Groundwater: the water stored beneath Earth’s surface in aquifers (layers of water-bearing rock or sand). It is used for human and stock drinking water, irrigation, and industry, and also has a role in sustaining some rivers, lakes, and wetlands, especially during low-flow periods. The health of surface-water ecosystems also depends on groundwater.
Groundwater volumes are based on estimates of:
- aquifer locations and extents, based on outlines described by White (2001) and Moreau and Bekele (2015)
- the depth or thickness of the aquifer, based on measurements of the winter groundwater level in at least one indicator well for each aquifer (monitored by regional councils)
- an aquifer’s porosity (how much water can be stored per volume of aquifer material; eg gravels with large spaces between pebbles can store more water than a tightly packed material without any void space).