River and Groundwater Basins of the World

Background:
Global population growth, rapid urbanisation, increasing industrialisation, agricultural intensification and tourism are all putting water resources under increasing stress - and this situation is further aggravated by accelerating climate change. Such pressures are impacting on water resource availability, whilst at the same time water resource degradation is having significant negative feedback for economic production, public health and livelihoods, and the natural environment. By 2025, 1.8 billion people are predicted to be living in regions with pressing water-scarcity problems and it is thus vital that all freshwater resources are managed effectively. This is becoming an increasingly challenging task, because hydrologically-extreme events (droughts and floods) are expected to become more frequent.

Map "River and Groundwater Basins of the World"

In order to address the multi-faceted nature of water resources management, many countries are endeavouring to apply the Integrated Water Resources Management (IWRM) process at both the national and river-basin level in a complementary tiered fashion. IWRM aims to improve institutional arrangements and working practices, promote good governance including stakeholder participation, and to consider equity issues, gender concerns, environmental needs and economic assessments. For this purpose coordinated cross-sector action on water resources is required to deal explicitly with land-use management - since urbanisation, agriculture, forestry, industrial development (all of which can seriously impact water resources) are usually governed by policies and drivers outside the conventional scope of water policy.

In some parts of the world at least, the management of river basins is progressing, and in others the path that needs to be followed is clear - but all too widely underlying groundwater resources are neglected. Thus the interdependence of the surface and subsurface part of the natural water-cycle requires more attention.

Many basins extend over more than one country, and these present unique challenges. Historically, the transboundary nature of flowing water has generally encouraged cooperation, but there is the risk that with increasing scarcity there will be potential for discord. However, the ‘Convention on the Law of the Non-navigational Uses of International Watercourses’ as well as the ‘Law of Transboundary Aquifers’ adopted by the UN provide guidance to countries to address their shared water resources in harmony.

The present map has been prepared to help water resource policy makers and planners visualise, at the broad scale and in general terms, the potential interaction between surface water systems and the underlying groundwater resources for their conjunctive uses.

Groundwater in Integrated Water Resource Management:
Taking an ‘integrated approach’ to water management not only involves looking at resource issues from both cross-sectoral and multi-disciplinary perspectives, but also requires coherent and consistent consideration of both surface water and groundwater. Organisations implementing the IWRM process usually have as their focus major river or lake basins, with the management unit usually being the corresponding surface water catchment area. Thus, while the IWRM concept explicitly includes groundwater, aquifer systems are often neglected in the practice. The reason for this can be that awareness of this ‘invisible resource’ is sometimes insufficient or that the complexity of groundwater flow systems is inadequately understood. The reality is that groundwater has to be conjunctively managed with all water resources, since aquifers are a major source for drinking water-supply, agricultural irrigation and industrial production, and are vital for sustaining the natural environment.

Interaction of groundwater and surface water Source: USGS

Shallow groundwater and surface water are usually intimately linked facets of the natural water cycle, with groundwater discharging into surface water bodies or receiving recharge from them, according to local conditions. Groundwater system ‘health’ is thus, at the same time, critical for river baseflow and other ‘dependent’ aquatic ecosystems, and also critically affected by the status of ecosystems (including streams) in aquifer recharge areas.

The dynamics of the range of possible interactions between rivers and shallow groundwater bodies is schematically illustrated in the figure left, but such variations are impossible to represent on maps at the global scale, and considerable variation can even occur over short distance, and also seasonally at the same location.

On the broader scale, the lower tracts of major alluvial basins are predominantly ‘groundwater discharging environments’ but as such systems are traced upstream to more elevated terraces these are important areas for groundwater recharge from surface water, especially in more arid climates and at the foot of mountains.

The level of interaction between surface water and groundwater tends to decrease in volume and change radically in form from humid to arid climatic settings, and in deeper confined aquifer systems. In humid areas shallow aquifers are annually replenished and have shallow water tables, with multiple discharges to the surface water environment, whilst in more arid regions significant groundwater replenishment to deeper water tables can have a return-time of decades or centuries (and in some cases groundwater resources can be essentially non-renewable), and groundwater discharge is limited to isolated oases and salt marshes.

An important outcome of this is that the areal extent of surface water drainage basins and underlying groundwater systems often differs radically in such regions, and this necessitates a variation of the physical framework appropriate for IWRM.

Diagram of river and groundwater basins in Africa Source: BGR & UNESCO

This is well illustrated at continental scale from Africa, by comparison of the surface water drainage basins (figure right – upper right) and the underlying groundwater basins (figure right – lower left), with the pseudo-perspective diagram in the centre highlighting the complexity of their detail.

Considerable differences between the surface water and groundwater settings can also occur at the more local scale in karstic limestone terrains where geological structure (and not land topography) predominates over the form of ‘drainage basins’.

