides, dolomitic limestones form important aquifers for local GW resources in the Maghreb region. Harder sandstone, sandy shale, and quartzite aquifers in Central, Eastern, and Western Africa rely on secondary porosity from weathering and fracturing. The humid areas of Eastern Africa are characterized by discrete aquifers of limited extent and low storage potential with weathered or fractured crystalline rocks. Crystalline igneous and metamorphic aquifers dominated by Precambrian rocks occupy about 40% of SSA (MacDonald et al. 2005). These aquifers have substantial permeability within the weathered overburden and fractured bedrock. The weathered zone thickness can exceed 90 m in the humid regions of Africa with porosity generally decreasing with depth, whereas permeability depends on the extent of fracturing and clay content (Chilton and Foster 1995). Volcanic rocks occupy about 6% of SSA, the majority of these are located in Eastern and Southern Africa (MacDonald et al. 2005). Furthermore, in Southern Africa, chalky shales and dolomitic limestones form substantial aquifers, e.g., underlying Zambia and South Africa. A complex sequence of lava flows and sheet basalts are interbedded with pyroclastic rocks that depend mainly on fractures for permeability. Coastal aquifers of SSA consist of sandstone, limestone, and sand and gravel sediments and have low storage potential.
Decadal aquifer recharge, which is approximately 2% of the estimated GW storage for the whole continent, has high spatial variability with low recharge rates (< 50 mm per decade) for the deep aquifers in North Africa and high recharge rates (> 1000 mm per decade) for aquifers in the humid climate zones of equatorial Africa (MacDonald et al. 2021). High storage and low recharge rates imply resilience of aquifers to short-term climate changes, but the danger of irreversible long-term depletion. In contrast, aquifers with low storage and high recharge rates are more vulnerable to droughts, but less sensitive to long-term depletion. In both cases, careful and sustainable GW management based on continuous aquifer monitoring is necessary to improve current water security taking into account also the needs of future generations.
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