Five main types of aquifer can be found (Box 2.1): sand and gravel, sandstone, karst, volcanic and basement aquifers (Margat and van der Gun, 2013). Each of these types is associated with specific physical properties, such as porosity, hydraulic conductivity and thickness that determine the flow and storage aquifers can allow. The two first types include the most conducive agriculture irrigation systems, and some of the most fertile land.