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Irrigation and drainage

by  Terry Heiler

Too much water or not enough water can both cause problems for farming. In drier areas of New Zealand irrigation schemes have made farmland lush and productive, while drainage of waterlogged land has made it possible to grow crops and pasture. However, both irrigation and drainage can have negative environmental consequences.

Regional characteristics

Regional rainfall

In general, New Zealand gets plenty of precipitation in the form of rain, snow and ice. However, it is not evenly spread, and water is not always in the right places at the right time.

Rain, rain, rain

At 25°C air can hold three times as much water as air at 10°C. As the westerly airstream climbs to go over the Southern Alps, it cools and drops a lot of rain. One millimetre of rain falling over 1 square metre is 1 litre of water; on 1 hectare it is equivalent to 10,000 litres.

Regions to the east of the mountains in the South Island have a low annual rainfall, while those on the west have a very high rainfall. On a line from the west coast to the east coast through Queenstown, there are annual rainfalls ranging from 5,000 millimetres in the west to 350 millimetres in Central Otago. The variability is less in the North Island, but rainfall tends to be lower in the east, and higher in the centre and west of the island.

On the east coasts of both islands, annual rainfall ranges from 350 to 700 millimetres. Rainfall is generally about the same in winter as in summer, averaging 3–6 millimetres per day. However, in the hotter parts of the South Island evapotranspiration (loss of water from the soil due to evaporation and crop transpiration) ranges from about 1–2 millimetres per day in winter to 8–10 millimetres per day in summer. As a result, summer and autumn droughts are common, and irrigation is needed for crops and pasture to keep growing. Irrigation demand peaks in December and January.

Regional irrigation

Since the late 1970s the area of irrigated farmland has doubled every 12 years.

In 2006 irrigation consents had been granted for 972,653 hectares nationally. Of this, 88.9% was down the east coast of New Zealand, 66.5% in Canterbury, 14.5% in Otago, 4.1% in Hawke’s Bay and 3.8% in Marlborough. By 2007 about 80% of these consents had been actioned on an estimated area of 776,000 hectares.

Economic significance of irrigation

Irrigated land covers about 4% of New Zealand’s farmland. In dry land regions irrigated land produces three times as much animal or crop produce as an equivalent area that is not irrigated. The extra value is around $1 billion each year.

Drainage needs

In the west and centre of the North Island, the average annual rainfall ranges from 1,000 to 1,500 millimetres. With this high rainfall, the main tasks of early settlers and government were to protect settlements from flooding, improve agricultural land through drainage, and protect it from water erosion.

Some lowland soils receive runoff water from upland zones. Unless this is quickly drained, either overland or through the soil, the land becomes waterlogged. This slows the rate of vegetation growth and decomposition, and organic matter builds up on the surface. This is especially typical of land with peat soils such as the Hauraki Plains, or pakihi soil (from a Māori word that means open country) as on the West Coast.

Before human intervention, the soil of around 20% of New Zealand’s land area was wet, concentrated mainly in Northland, Waikato, Manawatū, Westland and the lowlands of Southland. Despite a low annual rainfall, the coastal areas of Canterbury and Hawke’s Bay also initially needed draining before they could be used for farming.

Beginnings of irrigation

Otago and Canterbury are the two driest regions in New Zealand, so the management of water has always been important there. They were the first places to begin irrigation.

Mining rights

In the 1880s, during the gold rush in Central Otago, water rights for sluicing were issued by the warden’s courts as property rights. Later, under the Mining Act 1898, the rights to take water from small burns, creeks and water races diverted from larger streams were classified as mining rights. When land use changed from mining to pastoral agriculture, these rights were used to irrigate grasslands by flooding them (wild-flood irrigation), and became a fiercely protected property right.

Government involvement

Irrigation trials started in mid-Canterbury as early as 1880, but large-scale irrigation did not start until well into the 1930s. The Public Works Act 1910 gave the government the power to construct irrigation systems.

Up until the end of the 1920s, the government assisted Central Otago farmers to use their water resources better by constructing storage dams and distribution channels. The facilities were owned and operated by the government, with minimal finance from water users for operation and maintenance.

The Rangitātā diversion race

In 1935 work began on constructing a large channel to take water from the Rangitātā River in Canterbury 67 kilometres north to the Rakaia River, near Methven. It was used for power generation at the Highbank power station, and for irrigation. It was opened in 1945. Irrigation water from this race was responsible for transforming a large part of mid-Canterbury into highly productive farmland.

