These three major classes are now used as a standard for international maps of groundwater resources (Figure 1.7). The International Groundwater Resources Assessment Centre (IGRAC) has defined 36 global groundwater regions divided into 217 groundwater provinces (Margat and van der Gun, 2013). These divisions were developed focusing on the predominant characteristics of groundwater systems in continental regions. There are four main categories of groundwater regions: basement regions (B), sedimentary basins (S), high relief folded mountains regions (M), and volcanic regions (V).