While the ‘water-cycle continuity’ paradigm represents a solid base for the integration of groundwater into IWRM, there are some additional issues, and differences between groundwater and surface water systems, that have to be specifically addressed:

• In potential groundwater recharge areas the ‘connectivity’ of surface water to groundwater exhibits extremely wide variation with aquifer type and present climatic setting - for example, in some shallow karstic aquifer systems groundwater residence times are less than 100 days (allowing the potential penetration and survival of fecal micro-organisms), whilst in the large sedimentary aquifers beneath parts of the Sahara groundwater with residence times well in excess of 10,000 years is still slowly circulating to desert oases.
• Surface water systems tend to be flow-dominated, whilst most aquifers are characterised by very large storage (stock) and much lower flow rates (flux), with only a small fraction of groundwater being replenished and discharged in the annual hydrological cycle - globally it has been very approximately estimated that surface drainage (calculated by deducting an allowance for groundwater baseflow from total surface run-off) is some 3-8 times greater than groundwater flow, but fresh groundwater storage is more (and perhaps very much more) than 50 times that of all surface water systems.

These differences have major practical implications for water resources management:

• Whilst for rivers upstream-downstream considerations are well defined, such relations may neither predominate nor are necessarily fixed for groundwater.
• The huge ‘storage buffer’ of aquifers makes their groundwater resources of strategic importance for drought mitigation and climate-change adaptation.
• For groundwater bodies uncertainties in management decision-making relating to water resource withdrawals are usually easier to accommodate.
• Very different time scales have to be contemplated in the management of rivers and groundwater bodies - decades or more for groundwater compared to months or less for rivers. This has important consequences, especially for groundwater quality with slow penetration but very long residence of non-degradable pollutants in groundwater systems.

Origins of groundwater salinity Source: World Bank

Thus groundwater resources management requires a monitoring plan and protection measures that cover a very wide range of temporal and spatial scales - and is as a result considerably more costly to establish and to operate. It also has to take into careful consideration the possible occurrence of elevated salinity in parts of some groundwater systems. This may arise from a variety of different mechanisms, from the mobilisation of paleo-saline or connate waters at depth to the intrusion of modern sea-water, or to essentially surface processes related to irrigation water returns or soil water-logging due to rising water table (see figure above) - which require in-depth investigation and careful diagnosis before they can be managed effectively.

Integrating groundwater into broader water resources management has to respect the above considerations and differences, and rise to this challenge. An approach to reconciling hydrogeological diversity with river basins would recognise at least four different conditions with implications for water resources management:

• the existence of important aquifers of limited extent underlying only part of river basins - requiring independent local groundwater management plans which are then integrated into the overall river-basin plan;
• the existence of river basins underlain by extensive shallow (but sometimes thick) aquifer systems - requiring a fully integrated appraisal of groundwater and surface water resources and their man-made perturbations, whilst recognising probable spatial variation in the groundwater/surface water relationship;
• the existence of extensive deep sedimentary aquifer systems in arid regions - where groundwater flow will predominate and the aquifer basin (not river basin) is the rational water resources management unit;
• the existence of minor aquifers of shallow depth and patchy distribution, where the impacts of groundwater development are very localised, and are unlikely to be significant in terms of river basin dynamics.

Once again it is not feasible to show all such variations on a global-scale map, but they need to be borne in mind when viewing the need for improved integration of surface water and groundwater management.

Beyond the issue of spatial definition of appropriate units for water management, a major practical outcome of the existence of large aquifer storage has been the spontaneous conjunctive use of groundwater as a ‘coping strategy’ in times of surface water drought, when groundwater systems show more resilience. This has occurred very widely for agricultural irrigation on the alluvial plains of more tropical latitudes with highly-seasonal rainfall of less than 1,000 mm/year. The move to planned conjunctive management in regions with this potential can be deduced from this map and it offers significant response to the challenges of climate-change. Managed aquifer recharge (MAR), which involves a variety of physical and management measures to enhance groundwater recharge rates during the wet season, is a significant further extension of this principle.

Approach to Comparative Mapping of Basins at Global Scale:
The present map delineates and superimposes river/lake basins on the land surface, and the hydrogeological class of underlying aquifer. This requires representing a three-dimensional reality on two-dimensional paper in an easily readable way - which is far from straightforward.

Hatching has been used to show the surface form of river/lake basins, and colour shading to indicate class of aquifer, albeit that the global representation only allows inclusion of extensive systems, together with some smaller systems of more local relevance without detail. Different hatching has been used to indicate probable linkages between groundwater and surface water systems:

• Dense hatching (SW-NE) symbolises areas with large river/lake basins within which underlying aquifers are contained. In these areas integrated management of surface water and groundwater can be more simply formulated, because the boundary of the river/lake basin encompasses the natural extent of underlying aquifers, and the recharge of surface water into the aquifer or the discharge from aquifers are fully contained within the river/lake basin.
• Lighter hatching (NW-SE) has been used for areas in which underlying aquifer basins depart considerably from those of the river/lake basins. The fact that an aquifer extends into an adjacent surface water basin is a challenge for integrated water resources management, since ‘external groundwater issues’ may influence the water budget or water quality situation. In these areas the interlinkages between surface water and groundwater regimes need to be mapped and monitored in detail in order to identify potential externalities.
• Stippling is used for limited surface water catchments, mainly in coastal regions, where the situation may be comparable to the previous category, but cannot be portrayed in detail.

