On an international scale, New Zealand has an abundance of fresh water. However, demand for water per capita is high and is growing. New Zealand’s water supply is not evenly distributed, and resource limits are being reached in some parts of the country. This can result in the loss of in-stream values, poor water quality, water shortages and water-constrained economic opportunities.
In 2006 almost 20,000 consumptive resource consents to take water were in place in New Zealand. Irrigation accounted for 78 per cent of these, 11 per cent were for manufacturing processes, nine per cent for public water supply and two per cent for stock watering. However, it should be borne in mind that the vast majority of water takes for stock consumption are not the subject of resource consents, as they fall under the permitted use category in the relevant regional plan. Sixty-six per cent of the resource consents related to groundwater takes, although the volume of water taken from surface water sources was higher. Added together, the total allocation of water in 2006 was 676 cubic metres every second, which is the equivalent of twice the average flow rate of the Waikato River.
In some areas, freshwater is fully or over-allocated. A study undertaken in 2000 identified 134 fully or over-allocated resources of which 112 were rivers or streams. However, consent holders often do not take the total amount of water they are allocated or extract the water at the same time. The proportions of allocated water actually taken are highly variable. Demand for water changes according to various factors, for example the time of year, crop types, growth stage of the crop and weather conditions.
Water that is taken from groundwater reserves can be replaced by rainwater, but if too much is taken, the level of water in the aquifer can permanently decrease. This is known as lowering the water table. Where the water table is permanently lowered, this may have effects on surrounding streams and wetlands, as many may rely on groundwater flow particularly in the dry season when there is not enough surface water available to support them. Land may subside and in coastal areas, salt water may infiltrate fresh water as the water table drops. In addition, users with shallower wells may lose their supply.
Abstraction of large amounts of water from a water body can have significant effects on its ecological health. When large amounts of water are removed from a river, not only does the level of flow decrease, but the natural variability of the flow may also be affected. These changes can destroy the river’s natural character, as the varying flow is part of what has caused the river to develop the characteristics that it has. Unusually high flood flows that occur during periods of heavy rain can be important for keeping the river downstream clean, reducing nuisance plant growth, clearing away weeds and helping maintain its braided nature.
Reducing flow variability and flow levels can impact on fish, invertebrates and bird life. It can also affect the recreational values of a river. For example, kayakers and boaters require a minimum water level in order to undertake their activity.
Some rivers are particularly susceptible to damage from abstraction. Decreasing the flow in a braided river may eliminate some side braids altogether, as the reduced flow cannot sustain more than the main channel. This results in a decline in available habitat for braided river species – for example the dwarf galaxias, a rare native fish, tends to live in these side braids. Many invertebrates live there and they can be an ideal foraging habitat for birds such as the rare black fronted tern.
Dams are often used to store water for abstraction. Their construction in a previously natural environment can have serious adverse environmental effects. Dams do not just prevent water from moving down a river, but also gravel and silt that would normally be carried along by the water. Sediment that would naturally be transferred downstream can thus build up behind the dam. This can exacerbate downstream and coastal erosion as gravel and silt that would normally replace eroded areas is not available. Erosion of the river bed and riparian areas results in the loss of habitat for many species, for example native fish that normally inhabit the crevasses between pebbles on the river bed. In addition, dam structures can prevent fish passage along the water body. This problem can be mitigated if effective fish passes are used to allow the fish through the structure.
Abstraction of geothermal fluid for energy generation can result in a reduction in the temperature and pressure in geothermal fields. This in turn can lead to a loss of geothermal features such as geysers as well as result in ground subsidence.
Last updated at 1:51PM on February 25, 2015