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.
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.
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.
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.