8 How does irrigation impact on environmental outcomes?

It is generally acknowledged that irrigation impacts on environmental, conservation, recreational and cultural values. There are many sources of information on these impacts and this section does not attempt to identify, summarise or quantify them. Therefore the costs and benefits of environmental externalities have not been incorporated into the analysis of the economic value of irrigation. Instead this section attempts to provide an understanding of the mechanisms by which they impact and therefore how they may be mitigated.

The main impact of irrigation application on environmental outcomes is via land use change and the intensification of land use. More intensive farming has a greater risk of adverse environmental impacts, and risks can be accentuated by the physical characteristics of the location, for example the soil types, the proximity of surface water and the slope. Some impacts will be negative and some positive. Most can be influenced significantly by positive management practices, for example cultivation practice, water application efficiency, fertiliser application practices and stocking practices. The risks can be managed but require a much higher level of management input than dryland systems.

Irrigation abstraction has impacts on environmental values associated with building and operating irrigation distribution and application systems, siting, developing and operating storage. There can also be changes to the balance of the ecosystem from which the water is being abstracted.

Irrigation in much of the world occurs in arid or semi-arid continental climates where there is very low natural rainfall. In many of these areas the combination of unsuitable soils and poor drainage has lead to severe problems with soil degradation and salinisation. With the exception of small pockets in Central Otago, these conditions don’t exist in New Zealand, so New Zealand is generally not at any risk of major soil degradation from this source.

Loosely the impacts fall into nutrient effects, micro-organism effects and broader conservation effects[1].

8.1 Nutrients

Soluble nutrients (mainly Nitrogen as nitrate ions) from both fertiliser and natural recycling processes in the soils are carried by water percolating through the soil profile or flowing across the ground surface[2]. Irrigation increases the risk of this occurring. Depending on the amount and timing of irrigation, varying amounts of nitrates find their way into underground aquifers or surface rivers and streams. The impact on the concentration of these nutrients is complex and can depend on the form in which the nutrient is supplied, the dilution provided by the waterbody and the balance between various nutrients (most notably nitrogen and phosphorus). It can also take many years before the impact of past nutrient additions become apparent, for example, Lake Taupo.

With irrigation there is generally land use change and/or intensification of use with a concomitant increase in nutrient application and use, and there is potential for higher concentrations of nutrients to occur in waterbodies and catchments that are irrigated than under land uses without irrigation. However, there is also generally a greater ability to manage both the amount and timing of water applied as irrigation, and the amount of nutrient applied than in natural systems. Mitigation measures are already being used and more are under development, such as decision support systems for more precisely applying fertiliser and water to match the plant demands, soil water monitoring equipment, and improved application technology using centre pivot irrigation.

The mechanisms of nutrient cycling and losses are reasonably well understood, but the relationship of this to nutrient levels in any given stream or groundwater system is not. For example, there appears to be a natural variation in nitrate levels in groundwater that is difficult to isolate from land use effects[3]. Considerable work is continuing to better understand this relationship, as the impact on the environment is of critical importance.

There are also some positive impacts of irrigation that help to reduce nutrient losses. The most common is soil organic matter build up on light stony soils[4] prevalent on the Canterbury Plains. This build up of organic matter improves water-holding capacity, reduces nutrient leaching and reduces soil loss from frost and wind events. The effect is to allow more intensive management of these soils than would otherwise be the case at a similar risk of nutrient or soil loss.

Furthermore, the stability of the farming system over time needs to be considered as an impact of irrigation. For example, during or shortly after droughts, dryland farming systems are often subjected to short term environmentally damaging practices that management would prefer not to have to do (for example, cultivation or mob stocking), just for the farm business to survive. Irrigated farmers are able to minimise such practices because of more reliable production patterns.

8.2 Micro-organisms

Irrigation allows pastoral farms to support greater livestock numbers than would otherwise be practical and sustainable for the same level of business risk. Studies completed show a direct link between intensification of livestock farming and greater loads of micro-organisms in water.

The irrigation type and the soil type considerably influence the degree of percolation through the soil into groundwater.  These in turn are significantly influenced by irrigation management practices, in the same way as nutrient percolation.

Surface runoff or shallow percolation tends to concentrate the micro-organisms, the impacts of which vary depending on the volume of the receiving water, the type of micro-organism, etc. This is generally not an issue for irrigation of crops, although most arable crop systems have livestock integrated into the system albeit at relatively low stocking rates. However, there are times when stocking is intensive and potential for high micro-organism loadings in receiving waters arises. 

Work is underway to quantify the impacts of land use change under irrigation e.g., the Lincoln University dairy farm project co-funded by the MAF Sustainable Farming Fund.

8.3 Conservation

Abstraction of water from groundwater or from streams and rivers reduces water flows, which can have a number of impacts on the waterbody concerned e.g., on the habitat provided by the river for flora and fauna, on the recreational value, or on the ability of the waterbody to assimilate waste. Irrigation supply structures and management of water delivery can reduce natural "storm pulses" through rivers – pulses that are often important for stream health (stops silting and removes vegetation). Establishing these structures has short-term environmental impacts and can result in fundamental change to the waterbody.

As well as these negative effects, irrigation can also have positive benefits (apart from the obvious economic ones). e.g., re-charge of groundwater aquifers, runs-off back into streams can maintain more even flows down stream. Dams/reservoirs can also have recreational/ environmental, as well as flood control benefits. As with many complex issues the quest of whether effects are positive or negative are often quite site specific. Costs have been incurred for some communities to ameliorate drainage problems resulting from increased irrigation of surrounding land.

8.4 Net environmental result

The net result of these impacts may be positive or negative. Furthermore, what is negative to one group may be positive to another, for example, storage development may impact on some habitats but may result in enhancement of some other water courses. Changes in environ­mental values due to natural processes that are occurring simultaneously may also further confuse the identification of the impact of irrigation. Information is seldom perfect and different sources of information often cannot be compared directly. As more information becomes available, the concept of environmental bottom lines[5] is being shown to be too simplistic. Rather, it is likely that there are further environmental benefits from maintaining the natural variable flows of the river system.

It is also important to consider the wider picture of how various water systems and irrigation systems interact when considering impacts. While local and regional impacts may focus at the individual catchment level, viewing the system more as one (while recognising the nature of cultural values that may work against this approach) may give an improved outcome. For example, Canterbury has been shown to have more than adequate water for all uses[6], but the water is in the wrong place at the wrong time. Studies in other regions have given similar results.

Storage of excess flows from some major waterbodies in these regions would significantly relieve the environmental pressure that some smaller rivers and groundwater systems are under. Research has helped in an understanding of the close relationship between alluvial groundwater levels on the plains and lowland spring-fed streams (for example, the Avon River in Christchurch) and wetlands. Now that the relationships are becoming better understood, it is evident that there are opportunities to incorporate water abstraction for various uses as part of the active management of environmental, recreational and aesthetic values.