Groundwater Resources of the World

The Groundwater Resources Map of the World at the scale of 1 : 25 000 000 and 1 : 40 000 000 combines the related data known or published so far. It shows various characteristic groundwater environments in their areal extent:

• Blue colour is used for large and rather uniform groundwater basins (aquifers and aquifer systems usually in large sedimentary basins that may offer good conditions for groundwater exploitation).
• Green colour symbolises hydrogeological environments of complex geological structure. These are areas in heterogeneous folded or faulted regions where productive aquifers (including karst aquifers) may occur in close vicinity to non-aquiferous strata. In these areas remote sensing techniques as well as detailed ground surveys coupled with spring and stream flow analysis may help identify zones of high yielding aquifers.
• Brown colour outlines areas with local and shallow aquifers in which relatively dense bedrock is exposed to the surface. In these areas groundwater is comprised to the alteration zone of the bedrock and overlying shallow layers of weathered bedrock.

Map "Groundwater Resources of the World"

Within the three main hydrogeological units up to five different categories are defined according to their modelled recharge rates from over 300 mm to less than 2 mm per year. Dark colours (dark blue, green and brown) represent areas with very high recharge rates while light blue, green and brown colours outline regions with very low recharge potential. The latter category is vulnerable to groundwater mining. Groundwater recharge rates refer to the period 1961 - 1990 and are derived from simulations with the global hydrological model WaterGAP, version 2.1f provided by the University of Frankfurt/Main, Germany (Doell et al., 2006).

Aspects of hydrodynamic conditions are addressed by outlining areas of groundwater discharge in arid zones (e.g. endorheic basins or “chotts” and “sebkhas”). Groundwater resources frequently assure important wetland ecosystems. Therefore, wetlands with a surface larger than 500 hectares which are supposed to be groundwater related have been abstracted from the existing data base listing wetlands according to the RAMSAR convention (rsis.ramsar.org). Rising demands from population growth and food production are calling for a closer look at the use of groundwater.

Therefore, known areas of heavy groundwater abstraction prone to over-exploitation and areas of groundwater mining are mapped. Cities shown on the map (mostly those with a population estimated at more than 1 million inhabitants in 2005 according to the United Nations Department of Economics and Social Affairs) known to have at least 25 % of the total water consumption supplied by groundwater have been identified by a special symbol. Groundwater quality is an important issue for the use of groundwater such as drinking water supply and irrigated food production. Orange hatching has been applied in areas where the salinity of the groundwater in the aquifers regionally exceeds 5 g/l. In these places the aquifer resources are generally not suitable for human consumption, but some livestock may find it drinkable.

The global Groundwater Resources Map contains only selected information related to groundwater. For reasons of clarity and readability important complementary information has been deferred to a set of four inset maps at the scale of 1 : 120 000 000. These thematic maps highlight the issues of:

• Mean Annual Precipitation
• River Basins and Mean Annual River Discharge
• Population Density
• Groundwater Recharge per Capita

Comparison between the main Groundwater Resources Map and the four thematic small-scale maps should help to understand the global picture of groundwater and surface water resources and provide insight into their pressures, in particular the priority use for drinking purposes.

It reveals essential geographic differences over the globe in the distribution and amount of rainfall, the most important input factor for both, surface water flow and groundwater replenishment. The latter is mirrored in the main map by various colour shades. The rainfall map is based on data of the Global Precipitation and Climate Centre (GPCC) in Offenbach, Germany.

To highlight the surface water situation, a map of major active water basins (surface water catchment areas) has been provided by the Global Runoff Data Centre (GRDC) in Koblenz, Germany. In addition, the surface water courses and lakes have been classified according to their mean annual discharge. This picture contrasts with the main map, particularly in the dry regions of the world, where no surface water is available but luckily some of the biggest aquifer systems are located.

Population density also varies largely on earth and is a key factor for the broad variation of water demand on the continents. This information combined with the amount of groundwater recharge modelled by Doell et al. (2006) provides categories of mean annual groundwater recharge per capita. On this map, large countries have been subdivided into individual sub-regions or states, if this was appropriate or known, to highlight the regional variation.

References:

• 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