Africa Groundwater Mapping Issues
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Please cite page as: Africa Groundwater Atlas. 2022. Groundwater and Hydrogeological Maps of Africa. British Geological Survey. Accessed [date you accessed the information]. Weblink.
This page gives a brief overview of some key concepts around groundwater mapping: its importance; different types and uses of groundwater maps; and important issues to consider when making and using hydrogeological maps; and also briefly discusses options for future hydrogeological mapping in Africa.
- 1 Some background and issues around groundwater mapping in Africa
Some background and issues around groundwater mapping in Africa
The importance of mapping groundwater
Despite its strategic importance in many parts of the world, including Africa, there are still many gaps in information and knowledge about groundwater. Because of this, investments in groundwater resource development and management are often based on inadequate or poor quality data.
Groundwater mapping can play a valuable role in improving the planning and implementation of groundwater resource development, and monitoring, evaluating and managing water supplies. Mapping is another way of saying spatial thinking: collecting and using spatial data and information to progress and share knowledge, and to provide the best available evidence to support decisions at all stages of projects. Groundwater, or hydrogeological, maps show the spatial distribution of, and relationships between, hydrogeological features, such as aquifers and their hydraulic properties, recharge, groundwater levels or groundwater chemistry/quality.
Good maps combine different sources and types of information to give an accurate picture of the reality on the ground. Maps showing aspects of hydrogeology and groundwater resources cover different areas, are at different scales, have different themes and show different combinations of features. Different maps are useful for different purposes.
Types of groundwater maps and some of their uses
Groundwater maps can be at many different scales, from a single small river catchment or local aquifer up to the entire continent. The scale of a map depends on what it is designed for, and on the available information for creating it. Different levels of detail in spatial mapping are needed to map aquifers and groundwater resources at continental, regional, national and sub-national scales.
For example, regional maps don't just combine existing national maps: they harmonise groundwater information in the existing maps, making it consistent so that hydrogeological parameters can be easily compared and contrasted across the region; sometimes collecting new data or reinterpreting existing data.
Continental maps can be useful to capture continental trends, such as changing rainfall patterns in response to climate change; and to inform policy development at the pan-African level.
Maps can show different themes related to groundwater. They can vary from simple to complex, from only two or three layers up to many layers of interpreted information. Traditional hydrogeological maps usually show some form of geological classification with varying degrees of hydrogeological interpretation (showing the aquifer potential of each rock type). They may show some kind of representation of three dimensional (depth) variations. Sometimes a basic aquifer map is overlain with ornament that shows additional groundwater-relevant information, such as rainfall or estimated recharge, groundwater quality, groundwater flow direction or the locations of selected water boreholes or springs.
Thematic groundwater maps are designed to highlight particular aspects of groundwater, rather than a general understanding of the hydrogeology of an area. They often show only one hydrogeological parameter. Some examples are groundwater vulnerability maps; maps showing particular groundwater quality indicators, such as the distribution of salinity, arsenic or fluoride in groundwater; or maps showing only groundwater levels in a deep aquifer.
Maps at different scales, or showing different themes, are useful for different activities.
Traditional hydrogeological maps can provide a general overview of the hydrogeology of an area, and can be used to support groundwater resource development and management, including planning water supply interventions, and groundwater monitoring. High level overviews might need continental or regional scale maps; while national assessments need national scale maps and siting new abstraction boreholes needs sub-national scale maps. Traditional hydrogeological maps showing groundwater resource potential can provide a general understanding of the hydrogeology of the area mapped. Depending on their scale, they can also be used for identifying regions that are more vulnerable to water scarcity and need more water supply interventions; or for practical siting of new abstraction boreholes.
Thematic groundwater maps are usually designed for particular uses. Examples include groundwater vulnerability maps, which can be used to help plan industrial or other potentially polluting activities, ensuring they are sited in low vulnerability areas; or a map showing selected groundwater quality parameters (e.g. fluoride or nitrate), which can be used to identify areas where new groundwater abstractions may need to be avoided, or areas to be targeted for contaminant mitigation.
Key hydrogeological mapping issues
Representing complex hydrogeology on a two dimensional map is difficult. Hydrogeological maps have evolved various ways to try and best represent real conditions on and in the ground, but it should be remembered that maps are only representations of complex reality.
