Our rivers, lakes, wetlands, and aquifers are affected by natural and human-caused pressures. Terrain and climate can influence the way rivers form, the way water flows, and how much water is in a waterway or aquifer. Natural erosion, especially from alpine areas, continually changes the shape of many New Zealand rivers. Rain, and snow and ice melt influence the amount of water carried by waterways. Most of these ‘natural’ pressures remain relatively constant over time (although climate change is expected to influence aspects such as rainfall).
Human-caused pressures include discharges of pollutants from agriculture, industry, and urban areas into waterways; human-caused erosion; and the taking of water for irrigation and hydroelectric power generation.
This section outlines some key pressures on our waterways and aquifers from land use and the taking of water for irrigation and other uses.
How we use land affects freshwater quality
Our land has dramatically changed since humans first settled in New Zealand 700–800 years ago (see Land chapter). This change accelerated with the spread of pastoral farming during the 19th century. Agriculture remains the dominant land use, and farming practices have further intensified over recent decades. Between 2002 and 2014, sheep numbers decreased 25 percent but dairy cattle numbers increased 30 percent.
Land use and population growth have placed increasing pressure on waterways. This is more evident with agricultural land because it surrounds 46 percent of New Zealand’s rivers. The main pressures from intensive agriculture are the taking of fresh water for irrigation, and an increase in the flow of nutrients and E.coli into waterways.
New Zealand’s population grew 35 percent in the 32-year period from 1981 to 2013 (Statistics NZ, 2013a), with most of the increase in the Auckland region. Population growth has increased pressure on urban water infrastructure and increased the level of run-off and pollutants entering urban rivers and streams.
The nutrients of most concern in New Zealand are nitrogen and phosphorus. These nutrients occur naturally in relatively low quantities in rivers, streams, and lakes, and are essential for plants to grow. However, an excess of these nutrients can lead to excessive growth of slime and algae (periphyton). This excessive growth can increase daily fluctuations in dissolved oxygen and acidity (pH), impede river flows, and smother the riverbed, which fish and other aquatic animals depend on for food and habitat. High levels of nitrogen can also be harmful to fish and other aquatic animals.
In catchments dominated by agriculture, nitrogen in rivers comes mainly from livestock urine, and from nitrogen-based fertiliser. Between 1990 and 2012, the estimated amount of nitrogen that leached into soil from agriculture increased 29 percent (about 1.5 million kilograms a year). This increase was mainly due to increases in dairy cattle numbers (and therefore urine which contains nitrogen) and nitrogen fertiliser use (see figure 22). Once in the soil, excess nitrogen travels through soil and rock layers, and eventually ends up in groundwater, rivers, and lakes.
This graph shows the estimated annual nitrogen leached from the soil between 1990 and 2012. Leached nitrogen is shown as a total and by source, including from sheep, dairy cattle, beef cattle, and nitrogen fertilisers. Visit the MfE data service for the full breakdown of the data.
In catchments dominated by agriculture, phosphorus mostly enters waterways as part of eroded sediment. It can also come from phosphorus-based fertiliser.
Urban land use also affects waterways. Run-off from roads and other human-made surfaces can wash heavy metals and other pollutants into drains, streams, and rivers. Untreated wastewater also carries pollutants into streams and rivers. We currently have insufficient data to assess the effect of urban and industrial discharges on the freshwater environment and how these pressures have changed over time. We will include this information in future reports.
For more detail see Environmental indicators Te taiao Aotearoa: Trends in nitrogen leaching from agriculture and Geographic pattern of agricultural nitrate leaching.
Most water taken from waterways is used for irrigation
Irrigation is the biggest consumptive user of water in New Zealand (consumptive use is when water is taken from a waterway but not returned). Based on resource consent information (which governs the amount of water a user is allowed to take), irrigation accounts for about three-quarters of consumptive water use. The remainder is used for industry, town water supply, and stock water (Aqualinc Research, 2010). There is strong regional variation in water use: about half the water allocated for consumption nationally is in Canterbury, and one-quarter is in Otago.
Hydroelectric power generation is a major user of fresh water, but most of this water is returned to the river or lake it was taken from and so is not included in the breakdown of consumptive use. The exception is the Manapouri power station, where water is diverted from the Waiau River into the sea.
We are currently collecting new data to enable more in-depth and accurate reporting on freshwater flows. The data includes information on the total irrigated area, actual water use from water meters, the reliability of supply, restrictions on water use to manage over-allocation or seasonal low river flows, and the impact of water use on instream flows, river width, and fish habitat.