Otago and Canterbury schemes in the 1930s

By about 1933, most irrigation came from government schemes in Central Otago. About 45,000 hectares was irrigated. From the early 1930s through to the post-war period, developing irrigation was seen as something government should fund because it made farmland more productive, and it also provided work for unemployed people through works programmes. Several small schemes in Canterbury – Redcliffs (1936), Levels (1937) and the Rangitātā diversion race (begun in 1935) – were primarily driven by this policy.

Over time, the Crown purchased many of the original mining rights and developed large irrigation schemes, often involving storage dams. The current mining rights based on these entitlements do not expire until 2021, and in parts of Central Otago, when put together, they actually exceed available river flow.

The post-war period

Until about 1960 there was a clear policy that increasing production through irrigation was in the national interest, and should be supported by government. Subsidisation of agriculture was seen as justified to increase productivity and exports, which increased New Zealand’s foreign exchange earnings. Economic development took precedence over conservation or environmental interests.

Canterbury irrigation schemes

Irrigation schemes opened after the Second World War were:

  • the Ashburton–Lyndhurst scheme, opened in 1945 and supplying 13,300 hectares
  • the Mayfield–Hinds scheme, opened in 1948 and covering 35,400 hectares
  • the Valetta–Tinwald scheme, opened in 1957 and covering 5,400 hectares.

By 1959 the total irrigated area within these schemes was about 62,500 hectares.

Border dyke

The name border dyke describes the channel that restricts the flow of irrigation water as it moves over farmland. A dyke is a bank that prevents water entering or leaving an area. Border dykes are no more than 20–30 centimetres high and built on slightly sloping ground. It can’t be steeper or water would erode the soil.

Research and advice

In 1946 the Department of Agriculture established the Winchmore Irrigation Research Station to investigate ways of improving the efficiency of irrigation. They looked at ways of making irrigation automatic, and evaluated the importance of irrigation in contrasting farm systems. By the mid-1960s almost all of the flood irrigation schemes in New Zealand used automatic systems. The Department of Agriculture employed irrigation advisory officers to provide information to farmers.

1960s to 1970s

From the 1960s there were rapid increases in irrigation development, largely supported by government subsidies. By 1970 the total area under government irrigation schemes was about 100,000 hectares.

Private developments also increased dramatically, mostly based on groundwater resources in Canterbury. These covered an additional 100,000 hectares. Irrigation was seen as a cost-efficient way of making farming more productive – especially dairying.

However, community concerns about negative impacts on the environment from farming began to emerge.

1980s to 2000

Since the mid-1980s farmers themselves, rather than the government, have led the demand for increased irrigation.

Concern about the environment, and the availability and quality of water, increased. The Resource Management Act 1991, which aims to promote the sustainable management of natural and physical resources, was introduced.

Irrigation after 2000

By 2006 about 776,000 hectares were under irrigation, three times as much as in 1985. This accounted for 77% of all water allocated for managed use. In Canterbury, which had 400,000 hectares of irrigated land in 2006, irrigation used 7–14% of the region’s total water resource.

Financial benefits

In 2002/3 irrigated land contributed a net $920 million to New Zealand’s economy, 11% of the total farming contribution to the gross domestic product. It has been projected that further irrigation developments by 2013 of up to 470,000 hectares could earn an additional $660 million per year.

Water resource investment

For irrigation to expand, it was recognised that dam storage needed to be increased, and river flows need to be sustained. However, it has proven difficult to identify water resources suitable for storage, the process of applying for consents to take water can be expensive, and the financial investments required are large, despite attractive economic returns.

Large-scale community projects also involve inter-generational issues, with large costs lumped onto the present generation, but benefits continuing for an unlimited period into the future.

Irrigation problems in Canterbury and Otago

In 2007 Canterbury had the largest irrigated area in New Zealand, and the largest annual volume of water consented for use. However, consent reviews were likely to reduce entitlements, and so new applications were unlikely to be successful.

In Otago in 2007 water resources were heavily over-allocated where traditional mining rights were still in place. When these expire in 2021, water allocation will probably be reduced to protect the environment.

The problems evident in Canterbury and Otago in the early 2000s are expected to occur also in the other irrigated regions of New Zealand.