Groundwater is largely an invisible resource, hidden below the ground, unlike surface water. The only places we can gather direct evidence for the presence, nature and behaviour of groundwater are where it appears naturally at the surface as springs, or where wells or boreholes have been dug or drilled into aquifers. Away from these places, we have to infer hydrogeological characteristics from secondary data (such as geology and rainfall) or from extrapolation of primary hydrogeology data over large areas (such as extrapolating transmissivity or storativity values from test pumping of individual boreholes over a whole aquifer). When we look at a groundwater map, we should remember that it always includes a large amount of inferring and extrapolation from secondary and incomplete hydrogeological evidence.
Maps are two dimensional, but groundwater and hydrogeology are four dimensional. Groundwater exists in aquifers that are three dimensional, varying physically with depth as well as laterally; and also varies over time (the fourth dimension), depending on rainfall and environmental changes, including those driven by human influence. Many aquifer characteristics are difficult to represent on a map, such as aquifer type and thickness, the presence of overlying or intervening non-aquifers, and the presence of more than one aquifer with depth. So too are fluctuating water tables over seasons or years, and changing groundwater chemistry over time because of pollution. Various strategies are used to try and represent hydrogeological features and groundwater resources on maps, including using graphical ornaments with different patterns or colours, point data, and accompanying information in cross sections.
Aquifers and groundwater resources are often highly heterogenous, and not all of this heterogeneity can be measured, characterised or shown on a map. For example, in a fractured aquifer, the flow and storage of groundwater depends on the frequency, size and connectivity of water bearing fractures across the aquifer, which is typically not uniformly distributed across the aquifer, but can’t be measured, and can only be seen by drilling. Another example is where a geological formation includes variably thick layers of both sandstone and mudstone, which have very different hydrogeological properties: sandstones form good aquifers and mudstones poor aquifers. In both examples, boreholes drilled into the aquifer can have very different sustainable yields depending on whether they intersect many or few connecting fractures, or mostly sandstone or mostly mudstone; but the whole aquifer is mapped as a single entity.
The accuracy, precision and reliability of maps are dependent on the availability of good geological and hydrogeological data. Useful and reliable hydrogeological interpretation requires enough geological information at the right scale and level of detail, including variations in geology with depth, and detailed enough lithological description. There must be enough hydrogeological data to make reliable interpretations and infer hydrogeological properties across mapped geological formations. The key hydrogeological data are aquifer properties, in particular transmissivity (the permeability of aquifers integrated over their thickness) and aquifer storage capacity, including effective porosity. Both of these can only be measured during borehole pumping tests, which are rare in much of Africa. Other data can be used as a substitute, including borehole yields measured during normal borehole operation, especially if borehole water levels are measured at the same time, which allows borehole specific capacity to be calculated. Another key hydrogeological parameter is the depth to groundwater level, or water table, below the ground surface, especially when it is monitored over time. This gives an indication not only of how deep boreholes must be to reach groundwater resources, but also the saturated thickness of aquifers (how thick the aquifer is below the groundwater level), and groundwater recharge over time. Reliable spatially distributed rainfall data is needed to provide estimates of groundwater recharge, and therefore of the long-term sustainability of groundwater resources, and these estimates can be improved by a good understanding of recharge processes in the mapped aquifers. Groundwater chemistry data is needed to assess if groundwater is of suitable quality for the desired use. There is little consistency in mapping approaches between different projects and countries. The choice of what is shown on a map, and how it is shown, depends on its intended use, on available data and technologies, and on the background, experience and style choices of the mapmakers. Hydrogeological features are shown on a map if they were considered the most important to the particular use for which they were designed. Even where hydrogeological features are common across different maps, they are often depicted in different ways, for example using different colour or ornament schemes, and by a wide range of symbols. This makes it difficult to compare groundwater resources and hydrogeology across maps – such as across neighbouring countries.
Digital mapping technologies have changed some things about groundwater maps, although by no means all. Geographical Information System (GIS) technology, in particular, has been an important tool in the development of groundwater mapping. It allows the easy overlay of different map layers, and the storage of more attribute information, making it easier for users to separate, combine and interrogate different types of input information, which in traditional paper maps must be shown together, and static. Hydrogeologists who use GIS typically use a series of individual map layers including geology, physical aquifer properties, rainfall, and groundwater levels, chemistry and vulnerability, as well as supporting other environmental information such as on rivers and lakes, soils and land surface elevation.
