A considerable amount of New Zealand’s fresh water is stored as groundwater in aquifers – underground layers of water-soaked rock or gravel.
Confined aquifers have an impermeable layer of rock or sediment above and below them; unconfined aquifers only have an impermeable layer below. Although they are deeper than unconfined aquifers, confined aquifers can store water under pressure, which can make extraction easier once a well has been drilled.
Aquifers can lie under any landscape, but those under relatively flat land (such as broad river flood plains) tend to have more water and be more accessible. For example, beneath the Canterbury Plains are layers of gravel aquifers containing a great deal of water from rainfall on mountains, foothills and plains, plus leakage from rivers. The water moves slowly eastward down the plains to the sea.
Aquifers are fed by rainfall and rivers. They also flow back into rivers and, on the lower plains, are the source of spring-fed rivers such as the Avon River in Christchurch.
Around one-third of the water used in New Zealand comes from groundwater. New Zealand’s groundwater storage has been estimated as 612 billion cubic metres.
Groundwater has been used as a source of high-quality drinking water since early European settlement, and is the source of the domestic water supply for cities such as Christchurch and Napier. Careful management of the source aquifers and their recharge zones (areas where water goes underground) is required to maintain this high quality.
Groundwater for Christchurch’s town supply is some of the purest in the world. Drawn from aquifers that are 20–200 metres deep, it is thought that some of it has been underground for as long as 800 years. Some of it is under pressure – a bore driven in 150 metres creates a 9-metre-high fountain.
New Zealand has more than 50,000 lakes, although fewer than 4,000 are more than 1 hectare in area. Lakes have an important role in water distribution as they tend to store river water and slow down the transport of water, solutes (dissolved substances) and sediments to the sea.
Dams and reservoirs
New Zealand is dotted with storage reservoirs (artificial lakes, or natural lakes with raised water-levels), ranging in size from small farm dams, through to the 7,500-hectare Lake Benmore.
Many of the larger reservoirs have been created for generating hydroelectricity. There are three main hydroelectric storage lakes in New Zealand, which hold 70% of the water used to generate electricity. Lake Pūkaki has the largest storage (around 35% of the national storage), followed by nearby Lake Tekapo (around 21%) and Lake Taupō (14%).
Hydroelectricity provides 70% of New Zealand’s electricity generation capacity. Hydroelectric power generation uses around 114 cubic metres of water per person per day. All of this water is returned to natural systems. However, dams are not without ecological impacts, as they trap sediment, stop the passage of fish and alter downstream flow.
Storage reservoirs are also used for irrigation, as they allow water to be harvested during times of plenty and released at times most beneficial to farmers. They can also provide other benefits – a report showed that Lake Ōpuha (an irrigation reservoir created by damming the Ōpuha River in 1999) provided 480 jobs and an extra $124 million per year to South Canterbury’s economy. It also has provided stable minimum flows in the Ōpihi River, sustaining a valued trout fishery.