In areas where the surface water situation is not relevant because of erratic rainfall and run-off, the underlying hydrogeological situation can be clearly seen on the map. In arid areas no active river/lake basins are present, but they can be underlain by important deep aquifers containing large groundwater reserves and some groundwater flow. Hence groundwater basins should form the basis for water resources management.

For groundwater, the aquifer class is indicated by colour shading, according to the classification of the WHYMAP Groundwater Resources Map of the World:

• Blue shading: large relatively uniform aquifer systems
• Green shading: areas with complex hydrogeological structure
• Brown shading: areas with minor, shallow and local groundwater.

The most important groundwater resources at global scale are related to the former (blue and green shaded) areas.

A number of surface water and geographical features are also shown on the map according to current global datasets - such as major rivers and freshwater lakes, saltwater lakes, continuous ice sheets as well as selected major cities and country borders to provide map orientation.

Key Messages:
Groundwater should play an equal part in the management of water resources, since aquifers are a major source for drinking water-supply, agricultural irrigation and industrial production, and are critical for sustaining the natural environment.

Groundwater is usually flagged-up as part of the river-basin agency mandate – however groundwater systems have to be different from the simple input-output controls as applied to surface watercourses.

Whilst many facets of surface water resources management do not pertain to groundwater (for example navigation, fisheries, and energy generation) the impact of related construction measures on the surface water regime (for example dams or irrigation canals) can also have a major impact on groundwater.

In the more arid regions of the world the boundaries and size of surface water drainage basins and underlying groundwater systems rarely coincide - and the more rational basis for IWRM in such areas is the groundwater basin.

Elsewhere, the ‘water-cycle continuity’ paradigm is generally a solid base for the integration of groundwater into IWRM, but there are a number of aquifer specific issues and differences that have to be specifically addressed.

Multiple time-scales have to be contemplated in the management of rivers and groundwater bodies (decades or more for groundwater compared to months or less for flowing rivers), which has very important implications for groundwater quality protection and management.

Groundwater system ‘health’ can be critical for river baseflow and other dependent aquatic ecosystems, and itself is critically affected by the status of ecosystems (including streams) in aquifer recharge areas.

Groundwater resources management requires a monitoring plan and protection measures that cover a very wide range of temporal and spatial scales - and also the possible occurrence of elevated salinity in parts of some groundwater systems has to be taken into careful consideration.

References:

• BGR & UNESCO (2006): Groundwater Resources of the World - Transboundary Aquifer Systems, 1 : 50 000 000. Special Edition for the 4th World Water Forum, Mexico City, March 2006. Hannover, Paris.
• BGR & UNESCO (2008): Groundwater Resources of the World, 1 : 25 000 000. Hannover, Paris.
• FOSTER, S., TUINHOF, A., KEMPER, K., GARDUÑO, H. & NANNI, M. (2003): Characterization of groundwater systems - key concepts and frequent misconceptions. GW-MATE Briefing Note Series 2. World Bank, Washington D.C.
• FOSTER, S. & STEENBERGEN, F. VAN (2011): Conjunctive groundwater use: a ‘lost opportunity’ for water management in the developing world? Hydrogeol J 19: 959-962; doi: 10.1007/s10040-011-0734-1
• FOSTER, S. & AIT-KADI, M. (2012): Integrated Water Resources Management (IWRM): how does groundwater fit in? Hydrogeol J 20: 415–418; doi: 10.1007/s10040-012-0831-9
• GARDUÑO, H., FOSTER, S., NANNI, M., KEMPER, K., TUINHOF, A. & KOUNDOURI, P. (2006): Groundwater dimensions of national water resource and river basin planning. GW-MATE Briefing Notes Series 10. World Bank, Washington D.C.
• GWP & INBO (2009): A handbook for integrated water resources management in basins. Stockholm, Paris.
• PURI, S. & STRUCKMEIER, W. (2010): Aquifer resources in a transboundary context: a hidden resource? - Enabling the practitioner to ‘see it & bank it’ for good use. In: Earle A, Jägerskog A & Öjendal J (Eds): Transboundary Water Management: Principles and Practice: 73-90.
• RICHTS, A., STRUCKMEIER, W. & ZAEPKE, M. (2011): WHYMAP and the Groundwater Resources of the World 1:25,000,000. In: Jones J. (Eds.): Sustaining Groundwater Resources. International Year of Planet Earth; Springer. doi: 10.1007/978-90-481-3426-7_10
• WINTER, T.C., HARVEY, J.W., FRANKE, O.L. & ALLEY, W.M. (1998): Groundwater and surface water: a single resource. USGS Circular 1139. Denver.