Groundwater physical stocks

  • Image, Groundwater physical stocks.

    Groundwater is 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.

    We classified Groundwater physical stocks as a case study.

    Key findings

    In 2014, New Zealand’s aquifers held an estimated 711 billion cubic metres of groundwater.

    • Canterbury held 519 billion cubic metres (73.2 percent) of the total volume of groundwater.
    • Between 1994 and 2014 in all regions, groundwater volumes are estimated to have varied by less than 2 percent.
    • Unconfined aquifers (which receive water directly from the overlying land surface) hold approximately 96 percent of New Zealand’s groundwater.

    Figure 1

    Note: Estimates are based on water levels at 37 wells. Gisborne and Marlborough have estimated average groundwater volumes too small to be seen on the scale of this graph.

    Figure 2

    Note: Data for estimated variation in groundwater volume are from 15 consistent indicator wells with long-term data covering the 1994–2014 period (White & Reeves, 2002; White, 2007; Moreau & Bekele, 2015). Changes in total volume are less than 1.7 percent.

    Definition and methodology

    Groundwater is the water located beneath Earth’s surface in pores and fractures in sediment or rock formations. A unit of rock or sediment is called an aquifer when it can yield a usable quantity of water. Aquifers store water that can feed some lakes and rivers. They can also provide water to towns, cities, farms, and industry through wells and bores.

    Aquifers can be unconfined or confined. An unconfined aquifer is separated from the land surface only by sediment or rock, through which water and any contaminants can freely percolate down from the surface. A confined aquifer is contained between two units of impermeable material, such as unfractured rock or clay. Water held in confined aquifers is under pressure and may originate from sources located some distance away.

    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).

    While large volumes of water are estimated to be stored in the ground, they do not represent the total water available to be used. The useful volume will be a proportion of this total. For instance, to be able to access and use the water, it needs to be at a depth that can be reached and its quality must be fit for purpose. It also has to be stored in an aquifer with enough well-connected pore spaces to let the water flow through the rock and into the well at a suitable rate for the intended use.

    Data quality

    Topic Classification   Relevance Accuracy 
     Freshwater quality, quantity and flows  Case study

     

    Direct

     

    Medium

     See Data quality information for more detail.

    References

    Moreau, M & Bekele, M (2015). Groundwater component of the Water Physical Stock Account. GNS Science Consultancy Report 2014/290.

    White, PA (2001). Groundwater resources in New Zealand. In Rosen, MR & White, PA (Eds), Groundwaters of New Zealand (pp45–75). Wellington: New Zealand Hydrological Society.

    White, PA (2007). New Zealand groundwater volumes: Update for June 2005. Unpublished letter to Statistics New Zealand.

    White, PA & Reeves, RR (2002). The volume of groundwater in New Zealand, 1994 to 2001. GNS Science Client Report 2002/79.

    Archived pages

    See Groundwater physical stocks (archived April 2017).

    Updated 27 April 2017

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