Each rock has a characteristic set of properties that will affect how easily water flows through it. The properties include the size and shape of individual grains that make up the rock (e.g. sandstone), and the extent of fracturing in harder rocks (e.g. limestone and granite). Unfortunately, these properties are quite variable even within one rock type, so a range of values have been assigned.

This map shows the higher range of values.The ease with which water flows through a rock can be critical. In rocks where water flows easily, pollution incidents can quickly affect water supplies and rivers, but where flow is slower there is more opportunity for any pollution to be dispersed and reduced naturally. Rocks through which water flows easily are likely to allow ready recharge of their aquifers, and they provide better prospects for the drilling of wells and boreholes to provide water for drinking, agriculture or industry.

Assessment of water flow is based on integrating digital geological map data with point measurements made on individual rock samples and the results of field tests of water flow made on wells and boreholes.