Likely water management changes

Unless there are changes to current and planned water management, new irrigation operators are likely to find it increasingly difficult to gain access to water that is reliable enough to justify the investment.

Many farms are changing from the old border dyke system to centre pivot spray irrigation, which uses about half as much water to the same effect. The cost of this is about $7,000 per hectare.

In future, existing and new irrigation operators are likely to be subject to more onerous consent conditions, requiring improvements in water quality and more efficient use of water. More measurement, monitoring and reporting will probably be required, higher charges imposed by regional councils, and tougher penalties for non-compliance.

Water trading rights will probably be available in the future, and water meters required to measure water used. South Wairarapa plans for water meters to be in place for all users after 2009.

Irrigation and the environment

There are two main environmental concerns around irrigation:

  • the availability of water and the implications of extracting it from rivers
  • the nitrate leaching associated with increased stock numbers or crop production on irrigated land.

Legislative protection

The Water and Soil Conservation Act 1967 was the first legislation to recognise community concerns about land degradation, and the need to have minimum flow constraints on certain rivers. For the first time, water takes had to be registered and fixed-term permits issued. An amendment to the act in 1971 introduced water conservation orders to protect wild and scenic rivers.

Rakaia order

The first real test of this new approach to water allocation was the granting of a water conservation order on the Rakaia River in 1988. This prevented farmers south of the Rakaia developing irrigation schemes.

Water conservation orders are administered by the Ministry for the Environment. They are placed on water bodies of national significance to preserve them in their natural state (or as close as possible to it), and protect their recreational or natural features.

Resource Management Act

The Resource Management Act 1991 legislated for the sustainable management of natural and physical resources. Almost all irrigation proposals put forward since the 1980s, both private and communal, have met with vigorous opposition from environmentalist groups, fishing and recreation interests, local interest groups, and the Department of Conservation. Despite this, the total area under irrigation in New Zealand trebled between 1985 and 2007, almost entirely through private initiatives.

Sustainable Water Programme of Action

In 2006 the government announced an accelerated Sustainable Water Programme of Action (SWPoA). It called for progress to be made by 2007 on:

  • a national policy statement on water allocation
  • national environmental standards on water measurement, and acceptable ecological river flows and levels
  • identifying and protecting iconic water bodies
  • protecting water quality from the effects of urban and agricultural land use
  • identifying the role of water-user groups in water resources management
  • tools to assist regional councils to achieve the objectives of the SWPoA.

Nitrate leaching

National studies have shown a serious water quality problem developing in some streams and lakes, which was largely attributed to intensive land use. There have been concerns about nitrate contamination of groundwater from irrigated land use. Dairying has been particularly blamed, though nitrate losses are generally much greater under vegetable production than under dairying, but the area involved nationally is much smaller.

In Canterbury, from 1995 to 2004, nitrate concentrations in groundwater increased in about 20% of wells, most of which were in the lower plains. Lincoln University measurements concluded that nitrate leaching losses were lower from spray irrigation than from flood irrigation and that, if used correctly, irrigation can reduce nitrate losses by increasing pasture growth and nitrogen uptake.

Irrigation methods

Early irrigation methods

The earliest irrigation schemes in Central Otago used wild flooding, where water was diverted to flood paddocks. While initial costs were low, labour input was high, and water use was inefficient.

Before the mid-1940s the main method of irrigation was the border dyke system, where parallel, 12-metre-wide, shallow channels were constructed across a paddock, and water introduced from a head race. The sequence of watering was manually supervised and very labour intensive.

Automatic irrigation system technology was developed for border dyke irrigation at the Winchmore Irrigation Research Station in the 1950s and 1960s. This system allowed a farmer to set up a 17-day irrigation roster, and for the whole farm to be watered, paddock by paddock, by only one person.

More recently, the border dyke system has been improved through precision land levelling, introducing wider border widths and higher flows, and the use of electronic controllers to sequence the irrigation flows.

Modern developments

In the early 2000s most of the irrigated land in New Zealand used sprinkler systems. This is because most use groundwater, which cannot provide the flow rates needed for border dykes. Also, new sprinkler technologies reduced labour input, applied water more efficiently, allowed more land to be irrigated, and could correct soil moisture deficits each day. As a result, both crop and pasture yields are increased, compared with the same amount of irrigation water used in a border dyke system. However, sprinkler systems use more energy for pumping.

