Freshwater

Water use

Irrigation is increasingly used as a means to expand and intensify agricultural activity (including horticulture) in New Zealand. 4107  In 2006, the total consented irrigated land was just over 970,000 hectares, with 80 per cent of this being in Canterbury and Otago. 4108   The growth of irrigation in certain areas of the country is due to a desire to sustain and intensify farming enterprises in areas which are relatively water-poor. For example, the east cost of the South Island receives relatively little rainfall, because the location of the Southern Alps intercepts the prevailing weather, causing precipitation to fall before it reaches the region.

Issues arise where under low flow conditions, peak demand which is mainly for irrigation, cannot either be met by current abstraction methods or sustain the current methods of use. Methods to store surface water from lakes and rivers need to be found if the resource managers are to keep up with demand and if the environmental impacts on water quality of low flow levels are to be mitigated. However, suitable places to store water are few and far between, as they generally require the flooding of land. This is often land which has other important uses and environmental values. The impact of intensification on water quality must also be considered. A solution which addresses both water demand and avoids low flows in rivers addressing both water quality and quantity issues and futureproofing against climate change is a real challenge for this generation to overcome.

Water quality

Whilst pollution from point sources has been decreasing over the last decades, non-point source leaching of nutrients has been identified as an issue. The National River Water Quality Network indicated that in respect of the period from 1989 to 2007, there was a general trend towards improvement in visual clarity of water, but levels of nutrients such as phosphorus and nitrogen were  increasing in some catchments. Expansion and intensification of agricultural land use has caused increases in nutrients in waterways through leaching and runoff in some areas. 4109

Motutapu Cows (Credit: Raewyn Peart)

Agricultural has been identified as a source of increasing nitrogen in waterways. As native vegetation is turned into pasture, wetlands drained and stream bank vegetation removed, the land becomes less able to absorb water. As a result more nutrients are introduced into waterways from surface run off, delivering sediment, organic matter (effluent) and nutrients that have been deposited on the land. Leaching of contaminants through the soil profile further increases the nutrient loadings in water systems.

The issues relating to nitrogen especially have created an increased awareness of the effects that agricultural activity can have on waterways and how it is dealt with at a national level under the RMA.

Nutrient pollution

Increase nutrients in waterways, predominantly in the form of nitrogen and phosphorus, is one of the most significant byproducts of agricultural activity.

Horticultural and arable land uses make up less than two per cent of the country’s land area, a tiny portion compared with other agricultural land uses. Most of the land used for horticulture is flat or slightly undulating, which means that it does not produce large amounts of runoff. Although on modest slopes soil loss is often an issue in heavy rainfall events. Runoff occurs where water is applied at a rate faster than the soil can absorb. Nitrogen and in some cases phosphorus from fertilisers can also leach through the soil into freshwater systems. Run off and soil leaching can have marked effects on freshwater quality in some areas. 4110

Sedimentation

The removal of hill and riparian forests for agricultural use increases the amount of rainfall running off the land into streams and rivers. The tree canopy in a mid-latitude forest typically intercepts and then evaporates away up to one third of the rain that falls on that canopy, effectively reducing the rain that reaches the ground. Canopy interception losses are a function of effective leaf area and canopy roughness, and in west coast locations with frequent rain, can be significantly greater than water losses to transpiration (water drawn up through the tree roots via photosynthesis). There is little difference in canopy interception losses between native and exotic forests, tall manuka and kanuka or dense infestations of gorse or broom, provided that a closed canopy exists.

But such interception losses are far greater than that occurring under tussock, short pasture grasses or crops. In contrast to the highly compacted soils often found under pastoral or arable agricultural systems, forest soils have much higher infiltration capacities and seldom generate overland flow or surface run-off. When forest is replaced by short-rooted pasture grass, increased amounts of rainfall are allowed to run off the land. In the absence of tree roots to hold the soil together, greater amounts of sediment made up of inorganic matter (rock and soil) and organic matter (mostly animal faeces), are washed from the land into groundwater, streams, rivers and wetlands.

Although most forest in New Zealand were removed between 800 and 80 years ago (in two waves with initial Maori impacts followed by extensive impacts of European settlement), the effects of these changes in the landscape continue to be felt, as the hills and riparian areas continue to be used for agricultural production.

  1. Ministry for the Environment, 1997, The state of New Zealand’s environment 1997, 42

  2. Ballantine D J and R J Davies-Colley, 2009, Water quality trends at National River Quality Network sites for 1989-2007, Ministry for the Environment, Wellington, available at www.mfe.govt.nz/publications/water/water-quality-trends-1989-2007/water-quality-trends.pdf

  3. Ballantine D J and R J Davies-Colley, 2009, Water quality trends at National River Quality Network sites for 1989-2007, Ministry for the Environment, Wellington, available at www.mfe.govt.nz/publications/water/water-quality-trends-1989-2007/water-quality-trends.pdf

  4. http://www.mfe.govt.nz/environmental-reporting/land/use/fertilisers.html

Last updated at 11:04AM on November 27, 2015