Groundwater mapping has its origins at the level of individual groundwater exploration projects. Maps have emerged as a tool to understand the spatial variation of observed groundwater parameters, to interpolate between observational locations, to forge a common understanding of the groundwater systems concerned and to preserve information in a format convenient for later activities. Over time, many countries and states have developed groundwater mapping programmes for their entire territory or for areas in which their most important groundwater resources are located. Little uniformity existed initially between these programmes, both on the parameters mapped and on the way information was presented. The desire to improve methodologies and to 'speak a common cartographic language', especially in cases of groundwater bodies crossing international boundaries, triggered international cooperation to develop a uniform internationally applicable methodology for hydrogeological mapping. As a result, IAH, UNESCO and FAO jointly produced 'A Legend for Hydrogeological Maps' (Red Books Series IASH, 1962), which was followed by a coproduction by IGS, IAH, IAHS and UNESCO/IHD entitled 'International Legend for Hydrogeological Maps' (1970, in four languages). In 1974, UNESCO published a specialized quadrilingual supplement to the 1970 legend entitled 'Legend for geohydrochemical maps'. The 1970 and 1974 legends have been adopted and further elaborated by Struckmeier and Margat (1995). A related mapping guideline on groundwater vulnerability has been published in the same series (Vrba and Zaporozec, 1994).

The hydrogeological mapping methodology and standard legend were tested extensively in the project 'International Hydrogeological Map of Europe'. At its start in 1960, this project was a forerunner among regional hydrogeological mapping projects and in scale is still the most detailed. Several other regional mapping projects followed, most of them using the international legend mentioned earlier. Table 16.2 gives an overview of existing regional, continental and global ground-water maps.

Figure 16.1 is a simplified version of the world map on groundwater resources produced by WHYMAP. The map was produced at a scale of 1:50 million, but a more detailed version at a scale of 1:25 million is in preparation. This map is meant for educational purposes and for raising awareness rather than for assisting hydrogeologists in their daily activities. The legend was derived from the international legend for groundwater maps, but with some modifications and simplifications.

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