A forward look for hydrogeological and groundwater mapping in Africa
It might be that the map you need for your work has already been developed, and is listed in these pages on Groundwater and Hydrogeological Maps of Africa. But in many cases, the right groundwater maps for many uses aren’t yet available.
Most countries in Africa have a national hydrogeological or groundwater map, although not all - even those countries that depend significantly on groundwater resources (e.g. Mali). There is much scope for the development of new national hydrogeological maps where these are not yet available.
In many countries that do have a national hydrogeological map, this is decades old, and often based on limited information available at the time of making. In most countries, much more hydrogeological data and information has been collected in the years since the national map was created, and there is great scope for updating older national maps with new and more extensive data.
Many older hydrogeological maps are also hard to access. They were made in the pre-GIS, and often the pre-computer, era, and are available only as hard (paper) copies, which may be out of print, expensive or otherwise difficult to acquire for general use.
There is a growing demand for easily accessible - which today means digital – country-scale maps to support effective groundwater resource planning and management - maps that provide a reliable, up to date overview of national groundwater resources. There is scope for digitizing and georeferencing existing maps to make them more widely available: scanned pdfs available to view and download online; and georeferenced maps available to be viewed in interactive online viewers, and even to download in a format that is compatible with modern GIS software for subsequent use and analysis. Examples of where existing maps have been digitally georeferenced and made available to view online are eSwatini and Lesotho. Another approach to making national scale hydrogeological maps more widely available is the Africa Groundwater Atlas Country Hydrogeology Maps, which provide simple overviews of aquifer type and productivity for most countries in Africa, and are available to download as digital shapefiles for use in GIS.
Going further than this, in order to progress successful surveying, development and monitoring of groundwater resources, there is a great need and scope for updated, detailed, high quality, national and sub-national scale hydrogeological maps in many countries in Africa. This would make use of updated geological data and mapping, and the most up to date available hydrogeological data, such as aquifer properties from borehole testing, and groundwater levels and groundwater quality information from monitoring networks. Such high quality mapping relies on the rigorous collection, storage and interpretation of representative groundwater data during ongoing groundwater investigation and development of new water supplies. As understanding progresses, maps can be refined, upgraded and progressed even further to improve their usefulness. Groundwater mapping is not a one-off task but part of an ongoing process towards better understanding, development and management of groundwater resources. Some of the key issues involved in hydrogeological mapping, at all scales, are briefly discussed on this page.
There is also an increasing demand for regional hydrogeological maps, to support regional cooperation, for instance on transboundary aquifers, or for integrated water resources management (IWRM) by Lake and River Basin Organisations, or by regional economic communities. Examples of this are the SADC hydrogeological map (2010), developed by a consortium from South Africa, Botswana and Sweden under the auspices of SADC; and the Groundwater Resources in the ECOWAS Region map (2022), developed by a consortium of groundwater experts in BGR, BRGM, BGS, EAWAG and UNESCO under the auspices of the African Ministers' Council on Water (AMCOW)Pan-African Groundwater Program, the Economic Community of West African States (ECOWAS) Water Resources Coordination Centre (WRCC), and the Niger Basin Authority (NBA). Building on readily available data, the new ECOWAS map shows the potential for assimilating and harmonising existing hydrogeological information to improve regional groundwater mapping across Africa.
Another future direction for groundwater mapping in Africa may be developing new national, regional or even continental thematic groundwater maps to advocate for the needs of groundwater development and management related to specific issues. This might be highlighting regions where water resources or quality are at particular risk from climate change or urbanisation or other human pressures; or the water needs of IDPs and refugees; or the risks to wetlands and other natural environmental features from human development.
Maps are a part of the future as they are the past of groundwater understanding in Africa. Groundwater and hydrogeological maps, developed and used in the most effective ways, will support the sustainable development and management of water supplies for people, economic progress and environmental protection in Africa for many decades to come.
For general references, see Groundwater and Hydrogeological maps of Africa.