Orchard and vegetable irrigation

A reliable and effective irrigation system is an essential part of growing orchard and vegetable crops in the drier parts of New Zealand.

Developments in orchard and vegetable irrigation progressed from surface flooding and furrow systems, to drip irrigation and permanent sprinklers set within rows under trees. In the early 2000s sophisticated computer-controlled drip and micro-sprinkler systems, capable of applying plant nutrients with the irrigation water (fertigation), were increasingly used.

Irrigation scheduling and control technology

Soil moisture is measured to make sure that plants are only watered when they need it. Private contractors operate soil moisture and irrigation scheduling services. They measure soil moisture using devices including TDR (time domain reflectometry) instruments, which send a pulse between two probes inserted into the soil. The strength of the reflected pulse indicates how much moisture is in the soil.

While the use of soil moisture scheduling is increasing, in 2008 it was probably used on only about 20% of irrigated land. However, it may become mandatory in the future as environmental pressures increase.

Water use measurement

By 2013 it will be mandatory for all of the 9,000 people with resource consents to take water to measure their water use to an accuracy of 5%. This is to improve the monitoring of the efficiency of irrigation and reduce energy consumption.


The need for drainage

If wet soils are to be used for agriculture or horticulture, some drainage is usually needed, because the roots of pasture or crops will not survive and grow in waterlogged soil. Even partly drained soils will slow pasture or crop growth, because wetter soils take longer to warm up, and so growth does not start until later in the spring. Grazing animals, especially cattle, damage the soil when walking over wet land, leaving deep hoof marks that trap rainwater. Farm or orchard machinery may become bogged in wet paddocks.

Drainage advice

Until the late 20th century Department of Agriculture drainage advisory officers advised farmers on drainage techniques, and until the 1980s Massey University operated a drainage extension service in Manawatū. Today commercial drainage companies service the farming community.

Peat drains for irrigation

In the development of a large area of peat land in Waikato and the Hauraki Plains, deep drainage channels were excavated to allow the water table to be lowered. However, it was found that in summer these areas dried out too much for continued pasture growth. The remedy was to block the drains up to a certain level in spring, so plants still had access to groundwater.

Open drains

The first aim of drainage is to lower the water table (the level at which groundwater sits in the soil) below the level of plant roots. Early settlers first tried digging deep drains at wide intervals, with a series of smaller shallow drains leading into them. On West Coast pakihi soils, the soil surface was reshaped into giant hump-and-hollow corrugations, so water ran into drainage channels.

Grassed waterways are wide, shallow channels constructed across almost-flat land to contain occasional floodwater, or to reshape a meandering stream. They are not normally cultivated, but can be used for grazing.

On hills, contour furrows or graded banks are sometimes used to lead water across the slope to prevent erosion, and stop water pooling on lower flat land.

Tile drains

Later, clay pipe drains began to be used. They were constructed at a depth of about 50 centimetres, and carried drainage water into the larger drains. More recently, clay pipes have been replaced with perforated, corrugated plastic tubing.

Mole drains

In clay soils, mole drains can be formed to lead water into clay tile or corrugated plastic drains. These tunnels are created by dragging a 75-millimetre-diameter torpedo-shaped device on the end of a blade through the soil. Mole drains are on a slight slope and at a shallower depth than tile drains.

Mole drains are created at about 5-metre intervals, forming an intensive drainage network under a paddock. Mole drains cannot be used in stony or sandy soils.

Mole draining is best done in late spring or summer, when the soil is moist enough to be shaped, but then can dry so the tunnel and the crack formed by the blade hold their shape.

Environmental issues

Nutrient runoff

The main role of subsurface drains is to remove excess water from the soil. However, any nutrients in the drainage water are also lost into main drains and waterways. This means that more careful fertiliser management may be needed on drained soils to avoid environmental problems, such as nitrate leaching.

Wetland conservation

Draining wetlands for farming had previously been seen as beneficial. In more recent years, the value of wetlands and habitats for wildlife has been recognised. Preserving wetlands may be achieved through a national conservation order, a legal ruling that prevents any changes to the existing condition of the area.

External links and sources

More suggestions and sources

How to cite this page: Terry Heiler, 'Irrigation and drainage', Te Ara - the Encyclopedia of New Zealand, (accessed 24 June 2024)

Story by Terry Heiler, published 24 November 2008